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Dated: Jan. 01, 2011

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TechiWarehouse Note: We at TW were browsing about the web when we stumbled upon Mark's work. Although it's much outdated, it's very fundamental. And please don't let the title fool you. This is not a small short article but a short OS introductory book.

1. What is an operating system?

An operating system is a layer of software which takes care of technical aspects of a computer's operation. It shields the user of the machine from the low-level details of the machine's operation and provides frequently needed facilities. There is no universal definition of what an operating system consists of. You can think of it as being the software which is already installed on a machine, before you add anything of your own. Normally the operating system has a number of key elements: (i) a technical layer of software for driving the hardware of the computer, like disk drives, the keyboard and the screen; (ii) a filesystem which provides a way of organizing files logically, and (iii) a simple command language which enables users to run their own programs and to manipulate their files in a simple way. Some operating systems also provide text editors, compilers, debuggers and a variety of other tools. Since the operating system (OS) is in charge of a computer, all requests to use its resources and devices need to go through the OS. An OS therefore provides (iv) legal entry points into its code for performing basic operations like writing to devices.

Operating systems may be classified by both how many tasks they can perform `simultaneously' and by how many users can be using the system `simultaneously'. That is: single-user or multi-user and single-task or multi-tasking. A multi-user system must clearly be multi-tasking. The table below shows some examples.

OS Users Tasks Processors
Windows 3x S QM 1
Macintosh System 7.* S QM 1
Windows 9x S M* 1
AmigaDOS S M 1
hline MTS M M 1
UNIX M M $n$
NT S/M M $n$
Windows 2000 M M $n$
BeOS (Hamlet?) S M $n$

The first of these (MS/PC DOS/Windows 3x) are single user, single-task systems which build on a ROM based library of basic functions called the BIOS . These are system calls which write to the screen or to disk etc. Although all the operating systems can service interrupts , and therefore simulate the appearance of multitasking in some situations, the older PC environments cannot be thought of as a multi-tasking systems in any sense. Only a single user application could be open at any time. Windows 95 replaced the old coroutine approach of quasi-multitasking with a true context switching approach, but only a single user system, without proper memory protection. Windows NT added a proper kernel with memory protection, based on the VMS system, originally written for the DEC/Vax. Later versions of Windows NT and Windows 2000 (a security and kernel enhanced version of NT) allow multiple logins also through a terminal server. Windows 2000 thus has comparable functionality to Unix in this respect.Operating systems

The Macintosh system 7 can be classified as single-user quasi-multitasking 1.1 . That means that it is possible to use several user applications simultaneously. A window manager can simulate the appearance of several programs running simultaneously, but this relies on each program obeying specific rules in order to achieve the illusion. The MacIntosh not a true multitasking system in the sense that, if one program crashes, the whole system crashes. Windows$9x$is purported to be preemptive multitasking but most program crashes also crash the entire system. This might be due to the lack of proper memory protection. The claim is somewhat confusing.

AmigaDOS is an operating system for the Commodore Amiga computer. It is based on the UNIX model and is a fully multi-tasking, single-user system. Several programs may be actively running at any time. The operating system includes a window environment which means that each independent program has a `screen' of its own and does not therefore have to compete for the screen with other programs. This has been a major limitation on multi-tasking operating systems in the past.

MTS (Michigan timesharing system) was the first time-sharing multi-user system 1.2 . It supports only simple single-screen terminal based input/output and has no hierarchical file system.

Unix is arguably the most important operating system today, and one which we shall frequently refer to below. It comes in many forms, developed by different manufacturers. Originally designed at AT&T, UNIX split into two camps early on: BSD (Berkeley software distribution) and system 5 (AT&T license). The BSD version was developed as a research project at the university of Berkeley, California. Many of the networking and user-friendly features originate from these modifications. With time these two versions have been merged back together and most systems are now a mixture of both worlds. Historically BSD Unix has been most prevalent in universities, while system 5 has been dominant in business environments. The trend during the last three years by Sun Microsystems and Hewlett-Packard amongst others has been to move towards system 5, keeping only the most important features of the BSD system. A standardization committee for Unix called POSIX, formed by the major vendors, attempts to bring compatibility to the Unix world. Here are some common versions of UNIX.

Unix Manufacturer Mainly BSD / Sys 5
BSD Berkeley BSD
SunOS (solaris 1) Sun Microsystems BSD/sys 5
Solaris 2 Sun Microsystems Sys 5
Ultrix DEC/Compaq BSD
OSF 1/Digital Unix DEC/Compaq BSD/sys 5
HPUX Hewlett-Packard Sys 5
IRIX Silicon Graphics Sys 5
GNU/Linux Public Domain Posix (Sys V/BSD)
SCO unix Novell Sys 5

Note that the original BSD source code is now in the public domain. Unix is generally regarded as the most portable and powerful operating system available today by impartial judges, but NT is improving quickly. Unix runs on everything from laptop computers to CRAY mainframes. It is particularly good at managing large database applications and can run on systems with hundreds of processors. Most Unix types support symmetric multithreaded processing and all support simultaneous logins by multiple users.

NT is a `new' operating system from Microsoft based on the old VAX/VMS kernel from the Digital Equipment Corporation (VMS's inventor moved to Microsoft) and the Windows32 API. Initially it reinvented many existing systems, but it is gradually being forced to adopt many open standards from the Unix world. It is fully multitasking, and can support multiple users (but only one at a time-- multiple logins by different users is not possible). It has virtual memory and multithreaded support for several processors. NT has a built in object model and security framework which is amongst the most modern in use.

The Be operating system, originally developed for a new multimedia computer called the BeBox, is also new and is a fully multitasking OS. It is optimized for multimedia and is now saleable software developed by Be.Com after the new computer concept failed due to lack of financial backing. BeOS has proper memory protection but allows direct access to video memory (required for fast video games). It also has virtual memory, is pre-emptive multitasking and is based on a microkernel design. Is shares little with Unix except for a Bash shell, a POSIX programming interface and about 150 Unix commands (including Perl).

1.1 Key concepts

Before discussing more of the details, let's review some key ideas which lie behind the whole OS idea. Although these ideas may seem simple, you will do well to keep them in mind later. Simple ideas often get lost amongst distracting details, but it is important to remember that the ideas are simple.

1.1.1 Hierarchies and black boxes

A hierarchy is a way of organizing information using levels of detail. The phrase high-level implies few details, whereas low-level implies a lot of detail, down in the guts of things. A hierarchy usually has the form of a tree, which branches from the highest level to the lowest, since each high-level object is composed of several lower-level objects. The key to making large computer programs and to solving difficult problems is to create a hierarchical structure, in which large high-level problems are gradually broken up into manageable low-level problems. Each level works by using a series of `black boxes' (e.g. subroutines) whose inner details are not directly visible. This allows us to hide details and remain sane as the complexity builds up.

This is the single most important concept in computing! It is used repeatedly to organize complex problems.

Figure 1.1: The hierarchy is the most important concept in computing.

1.1.2 Resources and sharing

A computer is not just a box which adds numbers together. It has resources like the keyboard and the screen, the disk drives and the memory. In a multi-tasking system there may be several programs which need to receive input or write output simultaneously and thus the operating system may have to share these resources between several running programs. If the system has two keyboards (or terminals) connected to it, then the OS can allocate both to different programs. If only a single keyboard is connected then competing programs must wait for the resources to become free.

Most multi-tasking systems have only a single central processor unit and yet this is the most precious resource a computer has. An multi-tasking operating system must therefore share cpu-time between programs. That is, it must work for a time on one program, then work a while on the next program, and so on. If the first program was left unfinished, it must then return to work more on that, in a systematic way. The way an OS decides to share its time between different tasks is called scheduling.

1.1.3 Communication, protocols, data types

The exchange of information is an essential part of computing. Suppose computer A sends a message to computer B reporting on the names of all the users and how long they have been working. To do this it sends a stream of bits across a network. When computer B receives a stream of bits, it doesn't automatically know what they mean. It must decide if the bits represent numbers or characters, integers or floating point numbers, or a mixture of all of them. These different types of data are all stored as binary information - the only difference between them is the way one chooses to interpret them.

The resolution to this problem is to define a protocol. This is a convention or agreement between the operating systems of two machines on what messages may contain. The agreement may say, for instance, that the first thirty-two bits are four integers which give the address of the machine which sent the message. The next thirty-two bits are a special number telling the OS which protocol to use in order to interpret the data. The OS can then look up this protocol and discover that the rest of the data are arranged according to a pattern of


where the name is a string of bytes, terminated by a zero, and the time is a four byte digit containing the time in hours. Computer B now knows enough to be able to extract the information from the stream of bits.

It is important to understand that all computers have to agree on the way in which the data are sent in advance. If the wrong protocol is diagnosed, then a string of characters could easily be converted into a floating point number - but the result would have been nonsense. Similarly, if computer A had sent the information incorrectly, computer B might not be able to read the data and a protocol error would arise.

More generally, a protocol is an agreed sequence
of behaviour which must be followed.

For example, when passing parameters to functions in a computer program, there are rules about how the parameter should be declared and in which order they are sent. This is a simple example of a protocol. Protocols are an important part of communication and data typing and they will appear in many forms during our discussion of operating systems.

1.1.4 System overhead

An operating system is itself a computer program which must be executed. It therefore requires its own share of a computer's resources. This is especially true on multitasking systems, such as UNIX, where the OS is running all the time along side users' programs. Since user programs have to wait for the OS to perform certain services, such as allocating resources, they are slowed down by the OS 1.3 . The time spent by the OS servicing user requests is called the system overhead. On a multi-user system one would like this overhead to be kept to a minimum, since programs which make many requests of the OS slow not only themselves down, but all other programs which are queuing up for resources.

In the UNIX C-shell (csh) environment, it is possible to find out the exact fraction of time spent by the OS working on a program's behalf by using the timefunction.

1.1.5 Caching

Caching is a technique used to speed up communication with slow devices. Usually the CPU can read data much faster from memory than it can from a disk or network connection, so it would like to keep an up-to-date copy of frequently used information in memory. The memory area used to do this is called a cache. You can think of the whole of the primary memory as being a cache for the secondary memory (disk).

Sometimes caching is used more generally to mean `keeping a local copy of data for convenience'.

1.2 Hardware

Here we list the main hardware concepts.

1.2.1 The CPU

The CPU, or central processor unit is the heart and soul of every computer. This is the part which does the work of executing machine instructions. Traditionally, it is just one microprocessor with lots of pins to connect is to memory and devices - usually identifiable by being the largest chip. On modern machines, there may be several CPUs which can work in parallel. Also VLSI or very large scale integration technology has made it possible to put very many separate processors and memory into a single package, so the physical distinction between the CPU and its support chips is getting blurred. Nevertheless, the CPU is still logically separate from the memory and devices.

The CPU is driven by a `clock' or pulse generator. Each instruction completes in a certain number of `clock cycles'. Traditionally CPUs are based on CISC (Complex Instruction Set Computing) architecture, where a single instruction takes one or more clock cycles to complete. A new trend is to build RISC (Reduced Instruction Set Computing) processors which aim to be more efficient for a subset of instructions by using redundancy. These have simpler instructions but can execute much more quickly, sometimes with several instructions per clock cycle.

1.2.2 Memory

The primary memory is the most important resource a computer has. Since CPUs are only made with instructions for reading and writing to memory, no programs would be able to run without it. There are two types of memory: RAM - random access memory, or read/write memory, which loses its contents when the machine is switched off, and ROM - read only memory, which never loses its contents unless destroyed. ROM is normally used for storing those most fundamental parts of the operating system which are required the instant a computer is switched on, before it knows about disks etc.

1.2.3 Devices

The concepts of a device really has two parts. There is the hardware unit which is connected to the machine, and there is the logical device which is a name given by the OS to a legal entry point for talking to a hardware-device. When a user writes to a logical device, the OS invokes a device driver which performs the physical operations of controlling the hardware. For example, when writing to a disk, the OS must control the movement of the read-write heads. When writing to a printer, the OS places the information in a queue and services the request when the printer becomes free.

Some common logical devices are: the system disks, the keyboard, the screen, the printer and the audio device.

Disks and tapes are often called secondary memory or secondary storage.

1.2.4 Interrupts, traps, exceptions

Interrupts are hardware signals which are sent to the CPU by the devices it is connected to. These signals literally interrupt the CPU from what it is doing and demand that it spend a few clock cycles servicing a request. For example, interrupts may come from the keyboard because a user pressed a key. Then the CPU must stop what it is doing and read the keyboard, place the key value into a buffer for later reading, and return to what it was doing. Other `events' generate interrupts: the system clock sends interrupts at periodic intervals, disk devices generate interrupts when they have finished an I/O task and interrupts can be used to allow computers to monitor sensors and detectors. User programs can also generate `software interrupts' in order to handle special situations like a `division by zero' error. These are often called traps or exceptions on some systems.

Interrupts are graded in levels. Low level interrupts have a low priority, whereas high level interrupts have a high priority. A high level interrupt can interrupt a low level interrupt, so that the CPU must be able to recover from several `layers' of interruption and end up doing what it was originally doing. This is accomplished by means of a stack or heap 1.4 . Moreover, programs can often choose whether or not they wish to be interrupted by setting an interrupt mask which masks out the interrupts it does not want to hear about. Masking interrupts can be dangerous, since data can be lost. All systems therefore have non-maskable interrupts for the most crucial operations.

1.3 Software

1.3.1 Resource management

In order to keep track of how the system resources are being used, an OS must keep tables or lists telling it what is free an what is not. For example, data cannot be stored neatly on a disk. As files become deleted, holes appear and the data become scattered randomly over the disk surface.

1.3.2 Spooling

Spooling is a way of processing data serially. Print jobs are spooled to the printer, because they must be printed in the right order (it would not help the user if the lines of his/her file were liberally mixed together with parts of someone elses file). During a spooling operation, only one job is performed at a time and other jobs wait in a queue to be processed. Spooling is a form of batch processing.

Spooling comes from the need to copy data onto a spool of tape for storage. It has since been dubbed S imultaneous P eripheral O peration O n- L ine, which is a pretty lousy attempt to make something more meaningful out of the word `spool'!

1.3.3 System calls

An important task of an operating system is to provide black-box functions for the most frequently needed operations, so that users do not have to waste their time programming very low level code which is irrelevant to their purpose. These ready-made functions comprise frequently used code and are called system calls.

For example, controlling devices requires very careful and complex programming. Users should not have to write code to position the head of the disk drive at the right place just to save a file to the disk. This is a very basic operation which everyone requires and thus it becomes the responsibility of the OS. Another example is mathematical functions or graphics primitives.

System calls can be thought of as a very simple protocol - an agreed way of asking the OS to perform a service. Some typical OS calls are: read, write (to screen, disk, printer etc), stat (get the status of a file: its size and type) and malloc (request for memory allocation).

On older microcomputers, where high level languages are uncommon, system calls are often available only through assembler or machine code. On modern systems and integrated systems like UNIX, they are available as functions in a high level language like C.

1.3.4 Basic command language

Commands like

dir                 ; list files (DOS)
ls                  ; list files (UNIX)
cd                  ; change directory
copy file prn       ; copy file to printer
myprog              ; execute program `myprog'

constitute a basic command language. Every computer must have such a language (except perhaps the Macintosh - yawn!). In microcomputer operating systems the command language is often built into the system code, whereas on larger systems (UNIX) the commands are just executable programs like the last example above.

The command language deals typically with: file management, process management and text editing.

1.3.5 Filesystem

In creating a system to store files we must answer some basic questions.

Should the filesystem distinguish between types of files e.g. executable files, text files, scripts. If so how? One way is to use file extensions, or a naming convention to identify files, like myprog, SCRIPT.BAT, file.txt. The problem with this is that the names can be abused by users. If one tries to execute a file which is not meant to be executed, the result would be nonsense and might even be dangerous to the point of crashing the system. One way around this problem is to introduce a protocol or standard format for executable files, so that when the OS opens a file for execution it first checks to see whether the file obeys the protocol. This method is used for binary files in UNIX, for instance.
Protection. If several users will be storing files together on the same disk, should each user's files be exclusive to him or her?
Is a mechanism required for sharing files between several users?
A hierarchical filesystem is a good starting point for organizing files, but it can be too restrictive. Sometimes it is useful to have a file appear in several places at one time. This can be accomplished with links. A link is not a copy of a file, but a pointer to where a file really is. By making links to other places in a hierarchical filesystem, its flexibility is increased considerably.

1.3.6 Multiple windows and screens

Multitasking cannot be fully exploited if each user has only one output terminal (screen). Each interactive program needs its own screen and keyboard 1.5 . There are three solutions to this problem:

  1. Several physical screens can be attached to the computer. This is expensive and probably wasteful.
  2. Toggling between `logical screens'. By pressing a key on the keyboard the user can switch between two different images, which are separately maintained in memory.
  3. Window system.

The technology for the last of these solutions has only been available for a few years. While it is clearly the best of the three (and can be combined with$[1]$), it requires a considerable amount of memory and CPU power to implement. The problem of overlapping windows requires there to be a manager which controls the sharing of space on the screen. All of the graphics must be drawn and redrawn continuously. The operating system must provide primitives for doing this.

We shall not consider windowing further in this text, but it is worth bearing in mind that the principles are very similar to those of operating systems. Sharing and management are the key concepts.


Before proceeding, you should note that the design of operating systems is an active area of research. There are no universal solutions to the issues that we shall discuss, rather OS design must be thought of as a study of compromises. Hopefully you will get a feel for this during the course of the tutorial.


  1. What are the key ingredients of an operating system?
  2. What is the usefulness of system calls?
  3. What is the difference between primary and secondary storage.
  4. What is a logical device?
  5. Should different users be able to change one another's data? If so, under what circumstances?
  6. How do hardware devices send signals to the CPU?

2. Single-task OS

Before tackling the complexities of multi-tasking, it is useful to think about the operation of a single-task OS without all the clutter that multi-tasking entails. In a multi-task OS the features we shall discuss below have to be reproduced$N$-times and then augmented by extra control structures.

2.1 Memory map and registers

The key elements of a single-task computer are shown in figure 2.1 . Roughly speaking, at the hardware level a computer consists of a CPU, memory and a number of peripheral devices. The CPU contains registers or `internal variables' which control its operation. The CPU can store information only in the memory it can address and in the registers of other microprocessors it is connected to. The CPU reads machine code instructions, one at a time, from the memory and executes them forever without stopping.

Here is a brief summary of the types of register a CPU has. Some microprocessors have several of each type.

Register Purpose
Accumulator Holds the data currently being worked on.
Program counter Holds the address of the next instruction
  to be executed
Index (addressing) registers Used to specify the address of data to be loaded into or
  saved from the accumulator, or operated on in some way.
Stack pointer Points to the top of the CPUs
  own hardware controlled stack.
Status register Contains status information after each instruction
  which can be tested for to detect errors etc.

The memory, as seen by the CPU, is a large string of bytes starting with address$0$and increasing up to the maximum address. Physically it is made up, like a jigsaw puzzle, of many memory chips and control chips. mapped into the diagram shown. Normally, because of the hardware design of the CPU, not all of the memory is available to the user of the machine. Some of it is required for the operation of the CPU.

The roughly distinguished areas in figure 2.1 are

Zero page: The first t `page' of the memory is often reserved for a special purpose. It is often faster to write to the zero page because you don't have to code the leading zero for the address - special instructions for the zero page can leave the `zero' implicit.
Stack: Every CPU needs a stack for executing subroutines. The stack is explained in more detail below.
User programs: Space the user programs can `grow into'.
Screen memory: What you see on the screen of a computer is the image of an area of memory, converted into colours and positions by a hardware video-controller. The screen memory is the area of memory needed to define the colour of every `point' or `unit' on the screen. Depending on what kind of visual system a computer uses, this might be one byte per character and it might be four bytes per pixel!
Memory mapped I/O: Hardware devices like disks and video controllers contain smaller microprocessors of their own. The CPU gives them instructions by placing numbers into their registers. To make this process simpler, these device registers (only a few bytes per device, perhaps) are `wired' into the main memory map, so that writing to the device is the same as writing to the rest of the memory.
Operating system: The operating system itself is a large program which often takes up a large part of the available memory.

Note that this figure is very simplified. It does not show, for instance, special memory which might be located inside the devices or CPU. Such memory is often used for caching. Also it does not show how the various components are connected together by means of a high speed data bus.


Figure 2.1: A simple schematic memory map of a microcomputer. The order of the different segments of memory can vary depending on the system.

2.2 Stack

A stack is a so-called last-in first-out (LIFO) data structure. That is to say - the last thing to be placed on top of a stack, when making it, is the first item which gets removed when un-making it. Stacks are used by the CPU to store the current position within a program before jumping to subroutines, so that they remember where to return to after the subroutine is finished. Because of the nature of the stack, the CPU can simply deposit the address of the next instruction to be executed (after the subroutine is finished) on top of the stack. When the subroutine is finished, the CPU pulls the first address it finds off the top of the stack and jumps to that location.

Notice that the stack mechanism will continue to work even if the subroutine itself calls another subroutine, since the second subroutine causes another stack frame to be saved on the top of the stack. When that is finished, it returns to the first subroutine and then to the original program in the correct order.

On many older microcomputers and in many operating systems the stack is allocated with a fixed size in advance. If too many levels of nested subroutines are called, the stack can overflow. Consider the following example code for a stack.

// A simple stack handler. 
// Use the commands "push" and "pop" to push onto the stack and to pop 
// "out" of the stack. The allocated stacksize is very small so that 
// an overflow can occur if you push too far!! e.g. input
//  push 23
//  push 4
//  pop
//  push 678
//  quit
//  In a real stack handler the numbers would be the address of the next
//  instruction to return to after completing a subroutine.
//  The program is compiled with
//        g++ stack.C
//  MB 1994

#include <iostream.h>
#include <strstream.h>
#include <string.h>

// Include file

const int forever = 1;
const int stacksize = 10;
const int bufsize = 20;


class Stack

   int stack[stacksize];
   void ShowStack();
   void Push(int);
   int  Pop();


   int stackpointer;

// Level 0

main ()

{ char input[bufsize];
  char command[5];
  int number, newnumber;
  Stack s;

cout << "Stack demo\n\n";


while (forever)
   cout << "Enter command: ";

   // Extract command
   istrstream(input,sizeof(input)) >> command >> number;

   // Interpret command
   if (strcmp(command,"push") == 0)
   else if (strcmp(command,"pop")==0)
      newnumber = s.Pop();
   else if (strcmp(command,"quit")==0)
      number = 0;
      cout << "Bad command\n\n";


// Class Stack


{ int i;

stackpointer = 0;

for (i = 0; i < stacksize; i++)
   stack[i] = 0;


void Stack::Push (int n)

cout << "Pushing " << n << " on the stack\n";

if (stackpointer >= stacksize)
   cerr << "Stack overflow!\n";

stack[stackpointer] = n;


int Stack::Pop ()

if (stackpointer == 0)
   cerr << "Stack underflow!\n";
   return 0;

cout << "Popped " << stack[stackpointer] << " from stack\n";

return (stack[stackpointer]);


void Stack::ShowStack ()

{ int i;

for (i = stacksize-1; i >= 0; i--)
   cout << "stack[" << i << "] = " << stack[i];

   if (i == stackpointer)
      cout << " <<-- Pointer\n";
      cout << endl;

In this example, only numbers are stored. At the hardware level, this kind of stack is used by the CPU to store addresses and registers during machine-code subroutine jumps. Operating systems also use software controlled stacks during the execution of users' programs. High level languages subroutines can have local variables which are also copied to the stack as one large stack frame during the execution of subroutines.

2.3 Input/Output

Input arrives at the computer at unpredictable intervals. The system must be able to detect its arrival and respond to it.

2.3.1 Interrupts

Interrupts are hardware triggered signals which cause the CPU to stop what it is doing and jump to a special subroutine. Interrupts normally arrive from hardware devices, such as when the user presses a key on the keyboard, or the disk device has fetched some data from the disk. They can also be generated in software by errors like division by zero or illegal memory address.

When the CPU receives an interrupt, it saves the contents of its registers on the hardware stack and jumps to a special routine which will determine the cause of the interrupt and respond to it appropriately. Interrupts occur at different levels. Low level interrupts can be interrupted by high level interrupts. Interrupt handling routines have to work quickly, or the computer will be drowned in the business of servicing interrupts. For certain critical operations, low level interrupts can be ignored by setting a mask (See also the generalization of this for multiuser systems in chapter 4).

There is no logical difference between what happens during the execution of an interrupt routine and a subroutine. The difference is that interrupt routines are triggered by events, whereas software subroutines follow a prearranged plan.

An important area is the interrupt vector. This is a region of memory reserved by the hardware for servicing of interrupts. Each interrupt has a number from zero to the maximum number of interrupts supported on the CPU; for each interrupt, the interrupt vector must be programmed with the address of a routine which is to be executed when the interrupt occurs. i.e. when an interrupt occurs, the system examines the address in the interrupt vector for that interrupt and jumps to that location. The routine exits when it meets an RTI (return from interrupt) instruction.

2.3.2 Buffers

The CPU and the devices attached to it do not work at the same speed. Buffers are therefore needed to store incoming or outgoing information temporarily, while it is waiting to be picked up by the other party. A buffer is simply an area of memory which works as a waiting area. It is a first-in first-out (FIFO) data structure or queue.

2.3.3 Synchronous and asynchronous I/O

To start an I/O operation, the CPU writes appropriate values into the registers of the device controller. The device controller acts on the values it finds in its registers. For example, if the operation is to read from a disk, the device controller fetches data from the disk and places it in its local buffer. It then signals the CPU by generating an interrupt.

While the CPU is waiting for the I/O to complete it may do one of two things. It can do nothing or idle until the device returns with the data (synchronous I/O), or it can continue doing something else until the completion interrupt arrives (asynchronous I/O). The second of these possibilities is clearly much more efficient.

2.3.4 DMA - Direct Memory Access

Very high speed devices could place heavy demands on the CPU for I/O servicing if they relied on the CPU to copy data word by word. The DMA controller is a device which copies blocks of data at a time from one place to the other, without the intervention of the CPU. To use it, its registers must be loaded with the information about what it should copy and where it should copy to. Once this is done, it generates an interrupt to signal the completion of the task. The advantage of the DMA is that it transfers large amounts of data before generating an interrupt. Without it, the CPU would have to copy the data one register-full at a time, using up hundreds or even thousands of interrupts and possibly bringing a halt to the machine!


  1. What is the program counter?
  2. Explain why a stack is used to store local variables.
  3. Some microprocessors (68000/Intel 386 upward) support multitasking internally. A separate stack is then needed for each process. How can this be achieved?
  4. Write a program to create a stack (LIFO) which can store any number of local variables for each subroutine. Hint: use a linked list for the stack and for the variables.
  5. Write a program to implement a buffer (FIFO).
  6. When a computer is first switched on, it executes a program called a bootstrap program. This comes from the expression `to lift oneself by one's own bootstraps'. The computer must begin to execute instructions and `get going'. Find out for yourself, or speculate on how this takes place.
  7. What is a stack-frame?
  8. What is memory mapped I/O?

3. Multi-tasking and multi-user OS


To make a multi-tasking OS we need loosely to reproduce all of the features discussed in the last chapter for each task or process which runs. It is not necessary for each task to have its own set of devices. The basic hardware resources of the system are shared between the tasks. The operating system must therefore have a `manager' which shares resources at all times. This manager is called the `kernel' and it constitutes the main difference between single and multitasking operating systems.

3.1 Competition for resources

3.1.1 Users - authentication

If a system supports several users, then each user must have his or her own place on the system disk, where files can be stored. Since each user's files may be private, the file system should record the owner of each file. For this to be possible, all users must have a user identity or login name and must supply a password which prevents others from impersonating them. Passwords are stored in a cryptographic (coded) form. When a user logs in, the OS encrypts the typed password and compares it to the stored version. Stored passwords are never decrypted for comparison.

3.1.2 Privileges and security

On a multi-user system it is important that one user should not be able to interfere with another user's activities, either purposefully or accidentally. Certain commands and system calls are therefore not available to normal users directly. The super-user is a privileged user (normally the system operator) who has permission to do anything, but normal users have restrictions placed on them in the interest of system safety.

For example: normal users should never be able to halt the system; nor should they be able to control the devices connected to the computer, or write directly into memory without making a formal request of the OS. One of the tasks of the OS is to prevent collisions between users.

3.1.3 Tasks - two-mode operation

It is crucial for the security of the system that different tasks, working side by side, should not be allowed to interfere with one another (although this occasionally happens in microcomputer operating systems, like the Macintosh, which allow several programs to be resident in memory simultaneously). Protection mechanisms are needed to deal with this problem. The way this is normally done is to make the operating system all-powerful and allow no user to access the system resources without going via the OS.

To prevent users from tricking the OS, multiuser systems are based on hardware which supports two-mode operation: privileged mode for executing OS instructions and user mode for working on user programs. When running in user mode a task has no special privileges and must ask the OS for resources through system calls. When I/O or resource management is performed, the OS takes over and switches to privileged mode. The OS switches between these modes personally, so provided it starts off in control of the system, it will alway remain in control.

At boot-time, the system starts in privileged mode.
During user execution, it is switched to user mode.
When interrupts occur, the OS takes over and it is switched back to privileged mode.

Other names for privileged mode are monitor mode or supervisor mode.

3.1.4 I/O and Memory protection

To prevent users from gaining control of devices, by tricking the OS, a mechanism is required to prevent them from writing to an arbitrary address in the memory. For example, if the user could modify the OS program, then it would clearly be possible to gain control of the entire system in privileged mode. All a user would have to do would be to change the addresses in the interrupt vector to point to a routine of their own making. This routine would then be executed when an interrupt was received in privileged mode.

The solution to this problem is to let the OS define a segment of memory for each user process and to check, when running in user mode, every address that the user program refers to. If the user attempts to read or write outside this allowed segment, a segmentation fault is generated and control returns to the OS. This checking is normally hard-wired into the hardware of the computer so that it cannot be switched off. No checking is required in privileged mode.

// Example of a segmentation fault in user mode

main()           // When we start, we are by definition in user mode.

{ int *ptr;

ptr = 0;         // An address guaranteed to NOT be in our segment.

cout << *ptr;

3.1.5 Time sharing

There is always the problem in a multi-tasking system that a user program will go into an infinite loop, so that control never returns to the OS and the whole system stops. We have to make sure that the OS always remains in control by some method. Here are two possibilities:

The operating system fetches each instruction from the user program and executes it personally, never giving it directly to the CPU. The OS software switches between different processes by fetching the instructions it decides to execute. This is a kind of software emulation. This method works, but it is extremely inefficient because the OS and the user program are always running together. The full speed of the CPU is not realized. This method is often used to make simulators and debuggers.
A more common method is to switch off the OS while the user program is executing and switch off the user process while the OS is executing. The switching is achieved by hardware rather than software, as follows. When handing control to a user program, the OS uses a hardware timer to ensure that control will return after a certain time. The OS loads a fixed time interval into the timer's control registers and gives control to the user process. The timer then counts down to zero and when it reaches zero it generates a non-maskable interrupt, whereupon control returns to the OS.

3.2 Memory map

We can represent a multi-tasking system schematically as in figure 3.1 . Clearly the memory map of a computer does not look like this figure. It looks like the figures in the previous chapter, so the OS has to simulate this behaviour using software. The point of this diagram is only that it shows the elements required by each process executing on the system.

Figure 3.1: Schematic diagram of a multitasking system.

Each program must have a memory area to work in and a stack to keep track of subroutine calls and local variables.

Each program must have its own input/output sources. These cannot be the actual resources of the system: instead, each program has a virtual I/O stream. The operating system arranges things so that the virtual I/O looks, to the user program, as though it is just normal I/O. In reality, the OS controls all the I/O itself and arranges the sharing of resources transparently. The virtual output stream for a program might be a window on the real screen, for instance. The virtual printer is really a print-queue. The keyboard is only `connected' to one task at a time, but the OS can share this too. For example, in a window environment, this happens when a user clicks in a particular window.

3.3 Kernel and shells - layers of software

So far we have talked about the OS almost as though it were a living thing. In a multitasking, multi-user OS like UNIX this is not a bad approximation to the truth! In what follows we make use of UNIX terminology and all of the examples we shall cover later will refer to versions of the UNIX operating system.

The part of the OS which handles all of the details of sharing and device handling is called the kernel or core. The kernel is not something which can be used directly, although its services can be accessed through system calls. What is needed is a user interface or command line interface (CLI) which allows users to log onto the machine and manipulate files, compile programs and execute them using simple commands. Since this is a layer of software which wraps the kernel in more acceptable clothes, it is called a shell around the kernel.

It is only by making layers of software, in a hierachy that very complex programs can be written and maintained. The idea of layers and hierarchies returns again and again.

3.4 Services: daemons

The UNIX kernel is a very large program, but it does not perform all of the services required in an OS. To keep the size of the kernel to a minimum, it only deals with the sharing of resources. Other jobs for operating system (which we can call services) are implemented by writing program which run along side user's programs. Indeed, they are just `user programs' - the only difference is that are owned by the system. These programs are called daemons. Here are some example from UNIX.

  • mountd : Deals with requests for `mounting' this machine's disks on other machines - i.e. requests to access the disk on this machine from another machine on the network.
  • rlogind : Handles requests to login from remote terminals.
  • keyserv : A server which stores public and private keys. Part of a network security system.
  • syslogd : Records information about important events in a log file.
  • named : Converts machine names into their network addresses and vice versa.

3.5 Multiprocessors - parallelism

The idea of constructing computers with more than one CPU has become more popular recently. On a system with several CPUs it is not just a virtual fact that several tasks can be performed simultaneously - it is a reality. This introduces a number of complications in OS design. For example - how can we stop two independent processors from altering some memory location which they both share simultaneously (so that neither of them can detect the collision)? This is a problem in process synchronization. The solution to this problem is much simpler in a single CPU system since no two things ever happen truly simultaneously.

We shall consider this in more detail in later chapters. For now it is useful to keep in mind that multiprocessors are an important element of modern OS design.


  1. Write a program to manage an array of many stacks.
  2. Describe the difference between the kernel and daemons in UNIX. What is the point of making this distinction?
  3. What is two-mode operation?
  4. What is the difference between an emulator or simulator and true multi-tasking?
  5. To prepare to for the project suggestion in the next chapter, write a program which reads fictitious commands in from a file. The commands should be of the form:
    operator operand
    load     12
    add      23
    store    1334
    jsr      5678
    wait     1
    fork     0
    etc. Read in the commands and print out a log of what the commands are, in the form "Executing (operator) on (operand)". You should be able to recognize the commands `wait' and `fork' specially, but the other commands may be anything you like. The aim is to simulate the type of commands a real program has to execute.

4. Processes and Thread

4.1 Key concepts

Multitasking and multi-user systems need to distinguish between the different programs being executed by the system. This is accomplished with the concept of a process.

4.1.1 Naming conventions

Before talking about process management we shall introduce some of the names which are in common use. Not all operating systems or books agree on the definitions of these names. In this chapter we shall take a liberal attitude - after all, it is the ideas rather than the names which count. Try to remember the different terms - they will be used repeatedly.

  • Process : This is a general term for a program which is being executed. All work done by the CPU contributes to the execution of processes. Each process has a descriptive information structure associated with it (normally held by the kernel) called a process control block which keeps track of how far the execution has progressed and what resources the process holds.
  • Task : On some systems processes are called tasks.
  • Job : Some systems distinguish between batch execution and interactive execution. Batch (or queued) processes are often called jobs. They are like production line processes which start, do something and quit, without stopping to ask for input from a user. They are non-interactive processes.
  • Thread : (sometimes called a lightweight process) is different from process or task in that a thread is not enough to get a whole program executed. A thread is a kind of stripped down process - it is just one `active hand' in a program - something which the CPU is doing on behalf of a program, but not enough to be called a complete process. Threads remember what they have done separately, but they share the information about what resources a program is using, and what state the program is in. A thread is only a CPU assignment. Several threads can contribute to a single task. When this happens, the information about one process or task is used by many threads. Each task must have at least one thread in order to do any work.
  • CPU burst : A period of uninterrupted CPU activity.
  • I/O burst : A period of uninterrupted input/output activity.

4.1.2 Scheduling

On most multitasking systems, only one process can truly be active at a time - the system must therefore share its time between the execution of many processes. This sharing is called scheduling. (Scheduling $\leftrightarrow$time management.)

Different methods of scheduling are appropriate for different kinds of execution. A queue is one form of scheduling in which each program waits its turn and is executed serially. This is not very useful for handling multitasking, but it is necessary for scheduling devices which cannot be shared by nature. An example of the latter is the printer. Each print job has to be completed before the next one can begin, otherwise all the print jobs would be mixed up and interleaved resulting in nonsense.

We shall make a broad distinction between two types of scheduling:

Queueing. This is appropriate for serial or batch jobs like print spooling and requests from a server. There are two main ways of giving priority to the jobs in a queue. One is a first-come first-served (FCFS) basis, also referred to as first-in first-out (FIFO); the other is to process the shortest job first (SJF).
Round-robin. This is the time-sharing approach in which several tasks can coexist. The scheduler gives a short time-slice to each job, before moving on to the next job, polling each task round and round. This way, all the tasks advance, little by little, on a controlled basis.

These two categories are also referred to as non-preemptive and preemptive respectively, but there is a grey area.

Strictly non-preemptive Each program continues executing until it has finished, or until it must wait for an event (e.g. I/O or another task). This is like Windows 95 and MacIntosh system 7.
Strictly preemptive The system decides how time is to be shared between the tasks, and interrupts each process after its time-slice whether it likes it or not. It then executes another program for a fixed time and stops, then the next...etc.
Politely-preemptive?? The system decides how time is to be shared, but it will not interrupt a program if it is in a critical section. Certain sections of a program may be so important that they must be allowed to execute from start to finish without being interrupted. This is like UNIX and Windows NT.

To choose an algorithm for scheduling tasks we have to understand what it is we are trying to achieve. i.e. What are the criterea for scheduling?

We want to maximize the efficiency of the machine. i.e. we would like all the resources of the machine to be doing useful work all of the time - i.e. not be idling during one process, when another process could be using them. The key to organizing the resources is to get the CPU time-sharing right, since this is the central `organ' in any computer, through which almost everything must happen. But this cannot be achieved without also thinking about how the I/O devices must be shared, since the I/O devices communicate by interrupting the CPU from what it is doing. (Most workstations spend most of their time idling. There are enormous amounts of untapped CPU power going to waste all over the world each day.)
We would like as many jobs to get finished as quickly as possible.
Interactive users get irritated if the performance of the machine seems slow. We would like the machine to appear fast for interactive users - or have a fast response time.

Some of these criterea cannot be met simultaneously and we must make compromises. In particular, what is good for batch jobs is often not good for interactive processes and vice-versa, as we remark under Run levels - priority below.

4.1.3 Scheduling hierarchy

Complex scheduling algorithms distinguish between short-term and long-term scheduling. This helps to deal with tasks which fall into two kinds: those which are active continuously and must therefore be serviced regularly, and those which sleep for long periods.

For example, in UNIX the long term scheduler moves processes which have been sleeping for more than a certain time out of memory and onto disk, to make space for those which are active. Sleeping jobs are moved back into memory only when they wake up (for whatever reason). This is called swapping.

The most complex systems have several levels of scheduling and exercise different scheduling polices for processes with different priorities. Jobs can even move from level to level if the circumstances change.

Figure 4.1: Multi-level scheduling.

4.1.4 Runs levels - priority

Rather than giving all programs equal shares of CPU time, most systems have priorities. Processes with higher priorities are either serviced more often than processes with lower priorities, or they get longer time-slices of the CPU.

Priorities are not normally fixed but vary according to the performance of the system and the amount of CPU time a process has already used up in the recent past. For example, processes which have used a lot of CPU time in the recent past often have their priority reduced. This tends to favour iterative processes which wait often for I/O and makes the response time of the system seem faster for interactive users.

In addition, processes may be reduced in priority if their total accumulated CPU usage becomes very large. (This occurs, for example in UNIX). The wisdom of this approach is arguable, since programs which take a long time to complete tend to be penalized. Indeed, they take must longer to complete because their priority is reduced. If the priority continued to be lowered, long jobs would never get finished. This is called process starvation and must be avoided.

Scheduling algorithms have to work without knowing how long processes will take. Often the best judge of how demanding a program will be is the user who started the program. UNIX allows users to reduce the priority of a program themselves using the nicecommand. `Nice' users are supposed to sacrifice their own self-interest for the good of others. Only the system manager can increase the priority of a process.

Another possibility which is often not considered, is that of increasing the priority of resource-gobbling programs in order to get them out of the way as fast as possible. This is very difficult for an algorithm to judge, so it must be done manually by the system administrator.

4.1.5 Context switching

Switching from one running process to another running process incurs a cost to the system. The values of all the registers must be saved in the present state, the status of all open files must be recorded and the present position in the program must be recorded. Then the contents of the MMU must be stored for the process (see next chapter). Then all those things must be read in for the next process, so that the state of the system is exactly as it was when the scheduler last interrupted the process. This is called a context switch. Context switching is a system overhead. It costs real time and CPU cycles, so we don't want to context switch too often, or a lot of time will be wasted.

The state of each process is saved to a data structure in the kernel called a process control block (PCB). Here is an example PCB from Mach OS:

typedef struct machpcb 
   char    mpcb_frame[REGOFF];
   struct  regs mpcb_regs;            /* user's saved registers */
   struct  rwindow mpcb_wbuf[MAXWIN]; /* user window save buffer */
   char    *mpcb_spbuf[MAXWIN];       /* sp's for each wbuf */
   int     mpcb_wbcnt;                /* number of saved windows in pcb_wbuf */
   struct  v9_fpu *mpcb_fpu;          /* fpu state */
   struct  fq mpcb_fpu_q[MAXFPQ];     /* fpu exception queue */
   int     mpcb_flags;                /* various state flags */
   int     mpcb_wocnt;                /* window overflow count */
   int     mpcb_wucnt;                /* window underflow count */
   kthread_t *mpcb_thread;            /* associated thread */

Below is a kernel process structure for a UNIX system.

struct	proc 
   struct  proc *p_link;        /* linked list of running processes */
   struct  proc *p_rlink;
   struct  proc *p_nxt;         /* linked list of allocated proc slots */
   struct  proc **p_prev;       /* also zombies, and free procs */
   struct  as *p_as;            /* address space description */
   struct  seguser *p_segu;     /* "u" segment */
   caddr_t p_stack;             /* kernel stack top for this process */
   struct  user *p_uarea;       /* u area for this process */
   char	   p_usrpri;            /* user-priority based on p_cpu and p_nice */
   char	   p_pri;               /* priority, negative is high */
   char	   p_cpu;               /* (decayed) cpu usage solely for scheduling */
   char	   p_stat;
   char	   p_time;              /* seconds resident (for scheduling) */
   char	   p_nice;              /* nice for cpu usage */
   char	   p_slptime;           /* seconds since last block (sleep) */
   char    p_cursig;
   int     p_sig;               /* signals pending to this process */
   int     p_sigmask;           /* current signal mask */
   int     p_sigignore;         /* signals being ignored */
   int     p_sigcatch;          /* signals being caught by user */
   int     p_flag;
   uid_t   p_uid;               /* user id, used to direct tty signals */
   uid_t   p_suid;              /* saved (effective) user id from exec */
   gid_t   p_sgid;              /* saved (effective) group id from exec */
   short   p_pgrp;              /* name of process group leader */
   short   p_pid;               /* unique process id */
   short   p_ppid;              /* process id of parent */
   u_short p_xstat;             /* Exit status for wait */
   short   p_cpticks;           /* ticks of cpu time, used for p_pctcpu */
   struct  ucred *p_cred;       /* Process credentials */
   struct  rusage *p_ru;        /* mbuf holding exit information */
   int     p_tsize;             /* size of text (clicks) */
   int     p_dsize;             /* size of data space (clicks) */
   int     p_ssize;             /* copy of stack size (clicks) */
   int     p_rssize;            /* current resident set size in clicks */
   int     p_maxrss;            /* copy of u.u_limit[MAXRSS] */
   int     p_swrss;             /* resident set size before last swap */
   caddr_t p_wchan;             /* event process is awaiting */
   long    p_pctcpu;            /* (decayed) %cpu for this process */
   struct  proc *p_pptr;        /* pointer to process structure of parent */
   struct  proc *p_cptr;        /* pointer to youngest living child */
   struct  proc *p_osptr;       /* pointer to older sibling processes */
   struct  proc *p_ysptr;       /* pointer to younger siblings */
   struct  proc *p_tptr;        /* pointer to process structure of tracer */
   struct  itimerval p_realtimer;
   struct  sess *p_sessp;       /* pointer to session info */
   struct  proc *p_pglnk;       /* list of pgrps in same hash bucket */
   short   p_idhash;            /* hashed based on p_pid for kill+exit+... */
   short   p_swlocks;           /* number of swap vnode locks held */
   struct  aiodone *p_aio_forw; /* (front)list of completed asynch IO's */
   struct  aiodone *p_aio_back; /* (rear)list of completed asynch IO's */
   int     p_aio_count;         /* number of pending asynch IO's */
   int     p_threadcnt;         /* ref count of number of threads using proc */
   int     p_cpuid;             /* processor this process is running on */
   int     p_pam;               /* processor affinity mask */

UNIX also uses a `user' structure to keep auxiliary information which is only needed when jobs are not `swapped out' (see next chapter).

4.1.6 Interprocess communication

One of the benefits of multitasking is that several processes can be made to cooperate in order to achieve their ends. To do this, they must do one of the following.

Communicate . Interprocess communication (IPC) involves sending information from one process to another. This can be achieved using a `mailbox' system, a socket (Berkeley) which behaves like a virtual communications network (loopback), or through the use of `pipes'. Pipes are a system construction which enables one process to open another process as if it were a file for writing or reading.
Share data. A segment of memory must be available to both processes. (Most memory is locked to a single process).
Waiting. Some processes wait for other processes to give a signal before continuing. This is an issue of synchronization.

As soon as we open the door to co-operation there is a problem of how to synchronize cooperating processes. For example, suppose two processes modify the same file. If both processes tried to write simultaneously the result would be a nonsensical mixture. We must have a way of synchronizing processes, so that even concurrent processes must stand in line to access shared data serially.

Synchronization is a tricky problem in multiprocessor systems, but it can be achieved with the help of critical sections and semaphores/ locks. We shall return to these below.

4.2 Creation and scheduling

4.2.1 Creating processes

The creation of a process requires the following steps. The order in which they are carried out is not necessarily the same in all cases.

  1. Name. The name of the program which is to run as the new process must be known.
  2. Process ID and Process Control Block. The system creates a new process control block, or locates an unused block in an array. This block is used to follow the execution of the program through its course, keeping track of its resources and priority. Each process control block is labelled by its PID or process identifier.
  3. Locate the program to be executed on disk and allocate memory for the code segment in RAM.
  4. Load the program into the code segment and initialize the registers of the PCB with the start address of the program and appropriate starting values for resources.
  5. Priority. A priority must be computed for the process, using a default for the type of process and any value which the user specified as a `nice' value (see Run levels - priorities above).
  6. Schedule the process for execution.

4.2.2 Process hierarchy: children and parent processes

In a democratic system anyone can choose to start a new process, but it is never users which create processes but other processes! That is because anyone using the system must already be running a shell or command interpreter in order to be able to talk to the system, and the command interpreter is itself a process.

When a user creates a process using the command interpreter, the new process become a child of the command interpreter. Similarly the command interpreter process becomes the parent for the child. Processes therefore form a hierarchy.

Figure 4.2: Process hierachies

The processes are linked by a tree structure. If a parent is signalled or killed, usually all its children receive the same signal or are destroyed with the parent. This doesn't have to be the case--it is possible to detach children from their parents--but in many cases it is useful for processes to be linked in this way.

When a child is created it may do one of two things.

Duplicate the parent process.
Load a completely new program.

Similarly the parent may do one of two things.

Continue executing along side its children.
Wait for some or all of its children to finish before proceeding.

4.2.3 Unix: fork() and wait()

As an example of process creation, we shall consider UNIX. The following example program is written in C++ and makes use of the standard library function fork(). The syntax of forkis

returncode = fork();

When this instruction is executed, the process concerned splits into two and both continue to execute independently from after the forkintruction. If fork is successful, it returns$0$to the child process and the process identifier or pid of the child process to the parent. It, for some reason, a new process cannot be created it returns a value of$-1$to the parent.

The following example does not check for errors if fork fails.

//*  A brief demo of the UNIX process duplicator fork().
//*  g++ unix.C to compile this.

#include <iostream.h>

extern "C" void sleep();
extern "C" int fork();
extern "C" int getpid();
extern "C" void wait();
extern "C" void exit();

void ChildProcess();


main ()

{ int pid, cid;

pid = getpid();

cout << "Fork demo! I am the parent (pid = " << pid << ")\n";

if (! fork())
   cid = getpid();
   cout << "I am the child (cid=" << cid << ") of (pid = " << pid << ")\n";

cout << "Parent waiting here for the child...\n";

cout << "Child finished, parent quitting too!\n";


void ChildProcess()

{ int i;

for (i = 0; i < 10; i++)
   cout << i << "..\n";

Here is the output from the program in a test run. Note that the parent and child processes share the same output stream, so we see how they are synchronised from the order in which the output is mixed.

Fork demo! I am the parent (pid = 2196)
I am the child (cid=2197) of (pid = 2196)
Parent waiting here for the child...
Child finished, parent quitting too!

Note that the child has time to execute its first instruction before the parent has time to call wait(), so the zero appears before the message from the parent. When the child goes to sleep for one second, the parent catches up.

4.2.4 Process states

In order to know when to execute a program and when not to execute a program, it is convenient for the scheduler to label programs with a `state' variable. This is just an integer value which saves the scheduler time in deciding what to do with a process. Broadly speaking the state of a process may be one of the following.

  1. New.
  2. Ready (in line to be executed).
  3. Running (active).
  4. Waiting (sleeping, suspended)
  5. Terminated (defunct)

When time-sharing, the scheduler only needs to consider the processes which are in the `ready' state. Changes of state are made by the system and follow the pattern in the diagram below.

Figure 4.3: Process state diagram.

The transitions between different states normally happen on interrupts.


From state Event To state
New Accepted Ready
Ready Scheduled / Dispatch Running
Running Need I/O Waiting
Running Scheduler timeout Ready
Running Completion / Error / Killed Terminated
Waiting I/O completed or wakeup event Ready


4.2.5 Queue scheduling

The basis of all scheduling is the queue structure. A round-robin scheduler uses a queue but moves cyclically through the queue at its own speed, instead of waiting for each task in the queue to complete. Queue scheduling is primarily used for serial execution.

There are two main types of queue.

First-come first-server (FCFS), also called first-in first-out (FIFO).
Sorted queue, in which the elements are regularly ordered according to some rule. The most prevalent example of this is the shortest job first (SJF) rule.

The FCFS queue is the simplest and incurs almost no system overhead. The SJF scheme can cost quite a lot in system overhead, since each task in the queue must be evaluated to determine which is shortest. The SJF strategy is often used for print schedulers since it is quite inexpensive to determine the size of a file to be printed (the file size is usually stored in the file itself).

The efficiency of the two schemes is subjective: long jobs have to wait longer if short jobs are moved in front of them, but if the distribution of jobs is random then we can show that the average waiting time of any one job is shorter in the SJF scheme, because the greatest number of jobs will always be executed in the shortest possible time.

Of course this argument is rather stupid, since it is only the system which cares about the average waiting time per job, for its own prestige. Users who print only long jobs do not share the same clinical viewpoint. Moreover, if only short jobs arrive after one long job, it is possible that the long job will never get printed. This is an example of starvation. A fairer solution is required (see exercises below).

Queue scheduling can be used for CPU scheduling, but it is quite inefficient. To understand why simple queue scheduling is not desirable we can begin by looking at a diagram which shows how the CPU and the devices are being used when a FCFS queue is used. We label each process by$P1$,$P2$... etc. A blank space indicates that the CPU or I/O devices are in an idle state (waiting for a customer).


CPU $P1$ - $P1$ - $P2$ -
devices - $P1$ - $P1$ - $P2$

This diagram shows that$P1$starts out with a CPU burst. At some point it needs input (say from a disk) and sends a request to the device. While the device is busy servicing the request from$P1$, the CPU is idle, waiting for the result. Similarly, when the result returns, the device waits idle while the next CPU burst takes place. When$P1$is finished,$P2$is started and goes through the same kind of cycle.

There are many blank spaces in the diagram, where the devices and the CPU are idle. Why, for example, couldn't the device be searching for the I/O for$P2$while the CPU was busy with$P1$and vice versa?

We can improve the picture by introducing a new rule: every time one process needs to wait for a device, it gets put to the back of the queue. Now consider the following diagram, in which we have three processes. They will always be scheduled in order$P1$,$P2$,$P3$until one or all of them is finished.


CPU $P1$ $P2$ $P3$ $P1$ $P2$-finishes $P3$ $P1$-finishes $P3$ - $P3$
devices - $P1$ $P2$ $P3$ $P1$ - $P3$ - $P3$ -

$P1$starts out as before with a CPU burst. But now when it occupies the device,$P2$takes over the CPU. Similarly when$P2$has to wait for the device to complete its I/O,$P3$gets executed, and when$P3$has to wait,$P1$takes over again. Now suppose$P2$finishes:$P3$takes over, since it is next in the queue, but now the device is idle, because$P2$did not need to use the device. Also, when$P1$finishes, only$P3$is left and the gaps of idle time get bigger.

In the beginning, this second scheme looked pretty good - both the CPU and the devices were busy most of the time (few gaps in the diagram). As processes finished, the efficiency got worse, but on a real system, someone will always be starting new processes so this might not be a problem.

Let us ask - how can we improve this scheme? The resource utilization is not too bad, but the problem is that it assumes that every program goes in a kind of cycle

\begin{displaymath}CPU \leftarrow \rightarrow I/O.\end{displaymath}

If one program spoils this cycle by performing a lot of CPU intensive work, or by waiting for dozens of I/O requests, then the whole scheme goes to pieces.

4.2.6 Round-robin scheduling

The use of the I/O - CPU burst cycle to requeue jobs improves the resource utilization considerably, but it does not prevent certain jobs from hogging the CPU. Indeed, if one process went into an infinite loop, the whole system would stop dead. Also, it does not provide any easy way of giving some processes priority over others.

A better solution is to ration the CPU time, by introducing time-slices. This means that

  1. no process can hold onto the CPU forever,
  2. processes which get requeued often (because they spend a lot of time waiting for devices) come around faster, i.e. we don't have to wait for CPU intensive processes, and
  3. the length of the time-slices can be varied so as to give priority to particular processes.

The time-sharing is implemented by a hardware timer. On each context switch, the system loads the timer with the duration of its time-slice and hands control over to the new process. When the timer times-out, it interrupts the CPU which then steps in and switches to the next process.

The basic queue is the FCFS/FIFO queue. New processes are added to the end, as are processes which are waiting.

The success or failure of round-robin (RR) scheduling depends on the length of the time-slice or time-quantum. If the slices are too short, the cost of context switching becomes high in comparision to the time spent doing useful work. If they become too long, processes which are waiting spend too much time doing nothing - and in the worst case, everything reverts back to FCFS. A rule of thumb is to make the time-slices large enough so that only, say, twenty percent of all context switches are due to timeouts - the remainder occur freely because of waiting for requested I/O.

4.2.7 CPU quotas and accounting

Many multiuser systems allow restrictions to be placed on user activity. For example, it is possible to limit the CPU time used by any one job. If a job exceeds the limit, it is terminated by the kernel. In order to make such a decision, the kernel has to keep detailed information about the cumulative use of resources for each process. This is called accounting and it can be a considerable system overhead. Most system administrators would prefer not to use accounting - though unfortunately many are driven to it by thoughtless or hostile users.

4.3 Threads

4.3.1 Heavy and lightweight processes

Threads, sometimes called lightweight processes (LWPs) are indepedendently scheduled parts of a single program. We say that a task is multithreaded if it is composed of several independent subprocesses which do work on common data, and if each of those pieces could (at least in principle) run in parallel.

If we write a program which uses threads - there is only one program, one executable file, one task in the normal sense. Threads simply enable us to split up that program into logically separate pieces, and have the pieces run independently of one another, until they need to communicate. In a sense, threads are a further level of object orientation for multitasking systems. They allow certain functions to be executed in parallel with others.

Figure 4.4: System and user level threads: suppose we think of a household kitchen as being a process, then each electrical appliance which contributes to the work of the kitchen is like a thread. In order to work, a thread needs power. The power sockets are like kernel threads or CPUs. A job like making a cake or tidying up might involve several threads (powered machinery), which might run in parallel or one after the other. Since there are more appliances than power points, we have to schedule the time each appliance gets power so as to share between all of them.

On a truly parallel computer (several CPUs) we might imagine parts of a program (different subroutines) running on quite different processors, until they need to communicate. When one part of the program needs to send data to the other part, the two independent pieces must be synchronized, or be made to wait for one another. But what is the point of this? We can always run independent procedures in a program as separate programs, using the process mechanisms we have already introduced. They could communicate using normal interprocesses communication. Why introduce another new concept? Why do we need threads?

The point is that threads are cheaper than normal processes, and that they can be scheduled for execution in a user-dependent way, with less overhead. Threads are cheaper than a whole process because they do not have a full set of resources each. Whereas the process control block for a heavyweight process is large and costly to context switch, the PCBs for threads are much smaller, since each thread has only a stack and some registers to manage. It has no open file lists or resource lists, no accounting structures to update. All of these resources are shared by all threads within the process. Threads can be assigned priorities - a higher priority thread will get put to the front of the queue.

In other words, threads are processes within processes!
Threads can only run inside a normal process.

Let's define heavy and lightweight processes with the help of a table.


Object Resources
Thread (LWP) Stack$+$set of CPU registers$+$CPU time.
Task (HWP) 1 thread$+$process control block,
  program code, memory segment etc.
Multithreaded task n-threads$+$process control block,
  program code, memory segment etc.


4.3.2 Why use threads?

From our discussion of scheduling, we can see that the sharing of resources could have been made more effective if the scheduler had known exactly what each program was going to do in advance. Of course, the scheduling algorithm can never know this - but the programmer who wrote the program does know. Using threads it is possible to organize the execution of a program in such a way that something is always being done, when ever the scheduler gives the heavyweight process CPU time.

Threads allow a programmer to switch between lightweight processes when it is best for the program. (The programmer has control.)
A process which uses threads does not get more CPU time than an ordinary process - but the CPU time it gets is used to do work on the threads. It is possible to write a more efficient program by making use of threads.
Inside a heavyweight process, threads are scheduled on a FCFS basis, unless the program decides to force certain threads to wait for other threads. If there is only one CPU, then only one thread can be running at a time.
Threads context switch without any need to involve the kernel - the switching is performed by a user level library, so time is saved because the kernel doesn't need to know about the threads.

4.3.3 Levels of threads

In modern operating systems, there are two levels at which threads operate: system or kernel threads and user level threads. If the kernel itself is multithreaded, the scheduler assigns CPU time on a thread basis rather than on a process basis. A kernel level thread behaves like a virtual CPU, or a power-point to which user-processes can connect in order to get computing power. The kernel has as many system level threads as it has CPUs and each of these must be shared between all of the user-threads on the system. In other words, the maximum number of user level threads which can be active at any one time is equal to the number of system level threads, which in turn is equal to the number of CPUs on the system.

Since threads work ``inside'' a single task, the normal process scheduler cannot normally tell which thread to run and which not to run - that is up to the program. When the kernel schedules a process for execution, it must then find out from that process which is the next thread it must execute. If the program is lucky enough to have more than one processor available, then several threads can be scheduled at the same time.

Some important implementations of threads are

The Mach System / OSF1 (user and system level)
Solaris 1 (user level)
Solaris 2 (user and system level)
OS/2 (system level only)
NT threads (user and system level)
IRIX threads
POSIX standardized user threads interface

4.3.4 Symmetric and asymmetric multiprocessing

Threads are of obvious importance in connection with parallel processing. There are two approaches to scheduling on a multiprocessor machine:

  • Asymmetric : one CPU does the work of the system, the other CPUs service user requests.
  • Symmetric : All processors can be used by the system and users alike. No CPU is special.

The asymmetric variant is potentially more wasteful, since it is rare that the system requires a whole CPU just to itself. This approach is more common on very large machines with many processors, where the jobs the system has to do is quite difficult and warrants a CPU to itself.

4.3.5 Example: POSIX pthreads

The POSIX standardization organization has developed a standard set of function calls for use of user-level threads. This library is called the pthread interface.

Let's look at an example program which counts the number of lines in a list of files. This program will serve as an example for the remainder of this chapter. We shall first present the program without threads, and then rewrite it, starting a new thread for each file. The threaded version of the program has the possibility of reading several of the files in parallel and is in principle more efficient, whereas the non-threaded version must read the files sequentially.

The non-threaded version of the program looks like this:

//  Count the number of lines in a number of files, non threaded
//  version.

#include <iostream.h>
#include <fstream.h>

const int bufsize = 100;

void ParseFile(char *);

int LINECOUNT = 0;


main ()

cout << "Single threaded parent...\n";


cout << "Number of lines = %d\n",LINECOUNT;


void ParseFile(char *filename)

{ fstream file;
  char buffer[bufsize];

cout << "Trying to open " << filename << endl;, ios::in);

if (! file)
   cerr << "Couldn't open file\n";

while (!file.eof())
   cout << filename << ":" <<buffer << endl;


This program calls the function ParseFile()several times to open and count the number of lines in a series of files. The number of lines is held in a global variable called LINECOUNT. A global variable is, by definition, shared data. This will cause a problem when we try to parallelize the program using threads. Here is the threaded version:

//  Count the number of lines in a number of files.
//  Illustrates use of multithreading. Note: run this program
//  several times to see how the threads get scheduled on the system.
//  Scheduling will be different each time since the system has lots
//  of threads running, which we do not see and these will affect the
//  scheduling of our program.
//  Note that, on a multiprocessor system, this program has a potential
//  race condition to update the shared variable LINECOUNT, so we
//  must use a mutex to make a short critical section whenever accessing
//  this shared variable.
//  This program uses POSIX threads (pthreads)

#include <iostream.h>
#include <fstream.h>
#include <pthread.h>
#include <sched.h>

const int bufsize = 100;
const int maxfiles = 4;

void *ParseFile(char *);      // Must be void *, defined in pthread.h !

int LINECOUNT = 0;


main ()

{ pthread_t tid[maxfiles];;
  int i,ret;

// Create a thread for each file

ret = pthread_create(&(tid[0]), NULL, ParseFile,"proc1");
ret = pthread_create(&(tid[1]), NULL, ParseFile,"proc2");
ret = pthread_create(&(tid[2]), NULL, ParseFile,"proc3");
ret = pthread_create(&(tid[3]), NULL, ParseFile,"proc4");

cout << "Parent thread waiting...\n";

 // If we don't wait for the threads, they will be killed
 // before they can start...

for (i = 0; i < maxfiles; i++)
   ret = pthread_join(tid[i],(void **)NULL);

cout << "Parent thread continuing\n";
cout << "Number of lines = " << LINECOUNT << endl;


void *ParseFile(char *filename)

{ fstream file;
  char buffer[bufsize];
  int ret;

cout << "Trying to open " << filename << endl;, ios::in);

if (! file)
   cerr << "Couldn't open file\n";
   return NULL;

while (!file.eof())
   cout << filename << ":" <<buffer << endl;

   // Critical section
   ret = pthread_mutex_lock(&MUTEX);
   ret = pthread_mutex_unlock(&MUTEX);
   // Try uncommenting this ....
   // Yield the process, to allow next thread to be run
   // sched_yield();


In this version of the program, a separate thread is spawned for each file. First we call the function pthread_create()for each file we encounter. A new thread is spawned with a pointer to the function the thread should execute (in this case the same function for all threads), called ParseFile(), which reads lines from the respective files and increments the global variable LINECOUNT. Several things are important here.

The main program is itself a thread. It is essential that we tell the main program to wait for the additional threads to join the main program before exiting, otherwise the main program will exit and kill all of the child threads immediately. Thread join-semantics are like wait-semantics for normal processes.

Each of the threads updates the same global variable. Suppose now that two threads are running on different CPUs. It is possible that both threads would try to alter the value of the variable LINECOUNTsimultaneously. This is called a race condition and can lead to unpredictable results. For this reason we use a mutex to lock the variable while it is being updated. We shall discuss this more in the next section.

A final point to note is the commented out lines in the ParseFile()function. The call sched_yield()tells a running thread to give itself up to the scheduler, so that the next thread to be scheduled can run instead. This function can be used to switch between several threads. By calling this function after each line is read from the files, we can spread the the CPU time evenly between each thread. Actually, it is difficult to predict precisely which threads will be scheduled and when, because the threads in our program here are only a small number, compared to the total number of threads waiting to be scheduled by the system. The interaction with disk I/O can also have a complicated effect on the scheduling. On a single CPU system, threads are usually scheduled FCFS in a queue. If we yield after every instruction, it has the effect of simulating round-robin scheduling.

4.3.6 Example: LWPs in Solaris 1

Early solaris systems had user-level threads only, which were called light weight processes. Since the kernel was single threaded, only one user-level thread could run at any given time.

To create a threaded process in solaris 1, one simply has to execute a LWP system call. The `lightweight processes library' then converts the normal process into a process descriptor plus a thread. Here is the simplest example

/*                                                                  */
/* Creating a light weight process in SunOS 4.1.3                   */
/*                                                                  */

#include <lwp/lwp.h>
#include <lwp/stackdep.h>

#define MINSTACKSZ   1024
#define STACKSIZE    1000 + MINSTACKSZ
#define MAXPRIORITY  10


stkalign_t stack[STACKSIZE];

/* Zone 0                                                            */

main ()

{ thread_t tid;
  int task();

pod_setmaxpri(MAXPRIORITY);               /* This becomes a lwp here */


printf("Done! - Now other threads can run...\n");

/* Zone 1                                                            */

task ()

printf("Task: next thread after main()!\n");

Here is an example program containing several threads which wait for each other.

/*                                                                  */
/* Creating a light weight process in sunos 4.1.3 (Solaris 1)       */
/*                                                                  */
/* Yielding to other processes                                      */
/*                                                                  */

#include <lwp/lwp.h>
#include <lwp/stackdep.h>

#define MINSTACKSZ   1024
#define STACKCACHE   1000
#define MAXPRIORITY  10
#define MINPRIORITY  1


stkalign_t stack[STACKSIZE];

/* Zone 0                                                            */

main ()

{ thread_t tid_main;
  thread_t tid_prog1;
  thread_t tid_prog2;
  int prog1(), prog2();

lwp_self(&tid_main);                               /* Get main's tid */
lwp_setstkcache(STACKCACHE,3);         /* Make a cache for each prog */


printf("One ");


printf("Four ");


printf("Six ");


/* Zone 1,2..                                                        */

prog1 ()

printf("Two ");

if (lwp_yield(THREADNULL) < 0)
   lwp_perror("Bad yield");

printf("Seven \n");


prog2 ()

printf("Three ");


printf("Five ");

4.4 Synchronization of processes and threads

When two or more processes work on the same data simultaneously strange things can happen. We have already seen one example in the threaded file reader in previous section: when two parallel threads attempt to update the same variable simultaneously, the result is unpredictable. The value of the variable afterwards depends on which of the two threads was the last one to change the value. This is called a race condition. The value depends on which of the threads wins the race to update the variable.

What we need in a multitasking system is a way of making such situations predictable. This is called serialization.

4.4.1 Problems with sharing for processes

It is not only threads which need to be synchronized. Suppose one user is running a script program and editing the program simultaneously. The script is read in line by line. During the execution of the script, the user adds four lines to the beginning of the file and saves the file. Suddenly, when the next line of the executing script gets read, the pointer to the next line points to the wrong location and it reads in the same line it already read in four lines ago! Everything in the program is suddenly shifted by four lines, without the process execting the script knowing about it.

This example (which can actually happen in the UNIX shell) may or may not turn out to be serious - clearly, in general, it can be quite catastrophic. It is a problem of synchronization on the part of the user and the filesystem 4.1 .

We must consider programs which share data.

  1. When do we need to prevent programs from accessing data simultaneously? If there are 100 processes which want to read from a file, this will cause no problems because the data themselves are not changed by a read operation. A problem only arises if more than one of the parties wants to modify the data.
  2. Is it even sensible for two programs to want to modify data simultaneously? Or is it simply a stupid thing to do? We must be clear about whether such collisions can be avoided, or whether they are a necessary part of a program. For instance, if two independent processes want to add entries to a database, this is a reasonable thing to do. If two unrelated processes want to write a log of their activities to the same file, it is probably not sensible: a better solution would be to use two separate files.
  3. How should we handle a collision between processes? Should we signal an error, or try to make the processes wait in turn? There is no universal answer to this question - in some cases it might be logically incorrect for two processes to change data at the same time: if two processes try to change one numerical value then one of them has to win - which one? On the other hand, if two processes try to add something to a list, that makes sense, but we have to be sure that they do not write their data on top of each other. The writing must happen serially, not in parallel.

4.4.2 Serialization

The key idea in process synchronization is serialization. This means that we have to go to some pains to undo the work we have put into making an operating system perform several tasks in parallel. As we mentioned, in the case of print queues, parallelism is not always appropriate.

Synchronization is a large and difficult topic, so we shall only undertake to describe the problem and some of the principles involved here.

There are essentially two strategies to serializing processes in a multitasking environment.

The scheduler can be disabled for a short period of time, to prevent control being given to another process during a critical action like modifying shared data. This method is very inefficient on multiprocessor machines, since all other processors have to be halted every time one wishes to execute a critical section.
A protocol can be introduced which all programs sharing data must obey. The protocol ensures that processes have to queue up to gain access to shared data. Processes which ignore the protocol ignore it at their own peril (and the peril of the remainder of the system!). This method works on multiprocessor machines also, though it is more difficult to visualize.

The responsibility of serializing important operations falls on programmers. The OS cannot impose any restrictions on silly behaviour - it can only provide tools and mechanisms to assist the solution of the problem.

4.4.3 Mutexes: mutual exclusion

Another way of talking about serialization is to use the concept of mutual exclusion. We are interested in allowing only one process or thread access to shared data at any given time. To serialize access to these shared data, we have to exclude all processes except for one. Suppose two processes A and B are trying to access shared data, then: if A is modifying the data, B must be excluded from doing so; if B is modifying the data, A must be excluded from doing so. This is called mutual exclusion.

Mutual exclusion can be achieved by a system of locks. A mutual exclusion lock is colloquially called a mutex. You can see an example of mutex locking in the multithreaded file reader in the previous section. The idea is for each thread or process to try to obtain locked-access to shared data:


// Update shared data


The mutex variable is shared by all parties (e.g. a global variable). This protocol is meant to ensure that only one process at a time can get past the function Get_Mutex. All other processes or threads are made to wait at the function Get_Mutexuntil that one process calls Release_Mutexto release the lock. A method for implementing this is discussed below. Mutexes are a central part of multithreaded programming.

4.4.4 User synchronization: file locks

A simple example of a protocol solution, to the locking problem at the user level, is the so-called file-lock in UNIX. When write-access is required to a file, we try to obtain a lock by creating a lock-file with a special name. If another user or process has already obtained a lock, then the file is already in use, and we are denied permission to edit the file. If the file is free, a `lock' is placed on the file by creating the file lock. This indicates that the file now belongs to the new user. When the user has finished, the file lock is deleted, allowing others to use the file.

In most cases a lock is simply a text file. If we wanted to edit a file blurb, the lock might be called blurb.lockand contain the user identifier of the user currently editing the file. If other users then try to access the file, they find that the lock file exists and are denied access. When the user is finished with the file, the lock is removed.

The same method of locks can also be used to prevent two instances of a program from starting up simultaneously. This is often used in mail programs such as the ELM mailer in UNIX, since it would be unwise to try to read and delete incoming mail with two instances of the mail program at the same time.

We can implement a lock very easily. Here is an example from UNIX in which the lock file contains the process identifier. This is useful because if something goes wrong and the editor crashes, the lock will not be removed. It is then possible to see that the process the lock referred to no longer exists and the lock can be safely removed.

// Example of a program which uses a file lock to ensure
// that no one starts more than one copy of it.

#include <iostream.h>
#include <fstream.h>

// Include file

extern "C" int getpid();
extern "C" void unlink(char *);

int Locked();
void RemoveLock();

const int true = 1;
const int false = 0;
const int exitstatus=1;

// Main program

main ()

if (Locked())
   cout << "This program is already running!\n";
   return exitstatus;

 // Program here


// Toolkit: locks

Locked ()

{ ifstream lockfile;
  int pid;"/tmp/lockfile",ios::in);

if (lockfile)
   return true;

if (! lockfile)
   cerr << "Cannot secure a lock!\n";
   return true;

pid = getpid();
lockfile.out << pid;


return false;


void RemoveLock()


4.4.5 Exclusive and non-exclusive locks

To control both read and write access to files, we can use a system of exclusive and non-exclusive locks.

If a user wishes to read a file, a non-exclusive lock is used. Other users can also get non-exclusive locks to read the file simultaneously, but when a non-exclusive lock is placed on a file, no user may write to it.

To write to a file, we must get an exclusive lock. When an exclusive lock is obtained, no other users can read or write to the file.

4.4.6 Critical sections: the mutex solution

A critical section is a part of a program in which is it necessary to have exclusive access to shared data. Only one process or thread may be in a critical section at any one time.

In the past it was possible to implement this is by generalizing the idea of interrupt masks, as mentioned in chapter 2. By switching off interrupts (or more appropriately, by switching off the scheduler) a process can guarantee itself uninterrupted access to shared data. This method has drawbacks: i) masking interrupts can be dangerous - there is always the possibility that important interrupts will be missed, ii) it is not general enough in a multiprocessor environment, since interrupts will continue to be serviced by other processors - so all processors would have to be switched off; iii) it is too harsh. We only need to prevent two programs from being in their critical sections simultaneously if they share the same data. Programs A and B might share different data to programs C and D, so why should they wait for C and D?

The modern way of implementing a critical section is to use mutexes as we have described above. In 1981 G.L. Peterson discovered a simple algorithm for achieving mutual exclusion between two processes with PID equal to 0 or 1. The code goes like this:

int turn;
int interested[2];

void Get_Mutex (int pid)

{ int other;

other = 1 - pid;
interested[pid] = true;
turn = pid;

while (turn == pid && interested[other])  // Loop until no one
   {                                      // else is interested

void Release_Mutex (int pid)

interested[pid] = false;

Where more processes are involved, some modifications are necessary to this algorithm. The key to serialization here is that, if a second process tries to obtain the mutex, when another already has it, it will get caught in a loop, which does not terminate until the other process has released the mutex. This solution is said to involve busy waiting--i.e. the program actively executes an empty loop, wasting CPU cycles, rather than moving the process out of the scheduling queue. This is also called a spin lock, since the system `spins' on the loop while waiting.

4.4.7 Flags and semaphores

Flags are similar in concept to locks. The idea is that two cooperating processes can synchronize their execution by sending very simple messages to each other. A typical behaviour is that one process decides to stop and wait until another process signals that it has arrived at a certain place.

For example, suppose we want to ensure that procedure1()in process 1 gets executed before procedure2()in process 2.

// Process 1                               // Process 2

   procedure1();                           wait(mysignal);
   signal(mysignal);                       procedure2();
   ...                                     ...

These operations are a special case of interprocess communication. A semaphore is a flag which can have a more general value than just true or false. A semaphore is an integer counting variable and is used to solve problems where there is competition between processes. The idea is that one part of a program tends to increment the semaphore while another part tends to decrement the semaphore. The value of the flag variable dictates whether a program will wait or continue, or whether something special will occur. There are many uses for semaphores and we shall not go into them here. A simple example is reading and writing via buffers, where we count how many items are in the buffer. When the buffer becomes full, the process which is filling it must be made to wait until space in the buffer is made available.

4.4.8 Monitors

Some languages (like Modula) have special language class-environments for dealing with mutual exclusion. Such an environment is called a monitor.

A monitor is a language-device which removes some of the pain from synchronization. Only one process can be `inside' a monitor at a time - users don't need to code this themselves, they only have to create a monitor.
A procedure or function defined under the umbrella of a monitor can only access those shared memory locations declared within that monitor and vice-versa.
Wait and signal operations can be defined to wait for specific condition variables. A process can thus wait until another process sends a signal or semaphore which changes the condition variable.

4.5 Deadlock

Waiting and synchronization is not all sweetness and roses. Consider the European road rule which says: on minor roads one should always wait for traffic coming from the right. If four cars arrive simultaneously at a crossroads (see figure) then, according to the rule all of them must wait for each other and none of them can ever move. This situation is called deadlock. It is the stale-mate of the operating system world.

Figure 4.5: Deadlock in the European suburbs.

4.5.1 Cause

Deadlock occurs when a number of processes are waiting for an event which can only be caused by another of the waiting processes.

These are the essential requirements for a deadlock:

  1. Circular waiting. There must be a set of processes$P_1..P_n$where$P_1$is waiting for a resource or signal from$P_2$,$P_2$is waiting for$P_3$... and$P_n$is waiting for$P_1$.
  2. Non-sharable resources. It is not possible to share the resources or signals which are being waited for. If the resource can be shared, there is no reason to wait.
  3. No preemption. The processes can not be forced to give up the resources they are holding.

There are likewise three methods for handling deadlock situations:

  1. Prevention. We can try to design a protocol which ensures that deadlock never occurs.
  2. Recovery. We can allow the system to enter a deadlock state and then recover.
  3. Ostrich method. We can pretend that deadlocks will never occur and live happily in our ignorance. This is the method used by most operating systems. User programs are expected to behave properly. The system does not interfere. This is understandable: it is very hard to make general rules for every situation which might arise.

4.5.2 Prevention

Deadlock prevention requires a system overhead.

The simplest possibility for avoidance of deadlock is to introduce an extra layer of software for requesting resources in addition to a certain amount of accounting. Each time a new request is made, the system analyses the allocation of resources before granting or refusing the resource. The same applies for waitconditions.

The problem with this approach is that, if a process is not permitted to wait for another process - what should it do instead? At best the system would have to reject or terminate programs which could enter deadlock, returning an error condition.

Another method is the following. One might demand that all programs declare what resources they will need in advance. Similarly all wait conditions should be declared. The system could then analyse (and re-analyse each time a new process arrives) the resource allocation and pin-point possible problems.

4.5.3 Detection

The detection of deadlock conditions is also a system overhead. At regular intervals the system is required to examine the state of all processes and determine the interrelations between them. Since this is quite a performance burden, it is not surprising that most systems ignore deadlocks and expect users to write careful programs.

4.5.4 Recovery

To recover from a deadlock, the system must either terminate one of the participants, and go on terminating them until the deadlock is cured, or repossess the resources which are causing the deadlock from some processes until the deadlock is cured. The latter method is somewhat dangerous since it can lead to incorrect program execution. Processes usually wait for a good reason, and any interruption of that reasoning could lead to incorrect execution. Termination is a safer alternative.

4.6 Summary

In this chapter we have considered the creation and scheduling of processes. Each process may be described by

A process identifier.
A process control block which contains status information about the scheduled processes.
A private stack for that process.

The scheduling of processes takes place by a variety of methods. The aim is to maximize the use of CPU time and spread the load for the devices.

Processes can be synchronized using semaphores or flags. Protocol constructions such as critical sections and monitors guarantee that shared data are not modified by more than one process at a time.

If a process has to wait for a condition which can never arise until it has finished waiting, then a deadlock is said to arise. The cause of deadlock waiting is often a resource which cannot be shared. Most operating systems do not try to prevent deadlocks, but leave the problem to user programs.


  1. Explain the difference between a light weight process and a normal process.
  2. What is meant by the critical section of a program?
  3. What is meant by deadlock?
  4. Explain why round-robin scheduling would not be appropriate for managing a print-queue.
  5. Devise a combination of first-come-first-serve (FCFS) and shortest-job-first (SJF) scheduling which would be the `fairest' solution to scheduling a print queue.


You can learn a lot by solving the following problem. The idea is to make a time-sharing system of your own.

  1. Make a fake kernel simulator which, instead of executing processes in memory, reads instructions from a number of files. You should aim to share the time spent reading each `process' equally between all tasks. The output of your kernel should show clearly what is being executed and when. You should give each process a process identifier (pid). The `command language' you are reading in contains instructions like `abcd 3', `wait 4' etc. i.e. four letters followed by a number.
  2. Add process priorities to each task. You can decide how these are assigned yourself. Keep a record of how long each process takes to complete and print status information when each process finishes. You can either call the real system clock to do this, or increment a counter each time an instruction is read. This is like counting `fake CPU cycles'.
  3. The input files contain ` wait <number>' instructions. Modify your program so that when one of the tasks reads an instruction `wait 5', for instance, it waits for process number 5 to finish before it continues. The output of the kernel should show this clearly. Hint: use a status variable which indicates whether the process is `ready' or `waiting'.
  4. Copy and modify the input files so that a deadlock can occur. Explain carefully how it occurs. For example, make two processes wait for each other. Add to your kernel a simple test to detect such deadlock situations. Decide for yourself how you wish to handle this situation. Explain what you have chosen to do in your solution.
  5. Some of the input files contain `fork' instructions. Modify your code so that when such an instruction is detected, the current process spawns a new copy of itself which begins executing from the instruction after the fork command. The new process should have a different pid and should have the same priority as the old one.

Try to make your program as structured as possible. The aim is to write the clearest program, rather than the most efficient one. When presenting your results, give a listing of the output of each part and explain the main features briefly.

5. Memory and storage

Together with the CPU, the physical memory (RAM) is the most important resource a computer has. The CPU chip has instructions to manipulate data only directly in memory, so all arithemtic and logic operations must take place in RAM.

5.1 Logical and Physical Memory

5.1.1 Physical Address space

Every byte in the memory has an address which ranges from zero up to a limit which is determined by the hardware (see below). Although bytes are numbered from zero upward, not every address is necessarily wired up to a memory chip. Some addresses may be reserved for

Memory mapped I/O - individual registers belonging to other chips and hardware devices.
The interrupt vector - the CPU itself requires some workspace. Usually the interrupt vector and sometimes the processor stack occupy fixed locations.
The operating system itself. This takes up a fair chunk of memory. On most microcomputers this is located in ROM. On multiuser systems upgrades are much more frequent and it is always loaded from disk into RAM.

The physical address space consists of every possible address to which memory chips are connected.

5.1.2 Word size

A word is a small unit of memory, normally just a few bytes. The size of a word on any system is defined by the size of the registers in the CPU. This determines both the amount of memory a system can address and the way in which memory is used.

Up to about 1985, all CPUs had eight bit (1 byte) registers, except for the program counter and address registers which were 16 bits. The largest address which can be represented in a 16 bit number is$2^{16}=65,535$or$64k$bytes, and so these machines could not handle more memory than this. Similarly, since the accumulator and index registers were all 8 bits wide, no more than one byte could be manipulated at a time. (This is why bytes have a special status.)

After that came a number of 16 bit processors with larger program counters. Nowadays most CPUs have 32 bit registers. The DEC alpha machines, together with the OSF/1 operating system are based on 64 bit technology. The possible address range and internal number representations are enormous. 64 bit versions of other versions of unix and NT are also starting to appear.

5.1.3 Paged RAM/ROM

The size of the physical address space is limited by the size of the address registers in the CPU. On early machines this memory was soon exceeded and it was necessary to resort to tricks to add more memory. Since it was not possible to address any more than the limit, these machines temporarily switched out one bank of memory with another. The new memory bank used the same addresses as the old, but only one could be accessed at a time. This operation is called paging. A special hardware paging chip was used to switch between banks, containing a register which could choose between$N$banks of memory.

Paging has obvious disadvantages - not all memory can be used at once and the method is seldom used nowadays since modern CPUs can address much larger memory spaces. As we shall see later, multi-user systems use paging to disk. Instead of switching between hardware banks of memeory, they copy the old contents to disk and reuse the memory which is already there for something else.

5.1.4 Address binding - coexistence in memory

When a high level language program is compiled, it gets converted into machine code. In machine code there are no procedure names, or variable names. All references to data or program code are made by specifying the address at which they are to be found. This immediately begs the question: how do we know what the addresses will be? How do we know where the program is going to be located in memory?

On microcomputers, this is very straightforward. A program is compiled to run starting from some fixed address. The system defines a certain range of addresses which can be used by user programs (See figure 2.1). Whenever the program is loaded from disk, it is loaded into the memory at the same address, so that all of the addresses referred to in the program are correct every time.

A problem arises if the system supports several programs resident in memory simultaneously. Then it is possible that the addresses coded into one program will already be in use by another. In that case there are three possible options

  1. Demand that programs which can coexist be compiled to run at different addresses. (This means that every program which is to be able to coexist must know about every other!)
  2. Relative addressing. Machine code uses addresses relative to the start address at which the program was loaded. The CPU must then add the start address to every relative address to get the true address. This incurs a performance penalty. Also, on some microprocessors (e.g. intel 6502), the relative addressing instructions available are limited to fairly small relative ranges, due to the size of the CPU registers.
  3. Use address binding. Here the idea is that ``dummy" addresses are used when code is generated. When a program is loaded in, the true addresses are computed relative to the start of the program and replaced before execution begins. This requires a special program called a loader.

Needless to say, it is the last of these methods which is used in modern systems. It introduces an important distinction between logical and physical addresses. A user program writes only to logical addresses, not knowing where the program will end up in the physical address space. The addresses are converted to physical addresses automatically.

Again there is a choice. When should this conversion take place?

  1. When the program is loaded into memory, once and for all?
  2. While the program is being executed?

Initially it would seem that 1. is the better alternative, since 2 incurs a runtime overhead. In fact 2. is the more flexible option for reasons which will become more apparent when we consider paging to disk. By performing the distinction at runtime, we have the freedom to completely reorganize the use of physical memory dynamically at any time. This freedom is very important in a multitasking operating system where memory has to be shared continually.

Figure 5.1: If a program hard codes addresses, there will be collisions when we try to load a second program into memory. It is therefore imporant to have a way of allocating addresses dynamically.

5.1.5 Shared libraries

The concept of shared libraries lies somewhere in the grey zone between compiling and linking of programs and memory binding. We introduce it here for want of a better place. The advantages of shared libraries should be clearly apparent by the end of this section. On windows systems, shared libraries are called dynamically loaded libraries or dll's.

On older systems, when you compile a program, the linker attaches a copy of standard libraries to each program. Because of the nature of the linker, the whole library has to be copied even though perhaps only one function is required. Thus a simple program to print ``hello'' could be hundreds or thousands of kilobytes long! This wastes considerable amount of disk space, copying the same code for every program. When the program is loaded into memory, the whole library is loaded too, so it is also a waste of RAM.

The solution is to use a run-time linker, which only loads the shared library into RAM when one of the functions the library is needed. The advantages and disadvantages of this scheme are the following.

  1. Considerable savings in disk space are made, because the standard library code is never joined to the executable file which is stored on disk, thus there is only one copy of the shared library on the system.
  2. A saving of RAM can also be made since the library, once loaded into RAM can often be shared by several programs. See under segmentation below.
  3. A performance penalty is transferred from load-time to run-time, the first time a function is accessed: the library must be loaded from disk during the execution of the program. In the long run, this might be outweighed by the time it would otherwise have taken to load the library for$n$programs, which now can share it. Also, the amount of RAM needed to support$n$programs is now considerably less.
Figure 5.2: Statically linked files append the entire library to each compiled program. With shared libraries we can save disk and memory by linking a program dynamically with a single copy of the library.

5.1.6 Runtime binding

Keeping physical and logical addresses completely separate introduces a new level of abstraction to the memory concept. User programs know only about logical addresses. Logical addresses are mapped into real physical addresses, at some location which is completely transparent to the user, by means of a conversion table. The conversion can be assisted by hardware processors which are specially designed to deal with address mapping. This is much faster than a purely software solution (since the CPU itself must do the conversion work). The conversion is, at any rate, performed by the system and the user need know nothing about it.

The part of the system which performs the conversion (be it hardware or software) is called the memory management unit (MMU). The conversion table of addresses is kept for each process in its process control block (PCB) and mmust be downloaded into the MMU during context switching (this is one reason why context switching is expensive!). Each logical address sent to the MMU is checked in the following way:

Does the logical address belong to the process? If not, generate an ownership error (often called a segmentation fault, as we shall see below).
Translate the logical address into a physical address.

The ownership checking is performed at the logical level rather than the physical level because we want to be able to use the physical memory in the most general possible way. If we bind physical addresses to a special user it means that we cannot later reorganize the physical memory and part of the point of the exercise is lost. On the other hand, if users are only bound to logical addresses, we can fiddle as much as we like with the physical memory and the user will never know.

One more question must be added to the above.

Are the data we want to access actually in the physical memory? As we shall see later in this chapter, many systems (the most immediate example of which is UNIX) allow paging to disk.

We shall return to this in the next section.

The conversion of logical addresses into physical
addresses is familiar in many programming languages
and is achieved by the use of


Instead of referring to data directly, one uses a
pointer variable which holds the true address at which
the data are kept. In machine language, the same scheme
is called ``indirect addressing''.
The difference between logical addresses and pointers
is that all pointers are user objects, and thus pointers
only point from one place in logical memory to another place
in logical memory. The mapping from logical to physical is
only visible to the designer of the system.

How is the translation performed in practice? To make the translation of logical to phyical addresses practical, it is necessary to coarse grain the memory. If every single byte-address were independently converted, then two$32$bit addresses would be required for each byte-address in the table and the storage space for the conversion table would be seven times bigger than the memory of the system!

To get around this problem, we have to break up the memory into chunks of a certain size. Then we only need to map the start address of each block, which is much cheaper if the blocks are big enough. There are two schemes for coarse graining the memory in this way:

  1. Give each process/task a fixed amount of workspace (a fixed size vector) which is estimated to be large enough to meet its needs. Only the base address of the workspace and the size need to be stored i.e. the whole vector in logical memory is mapped into a corresponding vector in physical memory. We don't know where it lies in the physical memory, but the mapping is one-to-one.

    The disadvantage with this scheme is that either too much or too little memory might be allocated for the tasks. Moreover - if only a small part of the program is actually required in practice, then a large amount of memory is wasted and cannot be reused.

  2. Coarse grain or ``quantize'' the memory in smallish pieces, calledpages. Each page is chosen to have the same fixed size (generally 2-4kB on modern systems), given by some power of$2$bits (this varies from system to system). The base address of each page is then stored in the conversion table (the length is known, since it is fixed). A unit of logical memory is called a page, whereas a unit of physical memory is called a frame. Apart from the difference in names, they must of course have the same size.

The second of these possibilities is an attractive propostion for a number of reasons. By breaking up the memory into smaller pieces, we have the possibility of reorganizing (reusing) each piece separately. Large programs need not be entirely in memory if they are not needed. Also, if two programs use the same code, they can share pages, so two logical pages map into the same physical frame. This is advantageous for shared-libraries.

Page numbers and addresses
Page addressing is a simple matter if the size of one page is a
power$2^n$. Since addresses are stored in bits, page numbers can be
assigned by simply throwing away the lower bits from every address.
It is analogous to counting in blocks of a thousand, in regular base$10$.
To number blocks of size$1000$in base 10, one simply has to
drop the lowest three digits. Thus to store the mapping from
logical to physical here, we must cover all addresses from$0000$
to$9999$. Without pages, this would require$9999$addresses.

with paging

we need only$9$addresses, since$7123$
and$7663$are both in page$7$, for instance.

An important consequence of the mapping of pages, is that what appears to the user as$10MB$of sequential memory may in reality be spread in some random order just about anywhere in physical memory. The tables which map logical to physical memory are called the page table and the frame table, and are stored per process and loaded as a part of context switching.

5.1.7 Segmentation - sharing

From the point of view of the system: sharing, process management and efficiency, it is highly convenient to view the memory for different processes as being segmented.

A segment is a convenient block of logical memory which is assigned to a process when it is executed. The memory given to any process is divided up into one or more segments which then belong to that process. The purpose of segments is to help the system administrate the needs of all processes according to a simple paradigm. Each segment of memory is administrated separately and all of the checks on valid addressing are made on each segment. It is therefore convenient to use separate segments for logically separate parts of a program/process.

Code segment - program code
Data segment - the program stack and dynamically allocated data.
Arrays can conveniently be placed in a segment of their own - that way, array bound-checking will be performed automatically by the hardware of the system.
Figure 5.3: The UNIX process model, showing the various segments used by each process. The stack contains all local (automatic) variables and the heap is allocated by malloc().

The segment idea can all be built on top of the page/frame concept above by demanding that segments be a whole number of pages. That way, we retain the advantages of the page system. Segmentation is an additional overhead which relates to the sharing of logical memory between processes. The page overhead relates to the mapping of logical to physical addresses.

Memory addressing with segments is like plotting points in a plane with coordinates$(x,y)$. Addresses are written (segment,offset).

5.1.8 The malloc() function

The$C++$operator new which dynamically allocates memory is a wrapper function for the C library function malloc(). When we use new, the compiler translates this into a call to malloc(). As an example, let's ask what happens when we call the function malloc(). mallocis part of the standard C library on any system, but we shall only be concerned with how it is implemented in BSD UNIX. The function is used to obtain a pointer to (the address of) a block of memory$n$bytes long. For example,

pointer= malloc(n);

Since mallocis a user-level command, it obtains logical memory for the caller. The acquisition of physical memory is taken care or by the system on behalf of malloc, by deeper level kernel commands.

In order to obtain$n$bytes of memory, malloc must normally acquire too much memory. This is because the smallest unit of memory is a page. This when malloc is called, it checks to see if the data segment of the current process has$n$free bytes. If the space already exists within the pages already allocated to the process, malloc uses this space and updates the free-memory list. If there is not sufficient space, mallocmakes a call to the brk()function, which tries to extend the size of the data segment. In most cases, not all the memory obtained is required. The most extreme example would be the allocation of one charvariable (one single byte). Then the remainder of the page is free, and is added to the free memory list.

The next time mallocis called, it tries to use the remainder of the last allocated page, or any memory in the same segment which it allocated earlier, but has since been freed.

The fact that mallocdivides up pages of logical memory is of no consequence to the memory management system, since each process maintains its own free memory list for the data segment. Since the segment always consists of a whole number of pages there is no conflict with the page mapping algorithm.

Figure 5.4: Levels of mapping from user allocation to physical memory.
            \setcoordinatesystem u...
            ...ectangle corners at 0.444 24.162 and 20.288 16.224

5.1.9 Page size, fragmentation and alignment

The process of allocating memory is really only half the story of memory management. We must also be able to de-allocate or free memory. When memory is freed from a segment, it leaves a hole of a certain size, which is added to the free-list. Eventually, the number of these holes grows quite large and the memory is said to become fragmented.

Fragmentation can lead to wasted resources. We would clearly like to re-use freed memory as far as possible, but if the holes are not big enough to fit the data we need to allocate then this is not possible.

Another technical problem which leads to fragmentation and wastage is alignment. Alignment is a technical problem associated with the word-size and design of the CPU. Certain memory objects (variables) have to be stored starting from a particular (usually even) address. This is because the multiple-byte registers of the CPU need to align their ``footprints'' to the addresses of the memory. Or, by virtue of the word-size of the system, the CPU regards the addresses as being effectively multiples of the word-size. In order to meet this requirement, memory sometimes has to be `padded' out with empty bytes - which are therefore wasted.

Fragmentation occurs at two levels:

  • Internal fragmentation. This is space wasted by mallocin trying to fit data into a segment (logical memory).
  • External fragmentation. This is space lying between segments in the physical memory. (There are never holes between segments in logical memory since we can always just renumber the logical addresses to remove them - they are not real anyway.)

See the figure below.

Figure 5.5: Fragmentation occurs because we allocate memory in blocks of different sizes and then free the blocks. Fragments are show as the white gaps between allocated objects. Internal fragmentation happens inside segments of logical memory when programs like malloc divide up the segment space. External fragmentation occurs in the mapping of logical segments to physical segments when there are gaps between the segments in physical memory. External fragmentation is cured by only mapping pages as in figure 5.4.

Note that external fragmentation is formally eliminated by the page concept. With pages, every object in physical memory is always the size of a page or frame, every hole must also be the size of a page and thus one is guaranteed to be able to fit a page block into a page hole. To some extent this is a cheat though, because the problem is only transferred from external to internal fragmentation - but such is the nature of definitions.

Internal fragmentation can be minimized by choosing a smaller page size for the system. That means that, on average, fewer bytes will be wasted per page. Of course, the system overhead grows larger as the page size is reduced, so as usual the size of pages is a tradeoff between two competing requirements.

At the user level, it is possible to avoid of the fragmentation problem when writing programs. For example, if a program allocates and frees memory objects of random sizes, it will be a random issue whether or not the holes left over can be used again. If, on the other hand, a program only allocates memory in fixed size structures (like C's structand unionvariable types), then every hole will be the same size as every new object created and (as with pages) it will always be possible to fit new data into old holes. This is a program design consideration. Unions were designed for precisely this kind of purpose.

5.1.10 Reclaiming fragmented memory (Tetris!)

There are two strategies for reclaiming fragmented memory.

  1. Try to fit data into the holes that already exist.
  2. Reorganize the data so that all the holes are regrouped into one large hole.

The second alternative clearly represents a large system overhead and is seldom used.

The first method can be implemented in one of three ways. Given a free-list of available holes, one may choose a space on the basis of

First fit. Choose the first hole which will do.
Best fit. Choose the smallest hole that will do.
Worst fit Choose the largest hole (which in some screwy sense leaves the biggest remainder - for what it's worth).

The first two are preferable, but neither works best in all cases. The criterea are i) minimization of fragmentation and ii) minimization of the allocation overhead. The first is perhaps preferable, since it is fastest.

5.2 Virtual Memory

5.2.1 Paging and Swapping

Virtual memory is a way of making the physical memory of a computer system effectively larger than it really is. Rather than using mirrors, the system does this by determining which parts of its memory are often sitting idle, and then makes a command decision to empty their contents onto a disk, thereby freeing up useful RAM.

As we noted earlier, it is quite seldom that every byte of every program is in use all of the time. More often programs are large and contain sections of code which are visited rarely if ever at all by the majority of users - so if they are not used, why keep them in RAM?

Virtual memory uses two methods to free up RAM when needed.

Swapping. An entire process, including code segment and data segments is expunged from the system memory.
Paging. Only single pages are swapped out.

Of course, the simplest way to clear a space in RAM is to terminate some processes, but virtual memory is more subtle than that. The idea is to free RAM only temporarily, with the intention of copying the data back again later. All of this should happen in such a way that the user of the system do not realize that it is happening.

Swapping and paging dump the system memory in special disk caches. Normally these disk areas are not part of the usual file system structure, since the overhead of maintaining a file system is inapropriate when only the system needs to use the disk. Instead, the system stores swap files in large contiguous blocks, sacrificing utilization of space for speed. Some systems also allow swapping to a special file in the normal filesystem, which has a reserved size.

In UNIX, there both methods are available. On BSD systems, normally a whole disk partition (see next section) is reserved for swapping and paging. (This is called the swap partition for historical reasons.) If this fails to provide enough space, under SunOS the system administrator can either add other partitions, or use the mkfilecommand to create a swap file on a normal in a part of the file system where there is sufficient space. In the system 5 based HPUX operating system, the normal swap area is invisible to the user. Additional swap space can simply be grabbed from some part of the filesystem, by the kernel, if the system goes short. Eventually this can lead to a paradoxical situation in which the user sees nothing on the disk, but the OS declares that the disk is full!

Early versions of UNIX used swapping exclusively when RAM was in short supply. Since BSD 4.3, all systems which have learned something from the BSD project use paging as their main method of virtual memory implementation.

5.2.2 Demand Paging - Lazy evaluation

You might ask - if a program has a lot of pages which do not get used, what is the purpose of loading them in the first place and then swapping them out? One could simply make a rule that no page should be brought into memory until it were needed. Such a scheme is possibile, but few systems allow a program to run if it cannot be loaded fully into memory on start-up. One argument against this extreme form of paging is that, it could be dangerous to start a program which was unable to complete because it was too large to run on the system, under the conditions of the moment. If it started to run and then crashed or exited, it could compromise important data. (The BSD UNIX system allocates sufficient space in its swap area to swap or page out each entire process as it begins. That way, none of them will ever run out of swap during execution.)

On the other hand, if a program can be loaded in, it is most likely safe - so if we then discover that large parts of the program are never used, we can page them out and never bother to page them in again.

This is an example of what is called lazy evaluation. A lazy pager never brings a page back into memory until is has to i.e. until someone wants to use it. This can save a considerable amount of I/O time. Another name for this is demand paging, since it only occurs on demand from user processes.

It is now easy to see how the paging concept goes hand in hand with the logical memory concept: each time the system pages out a frame of physical memory, it sets a flag in the page table next to the logical page that was removed. If a process attempts to read from that page of logical memory the system first examines the flag to see if the page is available and, if it is not, a page fault occurs.

A page fault is a hardware or software interrupt (depending on implementation) which passes control to the operating system. The OS proceeds to locate the missing page in the swap area and move it back into a free frame of physical memory. It then binds the addresses by updating the paging table and, when control returns to the waiting process, the missing page is automagically restored, as if it had never been gone.

Notice, that the location of the physical frame is completely irrelevant to the user process. A frame does not have to be moved back into the same place that it was removed from, because the runtime binding of addresses takes care of its relocation.

5.2.3 Swapping and paging algorithms

How does the system decide what pages or processes to swap out? This is another problem in scheduling. A multitude of schemes is available. Here we shall only consider some examples.

Consider the UNIX system a moment. Before paging was introduced, the only way that memory segments could increase their size was to

  1. Try to look for free memory at the end of the current segment and add it to the current segment.
  2. Try to allocate a new, larger segment, copy the data to the new segment and deallocate the old one.
  3. Swap out the process, reallocate and swap in again.

In this use of swap space, it is clear that a process is swapped out while it is waiting for a suitable hole in to appear in the memory. This might take a long time and it might be immediate. Another case for swapping out a job is if it has been idle (sleeping) for a long time.

On a BSD-like UNIX system, the first three processes to be started are ) the swapper,$2$) init, the and ) the pagedaemon. The pagedaemon is responsible for examining the state of the page-table and deciding which pages are to be moved to disk. Normally the swapper will not swap out processes unless they have been sleeping for a long time, because the pager will first strip them of their inactive pages. It will begin to swap out processes however, if the average load on the system is very high. (The load average is a number based on the kernel's own internal accounting and is supposed to reflect the state of system activity.) This gives `cheap' processes a chance to establish themselves. It is the pagedameon which makes the paging decisions. By copying read-only segments to the swap area at load time, the running overhead of paging out read-only data is removed, since the data are always where we need them in swap space and never change. In modernized versions of UNIX, such as the Solaris systems by Sun Microsystems, read only pages from the code segment are thrown away when they are selected for swap out and then read in from the filesystem if needed again. Moreover, data pages are only allocated swap space when they are forced out of physical memory. These optimizations reflect the fact that modern systems have more physical memory than previously; also disks are getting faster.

Let us now look more generally at how paging decisions are made. The most important aspect of paging is that pages can still be accessed even though they are physically in secondary storage (the disk). Suppose a page fault occurs and there are no free frames into which the relevant data can be loaded. Then the OS must select a victim: it must choose a frame and free it so that the new faulted page can be read in. This is called (obviously) page replacement. The success or failure of virtual memory rest on its abililty to make page replacement decisions. Certain facts might influence these algorithms. For instance, if a process is receiving I/O from a device, it would be foolish to page it out - so it would probably I/O locked into RAM. Here are some viable alternatives for page replacement. FIFO - first in first out

Consider the figure below. Here we see the frames in the physical memory of a paging system. The memory is rather small so that we can illustrate the principles of contention for pages most clearly.

Figure 5.6: Illustration of the FIFO page replacement scheme.

The simplest way of replacing frames is to keep track of their age (by storing their age in the frame table). This could either be the date, as recorded by the system clock, or a sequential counter. When a new page fault occurs, we can load in pages until the physical memory is full - thereafter, we have to move out pages. The page which has been in memory longest is then selected as the first to go.

This algorithm has the advantage of being very straightforward, but its performance can suffer if a page is in heavy use for a long period of time. Such a page would be selected even though it was still in heavy use. Second chance

A simple optimization we can add to the FIFO algorithm is the following. Suppose we keep a reference bit for each page in the page table. Every time the memory management unit accesses a page it sets that bit to . When a page fault occurs, the page replacement algorithm looks at that bit and - if it is set to - sets the bit to$0$but jumps over it and looks for another page.

The idea is that pages which are frequently use will have their bits set often and will therefore not get paged out. Of course, this testing incurs an overhead. In the extreme case that all pages are in heavy use the page algorithm must cycle through all the pages setting their bits to zero before finding the original page again. Even then, it might not find a page to replace, if the bit was set again while it was looking through the others. In such a case, the paging system simply fails. LRU - least recently used

The best possible solution to paging would be to replace the page that will not be used for the longest period of time - but unfortunately, the system has no way of knowing what that is. A kind of compromise solution is to replace the page which has not been used for the longest period (see the figure below). This does not require a crystal ball, but it does require some appropriate hardware support to make it worthwhile. As with all good ideas, it costs the system quite a lot to implement it.

Figure 5.7: LRU page replacement algorithm. When there is a tie, the algorithm uses FIFO.

Two possibilities for such an implementation are the following.

We record the time at which each page was last referenced. Unlike the FIFO scheme above, this means that we have to update the time-stamp every single time memory is referenced, instead of only each time a page is replaced. If the copying operation takes, say, five CPU instructions (jump to update routine, locate page table entry, load system clock time, store system clock time, return), this means - roughly speaking - that the system is slowed down by a factor of around five. This is an unacceptable loss, so unless the memory management unit can do something fancy in hardware, this scheme is not worth the system's time.
We keep a stack of page addresses, so that the page number of the most recently accessed page is always on the top of the stack. Although this sounds cheaper in principle, since the page replacement algorithm never has to search for a replacement - it just looks on top of the stack - it still results in a large system overhead to maintain the stack. We must update a data stucture which requires process synchronization and therefore waiting. Again, without special hardware, this is not economical.

In practice, many systems use something like the second-chance algorithm above. The UNIX pagedaemon uses this approach.

5.2.4 Thrashing

Swapping and paging can lead to quite a large system overhead. Compared to memory speeds, disk access is quite slow - and, in spite of optimized disk access for the swap area, these operations delay the system markedly. Consider the sequence of events which takes place when a page fault occurs:

  1. Interrupt / trap and pass control to the system interrupt handler.
  2. Save the process control block.
  3. Determine cause of interrupt - a page fault.
  4. Consult MMU - is the logical address given inside the process' segment i.e. legal?
  5. Look for a free frame in the frame table. If none is found, free one.
  6. Schedule the disk operation to copy the required page and put the process into the waiting state.
  7. Interrupt from disk signals end of waiting.
  8. Update the page table and schedule the process for running.
  9. (On scheduling) restore the process control block and resume executing the instruction that was interrupted.

Such a sequence of operations could take of the order or milliseconds under favourable conditions (although technology is rapidly reducing the timescale for everything). It is possible for the system to get into a state where there are so many processes competing for limited resources that it spends more time servicing page faults and swapping in and out processes than it does executing the processes. This sorry state is called thrashing.

Thrashing can occur when there are too many active processes for the available memory. It can be alleviated in certain cases by making the system page at an earlier threshold of memory usage than normal. In most cases, the best way to recover from thrashing is to suspend processes and forbid new ones, to try to clear some of the others by allowing them to execute. The interplay between swapping and paging is important here too, since swapping effectively suspends jobs.

5.3 Disks: secondary storage

The physical memory, as we have already seen, is not large enough to accomodate all of the needs of a computer system. Also, it is not permanent. Secondary storage consists of disk units and tape drives onto which data can be moved for more permanent storage. Apart from the actual physical differences between tapes and disks, the principles involved in controlling them are the same, so we shall only consider disk management here.

5.3.1 Physical structure

Even disks come in different shapes and sizes. The most obvious distinction is between floppy disks, diskettes and hard-disks. Floppy disks and diskettes consist of a single disk of magnetic material, while hard-disks normally consist of several stacked on top of one another. Hard disks are totally enclosed devices which are much more finely engineered and therefore require protection from dust. A hard disk spins at a constant speed, while the rotation of floppy drives is switched on and off. On the Macintosh floppy drives have a variable speed operation, whereas most floppy drives have only a single speed of rotation.

As hard drives and tape units become more efficient and cheaper to produce, the role of the floppy disk is diminishing. We look therefore mainly at hard drives.

Figure 5.8: Hard disks and floppy disks.

Looking at the figure, we see that a hard disk is composed of several physical disks stacked on top of each other. A separate read head is provided for each surface. Although the disks are made of continuous magnetic material, there is a limit to the density of information which can be stored on the disk. The heads are controlled by a stepper motor which moves them in fixed-distance intervals across each surface. i.e. there is a fixed number of tracks on each surface. The tracks on all the surfaces are aligned, and the sum of all the tracks at a fixed distance from the edge of the disk is called a cylinder.

To make the disk access quicker, tracks are usually divided up into sectors - or fixed size regions which lie along tracks. When writing to a disk, data are written in units of a whole number of sectors. (In this respect, they are similar to pages or frames in physical memory.) On some disks, the sizes of sectors are decided by the manufacturers in hardware. On other systems (often microcomputers) it might be chosen in software when the disk is prepared for use (formatting). Normally sectors are 512 bytes, or half a kilobyte.

Because the heads of the disk move together on all surfaces, we can increase read-write efficiency by allocating blocks in parallel across all surfaces. Thus, if a file is stored in consecutive blocks, on a disk with$n$surfaces and$n$heads, it could read$n$sectors sectors-per-track without any head movement.

When a disk is supplied by a manufacturer, the physical properties of the disk (number of tracks, number of heads, sectors per track, speed of revolution) are provided with the disk. An operating system must be able to adjust to different types of disk. Clearly sectors per track is not a constant, nor is necessarily the number of tracks. The numbers given are just a convention used to work out a consistent set of addresses on a disk and may not have anything to do with the hard and fast physical limits of the disk.

To address any portion of a disk, we need a three component address consisting of (surface, track, sector).

5.3.2 Device drivers and IDs

A hard-disk is a device, and as such, an operating system must use a device controller to talk to it. Some device controllers are simple microprocessors which translate numerical addresses into head motor movements, while others contain small decision making computers of their own.

The most popular type of drive for larger personal computers and workstations is the SCSI drive. SCSI (pronounced scuzzy) (Small Computer System Interface) is a protocol and now exists in four variants SCSI 1, SCSI 2, and fast SCSI 2, SCSI 3. SCSI disks live on a data bus which is a fast parallel data link to the CPU and memory, rather like a very short network. Each drive coupled to the bus identifies itself by a SCSI address () and each SCSI controller can address up to seven units. If more disks are required, a second controller must be added. SCSI is more efficient at multiple access sharing than other disk types for microcomputers.

In order to talk to a SCSI disk, an operating system must have a SCSI device driver. This is a layer of software which translates disk requests from the operating system's abstract command-layer into the language of signals which the SCSI controller understands. The operating system generally provides two logical devices for each SCSI address: a raw device and a buffered device. On BSD UNIX systems these are referred to as /dev/??and /dev/r??.

5.3.3 Checking data consistency and formatting

Hard drives are not perfect: they develop defects due to magnetic dropout and imperfect manufacturing. On more primitive disks, this is checked when the disk is formatted and these damaged sectors are avoided. If sector becomes damaged under operation, the structure of the disk must be patched up by some repair program. Usually the data are lost.

On more intelligent drives, like the SCSI drives, the disk itself keeps a defect list which contains a list of all bad sectors. A new disk from the manufacturer contains a starting list and this is updated as time goes by if more defects occur.

Formatting is a process by which the sectors of the disk are

  1. (if necessary) created by setting out `signposts' along the tracks,
  2. labelled with an address, so that the disk controller knows when it has found the correct sector.

On simple disks used by microcomputers, formatting is done manually. On other types, like SCSI drives, there is a low-level formatting already on the disk when it comes from the manufacturer. This is part of the SCSI protocol, in a sense. High level formatting on top of this is not necessary, since an advanced enough filesystem will be able to manage the hardware sectors.

Data consistency is checked by writing to disk and reading back the result. If there is disagreement, an error occurs. This procedure can best be implemented inside the hardware of the disk - modern disk drives are small computers in their own right. Another, cheaper way of checking data consistency is to calculate a number for each sector, based on what data are in the sector and store it in the sector. When the data are read back, the number is recalculated and if there is disagreement then an error is signalled. This is called a cyclic redundancy check (CRC) or error correcting code.

Some device controllers are intelligent enough to be able to detect bad sectors and move data to a spare `good' sector if there is an error. Disk design is still a subject of considerable research and disks are improving both in speed and reliability by leaps and bounds.

5.3.4 Scheduling

The disk is a resource which has to be shared. It therefore has to be scheduled for use, according to some kind of queue system. If a disk only had one customer at a time, a first-come first-served FCFS policy would be adequate. However - requests both to read and to write may come randomly from any user process or from the system on a multitasking system and so we must think carefully about how to service them.

Since a disk is hardware, and involves mechanical movement, it can literally be destroyed by asking it to do too much. One of the aims of scheduling a disk device is to minimize wear on the disk surface. Another aim is to maximize the speed of access. If the disk heads are being asked to go backwards and forwards randomly many times a second, much time can be lost. Floppy disks are particularly susceptible to errors caused by misalignment between disk and disk head. The more a hed moves rapidly backwards and forwards, the more likely it is to miss its intended location and misread data. When this happens the data have to be read again and the whole process takes much longer.

Hard disks are more robust than floppies, but the algorithms for scheduling the disk nevertheless take into account the physical issue of movement. FCFS

As always, the simplest option for scheduling is the first-come first-serve method. This can be thought of in two ways: i) that the first user to obtain the disk gets to use it uninterrupted until his or her file access is finished, or ii) every individual disk access can be sheduled on a FCFS basis. On a busy system, ii) can lead to wild thrashing of the disk heads as different processes first try to move them one way and then another.

The AmigaDOS system (at least up to 1.3) suffered from this problem even if there were only two processes. The system tried to time-share the disk which resulted in a more than fifty percent loss in performance. The user could wait for minutes while the system tried to thrash out a job which could have taken seconds if one job had been allowed to complete first without interruption. SSTF - Shortest seek time first

To get the fastest response (ignoring mechanical restrictions) we could try to sort disk requests according to those which will cause the smallest movements of the disk head. Again, this does not protect the head from direction reversals, only from large movements. Also, like all priority scheduling, it could lead to starvation of some requests. SCAN, C-SCAN and LOOK

The scanning method is to order requests so that we only move the disk head in one direction at a time. Since the disk heads only move if we need to change tracks, all requests are ordered according to which track they lie on. The heads start at the first track and move uni-directionally to the next request and then the next etc. When they reach the inside of the disk, they reverse direction and come back again. This is also called the elevator or lift algorithm since this is the way many elevators are scheduled.

The C-SCAN or circular scan is a slight variant: when the heads hit the end track, they come immediately back to the beginning and start again, so that they always move in the same direction.

Of course, neither algorithm needs to go as far as the last track if there are no requests for data there. The LOOK algorithm is the same as SCAN or C-SCAN but does not move into areas of the disk where no requests are waiting.

Figure 5.9: Scanning disk scheduling algorithms. Which method?

The choice of scheduling algorithm depends on the nature of disk usage. For heavily use disks the SCAN / LOOK algorithms are well suited because they take care of the hardware and access requests in a reasonable order. There is no real danger of starvation, especially in the C-SCAN case.

The arrangement of data on a disk play an important role in deciding the efficiency of data-retrieval. In the next section we shall look at the high-level structures which the operating system places on top of sectors. This determines the level at which we are most used to seeing the disk.

5.3.5 Partitions

For the purposes of isolating special areas of the disk, most operating systems allow the disk surface to be divided into partitions. A partition (also called a cylinder group) is just that: a group a cylinders which lie next to each other. By defining partitions we divide up the storage of data to special areas, for convenience.

For instance, it is quite normal to keep the system software in one partition and user data in another partition. That way, when one makes a back-up of the disk, user data can easily be kept separate from system data. The separation becomes a hardware matter.

Partitions are supported on MS-DOS, Macintosh, BSD UNIX, AmigaDOS etc. Remarkably there are versions of system 5 UNIX which do not support partitions. BSD UNIX partitions are a good example, and since we are focussing on UNIX we shall discuss its partitions in more detail.

BSD UNIX uses a special convention for the partitions on each disk. Each disk may have up to eight logical partitions which are labelled from ato h.

Partition Usage
a root and boot partition
b swap partition
c the whole disk
d anything
e anything
f anything
g anything
h anything


Each partition is assigned a separate logical device and each device can only write to the cylinders which are defined as being its own. Partitions can overlap, because they are just limits. Thus, if we read from logical device c, which is defined as the whole disk, we could, in principle read from the whole disk, whereas if we use logical device b we may only read from the swap partition.

To use a partition we have to create a filesystem on it. This involves reserving space workspace for the operating system and suitable markers for navigating over the surface of the disk.. Since partitions are defined for convenience, it does not matter that they overlap. What is important is that the filesystems on two partitions do not overlap! This is extremely important. If two filesystems overlap, they will destroy each other!

In BSD UNIX, partitions are created by editing a table which is downloaded into the device driver. In Sun's SunOS and Solaris operating systems, a special command formatis used to make partitions. The newfscommand is used to create a filesystem.

Once a partition has been created, it has to be mounted in order to be reachable from the directory structure of the filesystem. The mount action is analagous to the opening of a file. On the Macintosh and Amiga operating systems, new disks are immediately sensed by the system and are mounted. In the Macintosh case (which has only a pictoral graphic user interface) new partitions or disks are mounted on the desktop at the root level. Under AmigaDOS, each new partition becomes a logical device and is given a logical device name which identifies the disk. If the Workbench (graphical user interface) is running, the disks appear together with their device names on the workbench in the same way as the Macintosh. Otherwise they appear in the mountlist.

In UNIX a partition is mounted using the command mount. For example a command like

mount /dev/sd0g  /user-data

would mount partition g on disk number zero onto the directory /user-data. The result would be that all files on that partition would appear under the directory /user-data. A prerequisite for mounting a UNIX partition is that the partition must contain a filesystem.

5.3.6 Stripes

In recent years some UNIX systems (particularly Hewlett Packard) have experimented with disk striping. Disk striping is a way of increasing the disk transfer rate up to a factor of$N$, by splitting files across$N$different disks. Instead of saving all the data from a given file on one disk, it is split across many. Since the$N$heads can now search independently, the speed of transfer is, in principle, increased manifold. The disadvantage with disk striping is that, if one of the$N$disks becomes damaged, then the data on all$N$disks is lost. Thus striping needs to be combined with a reliable form of backup in order to be successful.

Figure 5.10: Disk striping: files are spread in parallel over several disks.

5.4 Disk Filesystems

A filesystem is a high level interface to the disk, which allows users of a system to give names to files, organize files in directories and separate off special areas using partitions. A filesystem is said to be created on a disk by running a special program. On many systems this is identified with formatting the disk and involves writing address data to the surface as well as reserving system workspace on the disk.

5.4.1 Hierachical filesystems and links

The most popular type of filesystem interface is the hierachical one. Earlier operating systems like MTS did not have a directory structure. Each user had a separate login area, but the login area was not able to hold subdirectories. The hierachical file structure is a very convenient way of organizing data in directories, sub-directories and so on. But this rigid preoccupation with a hierachical ordering is not always the most appropriate one.

Look at the diagram below.

Figure 5.11: Links - deviations from a strict hierachical filesystem. /usr/local/bin/prog is a link to /usr/local/mysoftware/prog and /local is a link to /usr/local

Suppose are in the directory /usr/local/mysoftware, which contains a complete package of software that we have obtained in all of its sub-directories. Since the package is a unit, we would like to keep all of its files together and preserve that unity - but it might also be necessary for some of the files in the package to be installed in special places, elsewhere in the file tree. For example, the executable binaries might have to be placed in /usr/local/bin, and some configuration files for the system might have to be placed in a special directory where the operating system can find them.

The conflict of interest can be solved by introducing links. Links are objects which appear in the file system and look just like files. In fact they are pointers to other files which are elsewhere in the strict hierarchy. Links enable one file to appear to exist at two or more places at the same time. A link is not a copy of a file, it is an alias for the true route to the file through the hierachical system, but for all intents and purposes it looks like another instantiation of the file. The Macintosh filesystem refers to such links as `aliases'.

The UNIX file system makes a distionction between hard links and symbolic links. A symbolic links is literally just a small file which contains the name of the true file. We can create a symbolic link to a file which does not exist, and delete the file to which a symbolic link points. A hard link is more permanent however. In order to delete a file with hard links, all of the hard links must be removed. This requires a list of links to be associated with each file. The special files `.' and `..' are hard links to their parent directories.

When links jump across different branches of the file tree, the directory structure is sometimes called an acyclic graph.

5.4.2 File types and device nodes

Extremely elaborate filesystem interfaces can be made, which distinguish between different types of file and which permit or disallow certain operations on the basis of file type. The Macintosh operating system determines whether files are executable or text files. Clicking on an executable file loads and runs the program, whilst clicking on an application file loads the application which created it and then tells the program to load that file.

MS-DOS distinguishes file types by using filename extensions like .EXE, .COM, .TXT for executable files, relocatable executables and textfiles.

The UNIX system does not make any particular distinction on the basis of filenames. Instead it keeps a flag to say whether a file is executable or not. If a file is not marked as executable, UNIX will not try to run it. If it is marked executable, it will try to execute the file. If the file is not a valid binary program, it will fail. Executable binary files must conform to a certain protocol structure which identifies them to the operating system as being fit for execution. If a text file is marked executable, UNIX will try to pass the lines of the file to the command line interpreter or shell.

Certain files in the UNIX operating system are not really files at all but `handles' to devices. They are called device nodes. A device node is a way `into' a device through a filesystem interface. It is convenient to be able to use normal filing commands to access devices. Not all devices can be accessed in this way, but the interface is useful for those that can. In the Solaris 2 operating system, the kernel process list is represented as such a directory of pseudo-files.

For user convenience, the filecommand attempts to guess the contents of UNIX files, but this is not used by the system.

5.4.3 Permissions and access

On multi-user systems we must have a mechanism to prevent one user from freely modifying the files of another user - while at the same time, keeping enough flexibility to enable groups of users to share certain files. It is also advantaegous to be able to lock files so that they cannot be deleted - even by their owner. This is normally done by giving files permissions or protection bits.

Files must be readable and or writable only to very specific users. In some operating system, like the Apollo Domain OS and the Andrew file system, there are very intricate schemes for protecting files, consisting of lists of users who are allowed or disallowed access to them. Here we shall briefly sketch out the simple system used by UNIX as an example.

Each file has one owner and belongs to one group. The owner of the file is the only one (apart from the system administrator) who can decide whether others can read or write to the file and to which group it belongs. If the owner wishes, he or she may open the file for reading or writing to i) the other members of the group to which the file belongs, or ii) anyone. Since only the system administrator can add users to a group, the file is secure, provided the user sets the correct protection bits.

When a new file is created by a given user, that user is automatically the owner of the file. The group ownership is determined differently for BSD and system 5 UNIX. In BSD, the group is normally set to a default group for that user, called the login group. In system 5, the file inherits the group ownership from the directory it is created in. (This can also be arranged in BSD by setting the `sticky bit'.)

More modern UNIX systems and other operating systems now provide access control lists or ACLs. This generalizes the notion of file owner and group by allowing a file to be accessible to a named list of users and a named list of groups, rather than just a single user or a single group. ACLs were first introduced in the DOMAIN operating system by Apollo and were later adopted by HPUX and then Solaris. Novell systems (based on Apollo NCS) also provide ACLs. Modern filesystems like NFS 3, AFS and DFS also provide ACL support, but there is currently no standard implementation and the different systems are not compatible.

5.4.4 File system protocols

To read or write to a file, all operating systems require that users formally open the file. When finished, they must close the file. This formal procedure has several purposes. It allows us to

  1. see whether the file is inaccessible, because we do not have permission to open it.
  2. see whether the file is inaccessible because it is being used by another user. When we open a file for writing, a lock is placed on the file to prevent others from writing to it simultaneously. This lock is removed by the closeoperation.
  3. obtain pointers to where the file exists physically within the secondary storage and set up a data structure called a filehandle which the system will use to describe the state of the file as we use it.
  4. set up any cached data which might be used by the OS.

Once a file is open, the system must present the user with a consistent picture of the filesystem. When a user program reads lines from a file, a pointer should be advanced so that every line is read exactly once. An end of file condition should be signalled when the file is read (this is usually achieved by storing an EOFcharacter at the end of the file) etc. These are all aspects of an agreed protocol defined by the filesystem.

A more complex situation is the following. Suppose one user is reading a file and another user wants to write to it.

  1. Should the user be allowed to write to the file while someone is reading it?
  2. If so, should the user be able to see the changes made to the file until after they have closed it?

There are two possibilities - either all users see changes immediately, or only users opening files after the changes were made see the changes. Both versions of this are in use by different filesystem implementations. In the latter case, the OS has to keep several copies of the file until all file handles are released and everyone agrees about the contents of the file.

It is difficult to say that one or the other type of behaviour is more correct. This is largely a subjective issue. What is important is that the filesystem defines its behaviour and sticks to it consistently. The behaviour of the filesystem is often called filesystem semantics.

5.4.5 Filesystem implementation and storage

Although a sector is the smallest unit of allocation for the physical disk, most filesystems create logical structures on top of sectors in order to optimize disk access. These are called blocks. A block can in principle be any size. Usually they are from 512 bytes (the same size as a sector) up to 8k. The larger the block size, the more efficient file transfers will be.

If we want to save a file which is only three bytes long, we normally have to allocate a whole block and the remainder of the block is wasted. Some systems, notably BSD UNIX's ufs filesystem, from release 4.2, solve this problem by using two block sizes: major blocks and fragments. A file is allocated in large blocks except for the last one which is allocated as a fragment. Typical sizes for large blocks are 4kB to 8kB, and a typical size for fragments is from 512 bytes to 1kB (eighths).

Figure 5.12: Blocks and fragments

We must now address the issue of how the blocks are allocated. This is the analogous problem to that of memory allocation in RAM. The principal difference is that disk memory is considerably larger than primary memory, so problems can be encountered in addressing all of the blocks on the disk. We shall briefly mention some general strategies below - and then look more closely at the UNIX ufs filesystem.

To use the space on a disk, we must make a choice about whether we wish files to be stored contiguously, or whether we wish to use a scheme of logical and physical addresses, as we did in primary memory and allow files to be spread liberally any on the disk. The problem with contiguous allocation is, of course, fragmentation. We have a much better chance of being able to fit files into the spaces om a disk if we can allocate space in small blocks. On the other hand, we know that large blocks are more efficient, since we can read or write a file in fewer operations in the block size is large.

Contiguous allocation is seldom used (except in the swap area) for filesystems because of the fragmentation problem. Instead files are divided up into blocks and each file consists of a list of blocks which may be scattered anywhere on the disk. Our problem is then to identify files amongst all of the blocks. There are three ways of doing this:

  1. Linked lists. Each block of data includes a pointer to the next block of data in a linked list. The difficulty with this method is that each block must be read in the correct order and the blocks might be spread randomly over the disk. Thus the retrieval of a file could require a lot of disk head movement which is slow.
  2. Linked table. A linked list of blocks for each file is stored in a file allocation table. All of the pointers for every file are collected together in one place. This table could also be cached in RAM for faster access. This method is used by MS-DOS and a number of other microcomputer operating systems.
  3. Indexing. Each file has an index containing a list of blocks which contain the file itself. This index might be stored anywhere in principle. Space for it is normally allocated on the disk itself, inside reserved disk blocks, and partly inside an index table which is built when the filesystem is created. The index blocks are grouped in one place for convenient access. This system is used in UNIX. Since the index table must contain pointers to disk blocks, a way of storing the pointers must be found. If the list is small and is held in a filesystem block, then most of the block will be wasted. This is a drawback of the index method, but the main advantage of this method is that it has few limitations.

5.4.6 The UNIX ufs filesystem

A file system under UNIX is created using the newfscommand. A separate filesystem must be created on each separate partition of the disk. To define a filesysytem we have to define the blocksize and numerous other parameters. Each system has its own defaults which inexperienced users - and most often experienced users are wise to use.

Two structures are created when a file system is created: inodes and superblocks. These are the most important objects in the filesystem. Both of these objects are information structures, in the sense of the C language and they are defined under the /usr/include/ufsdirectory in files fs.hand inode.h. It is instructive to look at these files. This is where the default blocksize etc will be defined on your system!

The blocksize is variable, but a minimum block size of bytes i.e. 4kB is stipulated so that the system can address bytes of memory without using three level indirection (see below). Also, the last block of a file can be allocated as a fragment of a block whose size is recorded in the inode. (It might be a half, a quarter or an eighth of a block.)

A superblock contains the information on the boundaries of the partition (cylinder groups) and information about where the inode table is and where datablocks start. If the superblock is lost or damaged, the whole filesystem would be unreadable. It is so important that, when a file system is created, superblock backups are made at regular intervals throughout a partition. Thus if one block is detroyed, another can be used to repair the damage. The UNIX filesystem check program fsckcan do this. fsckis run automatically on every boot of the system in case the system went down uncleanly. (UNIX uses buffered and cached I/O so data are not always written to the filesystem immediately. The program syncis run the the OS at regular intervals in order to synchronize the disk structure with the present state of the cache. If the system crashes or goes down without synchronizing the filesystems, the superblock will be invalid and will have to be repaired.)

Partition `a'on disk zero is special. This is the default boot device. On power up, the boot program (in ROM) looks to the first few sectors of this partition for a boot block. Sectors contain the boot-block. Sector 16 marks the start of the superblock.

An inode or index node is the data structure which holds the specific data about a particular file. Regardless of how large a file is, there is exactly one inode per file. The elements of an inode are drawn in the figure below.

Figure 5.13: UNIX inodes.

When file system is created, it creates a fixed number of inodes. It is not possible to create more files on the system than the number of inodes, so a limit is built into each file system. Usually the limit is no problem in practice - and anyway, it can always be changed by changing the parameters given to newfs.

Inodes which are not in use, are kept in a doubly linked list called the free-list. Filenames are stored in a directory structure, not in the inodes themselves, with pointers to the appropriate inodes for the start of the file. Each inode contains a plethora of information about the file: the device on which the file resides, the type of file and its protection bits, the user id and group id of the owner, timestamps indicating the last time the file was written to etc, the size of the file and of course pointers to the actual blocks of data.

Data blocks are (of course) addressed by indexing. As an attempt at optimizing the index, inodes use three separate ways of addressing data on the disk (in fact four different ways are built in to the inodes, but only three are used). The inode contains a list of twelve 32bit pointers to blocks on the disk. For small files this would be enough. Since the minimum blocksize is kB these pointers can address up to bytes i.e. kB.

For larger files, a system of indirect addressing is used. There are three levels of indirect addressing, though only two are used currently. In single-indirect addressing, the inode has a pointer which points to a file block (not another inode). This file block has room for 4kB at least. Those 4kb are used to store a sequential array of 32bit pointers to other data-blocks which contain the true data. Using this method, we have space for four-byte pointers in the address block - and each pointer can point to bytes (4kB), thus we have space for bytes per file. This must then be added to the 48kB of direct pointers.

In double-indirect addressing, the inode pointer points to a block of pointers (as before), but now these pointers point to blocks which also contain pointers - i.e. the pointers to the real data. The total space accessible per file is now multiplied by - i.e. the number of 32 bit pointers which will fit into the minimum block size, since every fourth byte of the single-indirect memory above now forms a pointer to a block of 4kB. The total size is bytes, which is roughly 4 giga-bytes. This should, again, be added to the single-indirect and direct memory above.

Although the inodes can span an address space which is larger than bytes, internal pointers in the file structures are still$32$bit (except in OSF/1) and so a limitation to bytes is imposed by the word size of the system hardware.


  1. Go back and think about shared libraries, in view of what you have learned about logical, physical and virtual memory. What are the advantages and disadvanteges of shared libraries?
  2. Write programs to code the page replacement algorithms discussed in above.


Write a program to model the behaviour of a hard disk. A disk drive contains a stepper motor which pushes the head one track at a time. You can model the tracks and segments of the disk as an array.

const int tracks = 20;
const int sectors_per_track = 20;
const int heads = 2;
const int bytes_per_sector = 64;

char harddisk[tracks][sectors_per_track][heads][bytes_per_sector];

Write a device-driver program which moves the head of the disk according to the LOOK scheme. You can choose yourself whether you base it upon SCAN or CSCAN.

Why is this array not exactly like a disk? (Hint: think geometry.)

Suppose you have four short files of data, two short and one long. Design a simple filesystem so that you can do the following:

  1. Save the two short files onto your `virtual disk' from the real disk.
  2. Retrieve the files again. Make sure that you can retrieve the files by name, as many times as you like.
  3. Delete the first file.
  4. Save the longer file now, using the space that was freed when you deleted the shorter file in .
  5. Plot the head movement of your disk on the screen using track number for the horizontal axis against time vertically, so that the output looks something like the following.
          Track ->
    1 2 3 4 5 6 7 8 9 ...
    Time is measured in units of one head movement - one click of the stepper motor. Show how the head moves when you save and retrieve your files. Hint: use separate output files to print the result of the head movement and the result of retrieving a file.

Note that you will have to design a `protocol' for saving the data into the array. The disk array is just an array of characters, you if you want to save a file, you need to know what is data corresponding to which file.

Hint: you might want to limit the filename size to, say, eight characters to make the problem easier, like in DOS.

Explain carefully how you locate files on your disk, and what scheme your filesystem uses to recover files in the correct order.

6. Networks: Services and protocols

In this section we shall consider how to use the concepts we have considered so far to make the task of implementing network communication as straightforward as possible.

Consider a large company or a university with thousands of users, many of whom have workstations or terminals on their desks - all of whom are connected to a network. In this situation it is natural to share certain resources so that they can be accessed from anywhere on the network, without having to be reproduced on every machine:

The printer,
User authentification data (password database),
Disks holding user data,
A reference clock which can be used to set the local clocks on all systems,
Addresses and telephone numbers.

To some extent, this idea of sharing was the idea behind multi-user systems. Not everyone can afford their own - so we share. What big multiuser mainframe machines have tought us, however, is that a single monolithic computer with$n$terminals is not a good solution.

Users demand more and more CPU power every day.
Scheduling the CPU, even if efficient on paper, can be spoiled in practice for most users by a few greedy users. Everyone wants their own private CPU.
Interactive I/O places a big load, proportional to the number of users. A large machine with a hundred keyboards attached to it can quickly become overwhelmed by keyboard I/O. More and more programs are interactive and the I/O overhead is much larger since mice and windows came along.

The solution which is popular at present is to give everyone a smaller machine with their own CPU, keyboard and screen. Although perhaps wasteful in theoretical terms, in practice it is one of those little luxuries, like owning a big car, which improves the quality of life for those who have it. What's more, since computing power has generally increased, software has grown to absorb that power - so it is not wasted for long.

By giving everyone their own machine, linked together by a network we

Spread the interactive I/O load over all the machines.
Allow machines to have public and private resources.
Introduce a new overhead: the network software.

6.1 Services: the client-server model

To share public resources on a network, we introduce the concept of a service. A service is simply a job done by one part of a system on behalf of another. The service is provided by a server on behalf of a client. This is what is known as the client server model 6.1 .

Figure 6.1: The client server model. The client and the server need not be on the same machine when there is a network present.

We have already encountered this kind of behaviour before in connection with system calls. The system kernel is a kind of server, which provides I/O services for all the processes on a system. Also daemons, in the UNIX terminology, are servers which answer requests or perform some house-keeping on behalf of other processes. The key idea is that there are always two elements: clients and servers.

On a network, we would like to arrange things so that the server and the client might be anywhere - on the same machine or on different machines. We would like a flexible system for sending out requests for services into a network and getting an answer without having to know too much about where the services are coming from.

To achieve these aims, we need:

Interprocess communication which works across machine boundaries.
Protocols - agreed systems of behaviour - for requesting services.
Services need to have names or numbers which identify them uniquely.
Network services need to be multi-threaded, since several clients might request services simultaneously. We don't want to keep clients waiting.

6.2 Communication and protocol

There are two ways of making a client-server pair. One is to use Berkeley sockets directly and the other is to use RPC - or Remote procedure call software package.

A `socket' is a communications link from one process to another. Sockets work over a network using the internet protocol set (see next chapter). Opening a `socket' to another machine is like opening a file to read from or write to. Data are transferred as streams or packets of raw, non-interpreted bytes. The interpretation of the data once they arrive at their detsination is a problem for the user to deal with.

RPC, on the other hand, is a high level software package which works on top of sockets and allows programs to send typed data using a protocol known as XDR - external data representation. It also has high level tools called protocol compilers which help programmers to write the code to interpret data at both ends of a client-server connection.

There are two main implementations of this software:

Sun Microsystems' RPC
Apollo's NCS system.

Most of the software was developed for the UNIX-like operating systems, but has since been adapted to all the popular systems in use. All of the software runs on top of the TCP/IP network protocol which we shall discuss in the next chapter.

6.3 Services and Ports

Services are a high level concept. When we ask for a service, we are not interested in how the message gets to the server over a network. We just want to be able to call some function DoService(myservice)and have the result performed by some invisible part of the system. To make this happen, a system of `handles' is used. It is rather like opening a file - but now we want to open a service. The terminology is also different.

To obtain a service, we do not request a file handle but a port. A port is a software concept - it should not be confused with the hardware connector which couples your machine to the network (which is also called a port on some systems). It is a number which an operating system uses to figure out which service a client wants.

We say that a particular service lives at port xxx.

Here is some important terminology.

Well-known ports. Every computer, all over the world has to agree on the port numbers for different services. A well-known port is a port number () which is reserved for a well-known service like ftpor telnet. It has been registered in a world-wide register.

RPC program numbers. Historically, we distinguish between services and RPC, although the effect of the two is the same. The system of calling RPC services is different to normal services - it uses program numbers first, and works out port numbers for itself.

6.4 UNIX client-server implementation

It is useful to describe how UNIX deals with services, since this is the model which has been adapted for other systems.

6.4.1 Socket based communication

To send data to a server using sockets, we need to know the port number at which the server lives. Port numbers are listed in the file /etc/services, which looks like this.

# Network services, Internet style
# This file is never consulted when the NIS are running
tcpmux		1/tcp				# rfc-1078
echo		7/tcp
echo		7/udp
ftp		21/tcp
telnet		23/tcp
smtp		25/tcp		mail
time		37/tcp		timserver
time		37/udp		timserver
name		42/udp		nameserver
whois		43/tcp		nicname		# usually to sri-nic
domain		53/udp
domain		53/tcp
hostnames	101/tcp		hostname	# usually to sri-nic
sunrpc		111/udp
sunrpc		111/tcp
login		513/tcp
shell		514/tcp		cmd		# no passwords used
printer		515/tcp		spooler		# line printer spooler
courier		530/tcp		rpc		# experimental
uucp		540/tcp		uucpd		# uucp daemon
biff		512/udp		comsat
who		513/udp		whod
syslog		514/udp
talk		517/udp
route		520/udp		router routed
ingreslock      1524/tcp
bootpc          68/udp                          # boot program client
bootp           67/udp          bootps          # boot program server

The file maps named services into port numbers and protocol type. The protocol type is also an agreed standard which is defined in the file /etc/protocols, which looks like this:

# Internet (IP) protocols
# This file is never consulted when the NIS are running
ip	0	IP	# internet protocol, pseudo protocol number
icmp	1	ICMP	# internet control message protocol
igmp	2	IGMP	# internet group multicast protocol
ggp	3	GGP	# gateway-gateway protocol
tcp	6	TCP	# transmission control protocol
pup	12	PUP	# PARC universal packet protocol
udp	17	UDP	# user datagram protocol
hmp     20      HMP     # host monitoring protocol
xns-idp 22      XNS-IDP # Xerox NS IDP
rdp     27      RDP     # "reliable datagram" protocol

In order to open a socket, we must know the name of the host on which the server lives. If we don't know this information in advance, we can send a broadcast request to all hosts, hoping that one of them will reply with their correct address (see next chapter).

Also, when the message arrives at a host which runs the server process, there are two possibilities.

The server process is always running.
The server process gets started when the request arrives.

Both methods are used in practice. If a server is expected to receive a lot of requests, it should be running all the time. If it spends long periods sleeping it should probably started when a request arrives.

The second of these possibilities is handled by a yet another server called the internet daemon or inetd. This is a kind of public server which works on behalf of any service. inetd reads a configuration file called /etc/inetd.conf. Here are a few typical lines from this file.

# Configuration file for inetd(8).  See inetd.conf(5).
# Internet services syntax:
#  <service_name> <socket_type> <proto> <flags> <user> <server_pathname> <args>
# Ftp and telnet are standard Internet services.
ftp     stream  tcp     nowait  root    /usr/etc/in.ftpd        in.ftpd
telnet  stream  tcp     nowait  root    /usr/etc/in.telnetd     in.telnetd
# Shell, login, exec, comsat and talk are BSD protocols.
shell   stream  tcp     nowait  root    /usr/etc/in.rshd        in.rshd
login   stream  tcp     nowait  root    /usr/etc/in.rlogind     in.rlogind
exec    stream  tcp     nowait  root    /usr/etc/in.rexecd      in.rexecd
comsat  dgram   udp     wait    root    /usr/etc/in.comsat      in.comsat
talk    dgram   udp     wait    root    /usr/etc/in.talkd       in.talkd

inetd listens on the network for service requests for all of the daemons which are in its configuration file, and - if such a request arrives - it starts the server for the duration of the request.

Notice the field `wait' and `nowait'. This tells inetdwhat to do if another request arrives while one request is being processed - should it wait for the first request to finish (single threaded) or should it start several processes (multi-threaded) to handle the requests 6.2 .

6.4.2 RPC services

In the RPC way of doing things, we called a service based on a program number, a procedure number and a version number. There is now an extra step in the chain of events - yet another common server which must be consulted. This is called the portmapper. When an RPC server starts up on its host, it registers itself with the portmapper, telling it which port it is listening to and what program number it is using.

When an RPC client wants a service, it sends a request to the portmapper on the server host asking for a server which can deal with program number (service) xxx. The portmapper replies by giving the port on which the RPC server is listening.

The advantage of this scheme is that RPC applications do not have to run on well-known ports. A suitable free port can be found at start-up. On the other hand, each type of server program must have a unique program number, which must be obtained from Sun Microsystems. The program numbers are stored in /etc/rpc.

The real benefit of the RPC packages is the high level concepts which they handle on behalf of the programmer. The protocol compilers and XDR protocols provide a set of `frequently need subroutines' which enhance the system of communication across a network.

6.5 The telnet command

The telnet command, as opposed to the telnet service, does not only contact the well-known port number , but can also be used to send a message to any port. For example, instead of the command

finger mark@mymachine

to get information on user mark from the finger database, we could contact the well-known port on host `mymachine' as follows:

anyon% telnet anyon finger
Trying ...
Connected to anyon.
Escape character is '^]'.
Login name: mark                        In real life: Mark Burgess
Directory: /mn/anyon/u2/mark            Shell: /local/bin/tcsh
On since Aug 14 11:59:39 on ttyp1 from :0.0
17 minutes Idle Time
Mail last read Sun Aug 14 14:27:02 1994
No Plan.

Or had finger not been in /etc/services, we could have written

telnet hostname 79

Not all services accept textual input in this way, but telnetwill try to contact their ports nevertheless.

6.6 X11

The X11 window system, used by Unix, is a client-server based application. A user's workstation runs a server process called X, whose job it is to display windows on the user's display. Each application the user wishes to run is a client which must contact the server in order to have its output displayed on the X-display.

Figure 6.2: The X windowing system.

By making this simple client-server abstraction, it makes no difference whether applications are running on the same host as the X-display, or whether they are running over the network. X uses its own system of protocols which is layered on top of socket communication. Strangely, X and Sun's variant News are the only window systems which have understood the point of networking. All other window systems require you to run programs on the computer at which you are sitting.

6.7 html: hypertext markup language

A further example of a protocol is the world wide web hypertext markup (formatting) language (html). This insists upon simple rules for formatting pages and references.


  1. Explain what `protocol' means.
  2. Describe briefly the client-server model.
  3. What role do dæmons play in with respect to the unix kernel? Why are servers daemons?


Make a simple client-server model which commuicates via unix files. The server should be sent an arithmetic problem to solve, for example: . The client should send this request to the server, and the server should send back the answer. The client must be able to exit gracefully if the server does not answer for any reason. (Hint: you could use the `sleep' command to wait for the server to reply.)

You will need to think of the following:

  1. What filenames should you use to send messages from the client to the server and from the server to the client?
  2. Since the client and the server are independent processes, you need to find a way of discovering when the client and the server have finished writing their replies, so that you don't read only half of the answer by mistake.
  3. The server should loop around and around, waiting for maultiple requests, while the client sends only one request and exits when it gets a reply.

7. TCP/IP Networks

In the last chapter we looked at some of the high level considerations for enabling transparent communication over a network. The next thing to look at is how such a scheme of protocols is achieved in practice.

7.1 The protocol hierarchy

7.1.1 The OSI model

We begin by returning to the `most important idea in computing' - namely hierarchies. As we have noted before, the most practical way of solving complicated problems is to create `layers of detail'. At any level in a hierarchy, the details of the lower levels are invisible - so we never see the irrelevant pieces of the computing puzzle we are trying to solve.

The International Standards Organization (ISO) has defined a standard model for describing communications across a network, called the OSI model, for Open Systems Interconnect (reference model).

The OSI model is a seven layered monster. It does not have to be taken literally - it might not be natural to separate all of these parts in every single program - but it is useful as a way of discussing the logically distinct parts of network communication. The layers are described as follows.

7 Application layer Program which sends data
6 Presentation layer XDR or user routines
5 Session layer RPC / sockets
4 Transport layer tcp or udp
3 Network layer IP internet protocol
2 Data link layer ethernet (protocols)
1 Physical layer ethernet (electronics)


At the lowest level, the sending of data between two machine takes place by manipulating voltages along wires. This means we need a device driver for the signaller, and something to receive the data at the other end - a way of converting the signals into bytes; then we need a way of structuring the data so that they make sense. Each of these elements is achieved by a different level of abstraction.

  1. Physical layer. This is the problem of sending a signal along a wire, amplifying it if it gets weak, removing noise etc. If the type of cable changes (we might want to reflect signals off a satellite or use fibre optics) we need to convert one kind of signal into another. Each type of transmission might have its own accepted ways of sending data (i.e. protocols).
  2. Data link layer. This is a layer of checking which makes sure that what as sent from one end of a cable to the other actually arrived. This is sometimes called handshaking.
  3. Network layer. This is the layer of software which remembers which machines are talking to other machines. It establishes connections and handles the delivery of data by manipulating the physical layer. The network layer needs to know something about addresses - i.e. where the data are going, since data might flow along many cables and connections to arrive where they are going.
  4. Transport layer. We shall concentrate on this layer for much of what follows. The transport layer builds `packets' or `datagrams' so that the network layer knows what is data and how to get the data to their destination. Because many machines could be talking on the same network all at the same time, data are broken up into short `bursts'. Only one machine can talk over a cable at a time so we must have sharing. It is easy to share if the signals are sent in short bursts. This is analogous to the sharing of CPU time by use of time-slices.
  5. Session layer This is the part of a host's operating system which helps a user program to set up a connection. This is typically done with sockets or the RPC.
  6. Presentation layer. How are the data to be sent by the sender and interpreted by the receiver, so that there is no doubt about their contents? This is the role played by the external data representation (XDR) in the RPC system.
  7. Application layer. The program which wants to send data.

As always, the advantage of using a layered structure is that we can change the details of the lower layers without having to change the higher layers. Layers to are those which involve the transport of data across a network. We could change all of these without doing serious damage to the upper layers - thus as new technology arrives, we can improve network communication without having to rewrite software.

Most of these layers are quite static - only the physical layer is changing appreciably.

7.1.2 Data encapsulation

Each time we introduce a new layer of protocol into network transport, we need to `package in' the information in some agreed format. This is called data encapsulation. Often when data are encapsulated, each `packet' (to use the word loosely) is given a few bytes of `header information'. This is information which includes, for example, what the information is for, where it is going and which piece of the total information the current packet represents.

At the level of the network layer, data might be divide up into numbered packets, each of which contain the address of the sender and receiver, the length of the packet and so on.

Suppose now that we were to `unpack' these data, removing their headers and reassembling the data. We might find that the data are structured at a higher level, namely the transport layer. The form of the data might be a sequence of messages, each of which has a header of its own containing a port number or RPC program number of the receiver application program, the length of the message and so on.

Notice the parallels between this and the system of segments and pages in the virtual memory concept of chapter 5. Each layer of abstraction we introduce requires a small overhead (the header) which gets added to the data so that the receiver can make sense of them. This is the essence of implementing a protocol in practice.

7.2 The internet protocol family

The set of protocols currently used on most networks is called the internet protocol family. This is divided into four layers which correspond roughly to a coarse OSI model.

4 Application layer user program 6,7
3 Host to host transport higher level data encapsulation 3,4,5
2 Internet layer lower level datagram transport 2
1 Physical layer Network 1


At the internet layer, we have the IP or internet protocol which includes a specification of addresses and basic units of data transmission. The official name for the lowest level data `packages' in the internet protocol is datagrams. Each datagram consists of a number of 32 bit words. The first six of these words consists of the IP header.

Figure 7.1: IP datagram format

The size of datagrams may be altered by the transport agents during the process of being sent. If a router transmits datagrams from one physical network to another, and the second network uses a smaller packet size, it will divide datagrams up into smaller datagrams called fragments. The above header is then reproduced in each fragment together with a `fragment offset' which determines the order in which the fragments should be reconstructed at their final destination. The packet size on different physical networks is part of the low-level protocol definition. This is chosen when the physical layer is designed, based on the efficiency and speed of the network. On a slow network, a small packet size would be used so that the multiuser sharing of network time is more equitable, i.e. a greater number of packets per unit time can be sent if the packets are smaller. On the other hand, if the packet size is too small, the overhead becomes a significant portion of the total packet size and the transfer is inefficient.

At the next level (the transport layer), there are two standard protocol types provided. These are called tcp for transmission control protocol and udp for user datagram protocol. They are sometimes called connection-oriented and connectionless protocols respectively, or reliable and unreliable. We shall explain these names below.

7.2.1 udp

The user datagram protocol is called unreliable because when an application chooses this protocol, the best it can do is to `throw its data out to the wind' and hope that it will arrive. When we use udp transport, there is no guarantee that data will arrive at the destination and no confirmation of receipt is sent by the receiver.

It is called connectionless because the messages are sent one by one, without any concept of there being an on-going connection between the sender and receiver. This is like sending a letter in the post.

Udp is the simpler of the two transport layer protocols, since it requires no handshaking by the system. It is useful for applications which either need or want to provide their own form of handshaking. For example, it would be natural to use the udp protocol for a `question-answer' type of client-server system. The client knows that its question arrived if it gets an answer from the server, so asking the network protocols to guarantee it would be a waste of time.

A single `message' of udp encapsulated datagrams is officially called a packet and is given a small header as shown in the figure below.

Figure 7.2: udp packet header

Notice that this header contains no ordering information - so the order in which the packets arrive at their destination is not guaranteed by the protocol itself. Only the integrity of the data are checked, using a checksum.

7.2.2 tcp

A single message of the transmission control protocol is called a segment. The tcp protocol is called reliable or connection-oriented because sufficient handshaking is provided to guarantee the arrival and the ordering of the segments at their destination. The ordering of each message implies a concept of two machines being continual contact with one another. This is like a telephone conversation: both parties are in contact all the time.

TCP connections are useful for sending data to servers, where no particular reply is required. For example, it would be used to send print jobs to a printer queue across a network. The sender receives no reply from the print spooler, and wants every single line of data to arrive in the correct order without having to worry.

Each tcp segment has a header as shown in the figure below.

Figure 7.3: TCP segment header

7.3 The physical layer

As an example of a physical layer, we can take a brief look at the ethernet. Ethernet is one form of cabling which is in common use. Other kinds of cable include fibre optics (FDDI) , 10BaseT or ISDN.

7.3.1 Network connectivity

To send messages from one computer to another, we have to connect computers together. One way of doing this would be connect every machine to every other machine in some bizarre `cat's cradle' of wiring. This would require$n$network connections per machine if there were$n$machines. It's pretty clear that this is not a good solution.

Another solution is to chain machines together (see figure below) or put them in a ring. This requires only two connections per machine.

Figure 7.4: Chains and rings.

The disadvantage with this scheme is that each machine has to send signals forward to the next one, until they arrive at the correct machine, which costs time and resources. FDDI fibre optic transmission works like this. It is called a token ring.

Modern ethernet uses neither method. Instead it uses a combination of two solutions. A basic ethernet network consists of a single cable or bus. Every machine listens into the same cable with one interface connector (see figure).

Figure 7.5: Ethernet

Since all machines share the same cable, only one machine can be talking at once. Each machine waits its turn to transmit data. Each host flashes its signals to all the hosts on the cable like sending Morse code with a torch. Every host sees every message but only the host with the destination address bothers to accept the message.

Ethernet comes in three flavours: thick ethernet, a fat yellow cable with black markings, thin ethernet a coaxial (usually black) cable a few millimetres thick and twisted pair ethernet. The latter comes out of an ISDN telephone connector, whereas the older types use coaxial and D-pin connectors. Twisted pair ethernet is usually structured in star formation. That is, at strategic places on a master cable (usually thick ethernet) a `hub' is attached. This is a device which converts one connection into many. From the hub there is one twisted pair wire to each machine. If there are many machines, we require many hubs, since the number of connections is limited to ten or so.

Figure 7.6: Star base networks

A similar arrangement can be achieved with thin ethernet using a multiport repeater, rather than a hub. A repeater is simply an amplifier, which is used over long stretches of cable. A multiport repeater combines amplification with dividing up a thin ethernet cable into$n$branches.

The twisted pair solution is the most modern of these solutions.

7.3.2 Ethernet addresses

An ethernet address is a number which is wired into every ethernet interface card. It is unique for every machine in the world. The first few hexadecimal digits of the ethernet address identify the manufacturer of the interface.

The ethernet address is the only piece of information a machine has before it is configured. It is only used by diskless machines and some x-terminals as part of an ARP/RARP ((Reverse) Address resolution protocol) request to get an IP address.

7.4 Internet Addresses and Routing

7.4.1 IP addresses, networks and domain names

The internet is, by now, a world-wide network. As of today, it is version 4 (IPV4) of the internet protocol which is in common use. Every host on the internet has to have a unique address so that network communication is unambiguous. This is given by a 4-byte word of the form


where xxxetc can be numbers from$0$to (Certain addresses are reserved). In addition to a numerical address, each host has a name. For example, the following are valid internet addresses and the names they correspond to.

The addressing scheme is based on a hierarchical splitting of networks, subnets and hosts. To arrive correctly at its destination an IP packet has to know exactly which network a host is connected to. This information is correctly coded into the numerical address, but is not contained directly in the textual name form.

In the textual form, each machine belongs to a logical domain which has a name. The address takes the form


Thus in the above examples, `anyon', `nexus' and `ftp' are host names and `', `' and `' are domain names.

The numerical form is strictly speaking a combination of a network address and a host address. The textual form is a combination of a hostname and a domain name.

There is a subtle difference between these. Given a numerical IP address, datagrams can find their way precisely to the correct network and machine. The textual information is not sufficient however because, while the hostname is unique, the remainder of the address (the domain name) is usually a generic name for a group of networks - and we don't know how to choose the right one.

A logical domain, like the above examples, can encompass any number of different networks. For example, the domain name `' encompasses all of the subnets under the address 129.240.*.*. The IP packets need to know which subnet the machine is on in order to get to their destination, because the text name only says that they should to to 120.240.*.host. The *is unknown.

To complete this information, we need a database which maps internet domain names to internet addresses. This mapping is performed by the Domain Name Service (DNS) or Berkeley Internet Name Domain (BIND) which we shall discuss below.

7.4.2 Netmask and broadcast address

Each address consists of two parts: a network address and a host address. A system variable called the netmask decides how IP addresses are interpreted locally.

The netmask decides the boundary between how many bits of the IP address will be kept for hosts and how many will be kept for the network location name. There is thus a trade off between the number of allowed domains and the number of hosts which can be coupled to each subnet. Subnets are usually separated by routers, so the question is how many machines do we want on one side of a router?

The netmask only has a meaning as a binary number. When you look at the netmask, you have to ask yourself - which bits are ones and which are zeroes? The bits which are ones decide which bits can be used to specify the domain and the subnets within the domain. The bits which are zeroes decide which are hostnames on each subnet. The local network administrator decides how the netmask is to be used.

Figure 7.7: The netmask sets the division between network address and host address in the 4-byte IP address.

The most common situation is that the first three numbers xxx.yyy.zzzrepresent the domain and the last number mmmrepresents the machine. In this case the netmask is, leaving the last byte for machine addresses. It is only possible to have 254 different machines in the domain with address xxx.yyy.zzzwith this netmask. If we wanted more, we would have to introduce a different domain name for the extra hosts!

If we wanted more machines on each subnet, we would have to change the netmask and the definitions of the domain address. By making the netmask, as in the figure above, we add an extra bit to the host part. Thus a total of

hosts could use the same domain name.

One address is always reserved by the internet protocol, namely the broadcast address. This is an address which is used like a wildcard - to refer to all machines in a given domain simultaneously. Another address is reserved as an address for the network itself. Usually xxx.yyy.zzz.0is the network address, and xxx.yyy.zzz.255is the broadcast address, but on older networks the address xxx.yyy.zzz.0was used for both of these.

7.4.3 Routers and gateways

A router is a device which connects two physically different segments of network. A router can be an ordinary workstation, or it can be a dedicated piece of machinery. If a router joins$n$different networks, it has$n$different network interfaces and forwards datagrams between them. The router must be able to understand internet addresses in order to do this - since it must know where packets want to go.

A gateway is another name for a router. Some authors distinguish between gateways which forward packets with different network protocols, and routers which just isolate different segments of network of the same type.

Roughly speaking, the network on the short end of a router is called a local area network (LAN) and the greater network on the long end is a wide area network (WAN), though these names are normally used as it suits.

7.5 Network Naming services

7.5.1 The Domain Name Service

Although the system of textual internet addresses is very convenient from a user point of view, it creates a problem. Users, on the one hand, would like to use names rather than numbers to talk about network hosts, but the name form is not sufficient in itself as an exact specification of a network and host addresses.

The solution to this problem is the domain name service or DNS. This is a service which takes a textual internet address of the form


and returns the numerical form of the IP address for that host. This is called resolving the name. Notice that no two machines in the same domain may have the same name, otherwise the DNS would not be able to resolve the IP address from the textual form.

The DNS also performs the reverse service, converting numbers into names and stores extra information about which hosts are mail-exchangers etc. The UNIX program nslookup can be used to browse in the Domain Name Service.

The domain name service is a daemon, called a nameserver, which runs on some chosen host (usually a UNIX machine, since the software was written for UNIX) and looks up names in a database. The host on which the nameserver runs is often called a nameserver too.

Each server covers only the list of hosts in its local domain, not those of other domains - but it has a knowledge of other nameservers which can answer queries in other domains. If a nameserver receives a request to resolve a name which is not in its own domain, it forwards the request to the official nameserver for that domain.

Nameservers update each other's information constantly about what official nameservers addresses are so that the data are always up to date. Each new network which is set up on the internet has to register its nameserver centrally so that this information is complete.

Every host on a network must know the name of its local nameserver in order to send requests for name resolution.

The DNS software which is in most widespread use is the Berkeley BIND software (Berkeley Internet Name Domains). Microsoft have their own implementation called WINS (Windows internet nameservice) as their own commercial solution but this will soon be abandoned in favour of DNS, since it lacks adequate functionality and security.

7.5.2 Network Information Service

The DNS is not the only database service which is in use on the internet. The Network Information Service (NIS), written by Sun Microsystems is another service which provides information on the network. NIS was formerly called the Yellow Pages, until Sun Microsystems were politely informed that Yellow Pages was a trademark of British Telecom. Many people still refer to NIS as YP.

NIS was designed for the UNIX operating system. It is nevertheless used by DOS and Macintosh machines which run software to communicate with UNIX servers on TCP/IP networks. The data it stores are commonly required configuration files for UNIX.

For example, the user registration database is contained in NIS, as is the list of all hosts on the local network. The hosts information actually reproduces the information which is stored in the DNS, but the information is not complete - since only host names are mapped to IP addresses - no domain names are included. A number of other databases are held in NIS, such as network-wide mail aliases and information about groups of users.

The advantage of NIS is that each user on a network can have the same login name and password on all of the machines which use the network information service - because they all read the same database. This NIS is simply a way of sharing the information which would otherwise have to be typed in separately to each machine.

Wheras each host must know the name of its nameserver, no host has to know the name of the local NIS server - that is because NIS uses the broadcast system. The software which connects clients to the server sends out a request to the broadcast address. The message is received by every host on the network that is listening. When a NIS server receives the messages, it replies to the sender with its IP address, so that the sender knows which host to query for NIS information. It will continue to use that address for a while (even though the server may crash in the mean time) and then it broadcasts its query again.

If no servers are available, a client may never get its information! Most networks have backup servers in case on should fail. That way if one doesn't answer, hopefully the other one will.

The advantage of the system is that a client always ends up asking the server which can answer quickest - and which, presumeably, has the least to do, so the load of answering the service is spread around.

7.6 Distributed Filesystems

Probably the first thing we are interested in doing with a network is making our files available to all hosts, so that - no matter where in our corporate empire we happen to be sitting - we always have access to our files.

The concept of a distributed filesystem is about sharing disks across a network. Many operating systems have

There are three main contenders for such a system in the UNIX world. Only one of these is in widespread use.

7.6.1 NFS - the network filesystem

NFS was historically the first distributed filesystem to be implemented, by Sun Microsystems. All manufacturers now support Sun's NFS.

NFS is based on Sun's own RPC system (Remote procedure call). The idea behind NFS is to imitate UNIX filesystem semantics as closely as possible from across a network. NFS works by implementing a number of servers which run on UNIX machines.

One problem with a network file system is what to do about machine crashes. Suppose we are in the middle of writing or retrieving a file and the server machine supplying the file crashes. We need some way of remembering where we were, so that when the machine comes back up, the operation can continue where it left off. In fact this is almost impossible to achieve in practice - NFS's solution works in many cases, but not in all.

In the UNIX filesystem, a user must obtain a lock on a file in order to read or write to it. In NFS, the same system applies. A lock is obtained from a lock server on the host where the real disk filesystem lies and the state of the filesystem is communicated by a state server. NFS is sometimes called a stateless protocol, but this is a misleading title. The state of the filesystem on the server is maintained on the server which owns the filesystem. If there is a crash, the server tries to reestablish the locks it held before the crash. If this is not possible because the filesystem has changed in the meantime or because of unfortunate timing, the result is a `stale NFS filehandle' - an unrecoverable error. The state information has to be cleared and restarted.

NFS is called stateless because the server does not record the requests which the client makes (except for locks). The server processes requests without caring about which client it is serving, or about what it has done in previous requests. It doesn't know how much of a file the client has read. In other words, it is the client's responsibility to keep track of what requests it sends to the server and whether or not it gets a reply.

NFS version 3 is now in use by some vendors and includes a number of improvements (and a few new bugs) over NFS. These include better caching, access control lists (ACLs) etc.

7.6.2 AFS - the andrew filesystem

Another filesystem which is increasingly discussed, is the Andrew file system. The CERN high energy physics (HEP) group use the AFS as a global filesystem and many other institutions are starting to follow suit. Whereas NFS tries to reproduce UNIX-like file semantics across a network, AFS is a different filesystem altogether. AFS solves the problem of user authentification between different sites. A problem in sharing files between different sites around the world is that usernames and passwords are local to each site. It is possible (though perhaps unlikely) that very different users around the world might have the same user ID and login name, and even the same password. Thus AFS has to take into account the username problem. AFS also has more advanced caching features to speed up file access and access control lists (ACLs). It is in many ways superior to NFS, but whereas NFS is free software, AFS is a commercial product maintained by Transarc and is therefore not in widespread use.

An improved version of AFS, called DFS has been incorporated into Digital's Distributed computing environment.

7.6.3 DCE - the distributed computing environment

The Digital Equipment Corporation's Distributed Computing Environment is, in fact, as complete substitute for Sun's NFS system from RPC up. Instead of using Sun's RPC software, DCE uses software originally developed for the Apollo Domain operating system, called NCS. DCE works on top of Domain sockets.

The open software foundation (OSF) has adopted DCE as its official network solution, though its on operating system NSF1 still supports NFS. One of the features of the DCE system is the concept of multiple backups of files. If one server fails, DCE allows another server to take over. This requires several servers to have disk-copies of the same files. This system is efficient on a read mostly filesystem. When a write is made to such a filesystem it must be made synchronously to$n$disks. Maintaining these copies requires complex algorithms and a time-consuming copying overhead. DFS/DCE is also now licensed by Transarc.

8. Security: design considerations

System security can mean several things. To have system security we need to protect the system from corruption and we need to protect the data on the system. There are many reasons why these need not be secure.

Malicious users may try to hack into the system to destroy it.
Power failure might bring the system down.
A badly designed system may allow a user to accidentally destroy important data.
A system may not be able to function any longer because one user fills up the entire disk with garbage.

Although discussions of security usually concentrate on the first of these possibilities, the latter two can be equally damaging to the system in practice. One can protect against power failure by using un-interruptable power supplies (UPS). These are units which detect quickly when the power falls below a certain threshhold and switch to a battery. Although the battery does not last forever - the UPS gives a system administrator a chance to halt the system by the proper route.

The problem of malicious users has been hightened in recent years by the growth of international networks. Anyone connected to a network can try to log on to almost any machine. If a machine is very insecure, they may succeed. In other words - we are not only looking at out local environment anymore, we must consider potential threats to system security to come from any source.

The final point can be controlled by enforcing quotas on how much disk each user is allowed to use.

8.1 Who is responsible?

System security lies with

The user.
The system administrator.
The system designer.

Many would prefer to write this list upside down - but we must be practical. Usually we are not in a position to ring to the system designer and say `Hey, that system modeule you wrote is not secure, fix it!'. The response would at any rate take some time. Rather, we have to learn to take the system as it comes (pending improvements in later releases) and make the best of it. All users of the system should be aware of security issues.

Ideally, if all users were friendly and thoughtful, everyone would think about the welfare of the system and try to behave in a system-friendly way. Unfortunately some users are not friendly, and accidents can happen even to friendly users.

8.2 Passwords and encryption

The first barrier to malicious users is the password. Every user on a multiuser system must have a password in order to log on. Passwords are stored in a coded or encrypted form so that other users cannot read them directly. Nevertheless, on very many systems, the coded passwords are readable to all users. Moreover, the algorithm which encrypts passwords is usable by all users. This means that anyone can try to crack the passwords by guessing.

8.2.1 UNIX passwords

In most UNIX systems, passwords and login information are stored in the file /etc/passwd. This files looks something like this:

sundiag:*:0:1:System Diagnostic:/usr/diag/sundiag:/usr/diag/sundiag/sundiag
sysdiag:*:0:1:Old System Diagnostic:/usr/diag/sysdiag:/usr/diag/sysdiag/sysdiag

The fields of the file are:

login name : password : user id: group id : full name : directory : shell

i.e. the encrypted password is readable as the second field. The UNIX standard library command crypt() converts a text string into this coded form.

When a user types in his or her password, the system does not try to decrypt the password, but rather encrypts the password and compares the coded forms. The reason for this is that there is no (publicly) known algorithm for decoding passwords encrypted using crypt(). Just to reverse the process would take hundreds of thousands of years of CPU time. crypt was designed to this way.

To encrypt a password, crypt takes the password string and a random number, known as a salt.

code_passwd = crypt (passwd_string,salt);

The salt ends up being the first two characters of the encrypted form of the password. (If we didn't know the salt, it would be impossible to compute the same encrypted form more than once!)

To try to guess passwords automatically, all we have to do is to send a whole list of guesses as passwd_string, take the first two characters of the encrypted password as the salt, and compare the result of the cryptfunction with the encrypted form from the password file.

Elaborate programs have been written to try to crack passwords in this way. Such programs are useful tools for the system administrator who should keep an eye on which users have poor passwords. It is better that the system administrator finds a bad password before a malicious user does.

On newer UNIX systems, passwords are stored in a shadow password file which is not /etc/passwdbut a different non-readable file. Since normal users cannot read this file, they can only try to log onto other users' accounts by trial and error. They cannot compare an encrypted list of their own to the password file.

8.2.2 Bad passwords

Surveys of user passwords show that very many users choose extremely simple passwords. Passwords should be a combination of large and small letters, numbers and special symbols like !@#$%^&*etc. Passwords should not be

  1. Your name or anyone else's name (your dog's name!)
  2. Names from books, place names or the name of your computer.
  3. Names of famous people like Einstein, Marx.
  4. Your phone number.
  5. Your birthday.
  6. Your car registration plate
  7. Any personal information which is easily obtained.
  8. Your login name!!
  9. Any word in an English or foreign dictionary.
  10. A keyboard sequence like qwerty.
  11. Any of the above spelled backwards.

Some enhanced systems take the view that users should not be able to choose an insecure password, and prevents them from doing so. Most commercial operating systems don't care whether users have no passwords at all.

8.3 Super-user, or system administrator

The super-user is a trusted user. The super-user has unlimited access to files on a system. He/she is the only user who can halt the system and is the only user who can make backups of system data.

Clearly such a user is required:

To maintain the system and deal with special circumstances which arise.
To create and destroy new and old users.
To make backups of the system.
To install software and system upgrades.

Often system administrators end up doing a lot more than this. What is important to understand is that the superuser has a highly responsible position which must not be compromised. The administrator's account must not be used authorized users. The password is of crucial importance. The designer of an operating system must be acutely aware of the need to preserve the security of privileged access.

Under the UNIX system, the superuser is called root.

8.3.1 Network administration

Networks make it possible to link computer systems in an unprecedented way. We can `mount' (see chapter 5) filesystems from one computer onto another computer across a network and log in to systems all around the world (if we have an account!). We must ask: what is the role of the superuser in a networked environment?

Consider the following. Suppose the administrator of one machine in Oslo gets permission from a system in California to access a filesystem on the Californian machine. When the Oslo administrator mounts the filesystem on his machine (without needing to give a password), he sees the files as though they were a part of his system. Now, since root has the rights to all files, it might seem natural that he would be able to read and modify the files of all users in California. But surely, this is wrong - the superuser of a machine in Oslo cannot be regarded as a trusted user for a system in California!

UNIX gets around this problem by mapping the user root(which has user id$0$and all rights) to the user nobody(which has user id$-1$and no rights) across a network. This means that the superuser has rights only on the local machine. To get rights on another machine, across a network, either special permission must be given by the remote machine - or the user must be able to log onto the machine by knowing the rootpassword.

As another example of network security - or lack of it - let us consider also the X-windows system. X is a windowing system which is designed to work transparently over a network. X works by connecting to a server, anywhere on the network. Normally the X-server only allows the machine on which it is running to access the display, but in a network situation it is not unusual to find users logged in on several different machines. Such a user wants all the windows to appear on his or her workstation, so the X server allows certain other named hosts to open windows on its display.

Before the introduction of the xauthority mechanism, all security was based on the xhost program. This was host based meaning that anyone using a named host could open windows on the server. Many users do not understand the X system (which is quite complex) and simply disable access control by calling xhost +. This allows any host in the world to connect to the user's server. In practice, this means that anyone in the world can view the picture on such a user's screen.

Many programs have not adopted the xauthority system which is user based, and so the xhost problem is still widespread,

8.3.2 Setuid programs in unix

The superuser rootis the only privileged user in UNIX. All other users have only restricted access to the system. Usually this is desirable, but sometimes it is a nuisance.

A set-uid program is a program which has its setuid-bit set. When such a program is executed by a user, it is run as though that user were the owner of the program. All of the commands in the program are executed by the owner and not by the user-id of the person who ran the program. If the owner of the setuid program id rootthen the commands in the program are run with root privileges!

Setuid programs are clearly a touchy security issue. When giving away one's rights to another user (especially those of root) one is tempting hackers. Setuid programs must be secure.

A setgid program is almost the same, but only the group id is set to that of the owner of the file. Often the effect is the same.

An example of a setuid program is the psprogram. pslists all of the processes running in the kernel. In order to do this it needs permission to access the private data structures in the kernel. By making pssetgid root, it allows ordinary users to be able to read as much as the writers of psthought fit, but no more.

Naturally, only the superuser can make a file setuid or setgid root.

Next, we have the problem of what to do with setuid programs which are read across the network. If we mount a filesystem across a network, we have no control over what goes into the file. Suppose then a stupid system administrator, angry at the world and dying for revenge, made a setuid root program which executed every command every user gave to it - then suddenly everybody who accessed this file over the network would have root access on their local machine!

Clearly careless setuid programs can be a security risk, so network-based filesystems give the option of disallowing setuid programs.

8.4 Backups

Accidents happen even to the most careful users. Users delete files without meaning to, power failure leads to disk corruption, software bugs can delete files, system administrators can make mistakes - and of course someone might actually steal your computer!

User data are the most important part of a computer system - anything else can be replaced. New disks can be bought, software can be loaded in afresh - but once user data are gone, they are gone. It is therefore important to backup user data regularly. From a network vantage point, it is useful to be able to take backups centrally. In BSD UNIX, this can be done using the rdump command.

Backing up data is expensive - both in terms of man-hours and in the cost of storage media. Some systems use secondary disks to keep backups of important data. The cheaper alternative is to use tape. Tape comes in many forms. the most common in use today are

Standard -inch tape cartidges.
EXABYTE 8mm (video tape!)
DAT (Digital audio tape)

Larger systems may also use half-inch tape. Tape machines are becoming more intelligent and often include compression software in their device drivers which packs more information into the same space on the tape.

An EXABYTE video tape with normal compression can hold up to 5GB of data. Newer drives support 10GB, but device drivers are not easy to come by.

Depending on how often users actually use the system, it is worth considering making backups

Every night. The most important data should be backed up at least as often as significant changes are made.
Every week. Less important data might only be worth backing up once a week.
Every month. For convenience you might want to record the setup of your system software once in a while - even though this can be loaded in again from source.

Backup software is usually intelligent enough to be able to extract only files which have been modified since the last backup, so daily backups need not copy every file every night.

How long should you keep a backup? It might take some time to discover that a file is gone. How long you keep backup tapes depends on how long you value your data. A year is not an unreasoable length of time.

8.5 Intruders: Worms and Viruses

Worms and viruses are intruder programs which enter a system illegally and take hold of the system in some way.

A virus is a piece of code which attaches itself to another program in order to get executed surreptitiously.

A worm is a program which propagates from computer to computer, without necessarily changing anything.

Other kinds of intruders are Trojan horses i.e. programs which masqeuerade as something they are not, bacteria, which simply copy themselves in order to overwhelm the system and logic bombs which go off when some condition is met. Multiuser systems are generally harder to affect with intruders than microcomputers since the operating system exercises a much greater level of control.

8.5.1 Back doors

Back doors or Trojan horses are faults in the system software, which devious programs can use to gain access to the system from outside. In most cases these are network based programs. Most intruders enter via a network, but on small computers which use floppy disks or diskettes, they can also enter on disk.

Some program have become well-known backdoors in the UNIX world. sendmail is one. Many backdoors are setuid root programs which contain bugs that can be exploited by clever users, so that these users can gain privileged access.

How do we know when an intruder is on the system? This is an extremely difficult problem - there are many ways to hide intruders, so that - if one is not specificaly thinking about the possibility of threats, it is easy to miss them.

On UNIX inspired systems, the command netstat shows a list of machines which are connected to the system. It gives a listing as follows:

Active Internet connections
Proto Recv-Q Send-Q  Local Address       Foreign Address        (state)
tcp        0      0  saga.1147           xantos-7.6000          ESTABLISHED
tcp        0      0  saga.1146           xantos-7.6000          ESTABLISHED
tcp        0      0  saga.1145           xantos-7.6000          ESTABLISHED
tcp        0      0  saga.1144           xantos-7.6000          ESTABLISHED
tcp        0      0  saga.1143           njal.6000              ESTABLISHED
tcp        0      0  saga.1141           njal.6000              ESTABLISHED
tcp        0      0  saga.1138           njal.6000              ESTABLISHED
tcp        0      0  saga.1132           xantos-2.6000          ESTABLISHED
tcp        0      0  saga.1130           xantos-2.6000          ESTABLISHED
tcp        0      0  saga.1125       FIN_WAIT_1
tcp        0      4  saga.1120       FIN_WAIT_1
tcp        0      0  saga.1022      ESTABLISHED
tcp        0      0  saga.1094           xantos-7.6000          ESTABLISHED
tcp        0      0  saga.1086           xantos-4.6000          ESTABLISHED
tcp        0      0  saga.1023      ESTABLISHED
tcp        0      0  saga.1080           xantos-4.6000          ESTABLISHED

This gives an indication of who is currently connected. Of course, intruders could connect when you are not watching, so another thing to do is to monitor all the connections made to your machine continuously and dump the result to a file. This requires a considerable amount of storage and some skill in interpreting the data. The program tcpdump will do this. Sun have their own version called etherfind.

On the other hand, we cannot live in a perpetual state of paranoia, thinking that everyone is out to get us. A balance must be struck by taking all reasonable precautions and being aware of the problem. Finally, the super-user should never install software which is of suspicious or unknown origin.

8.6 Firewall

One way of designing a network to protect it from attack is to use a machine as a ``firewall''. That is - a barrier to stop the spread of network threats. The idea is to isolate important machines by placing another highly secure machine between the outside world and the local network. The firewall is the only machine which is connected to a wide area network. It is also connected to the local area network, but it does not forward packets to the local network and vice versa. Thus sensitive data can be hidden behind the firewall, where they can be shared on the local network but not by the external network.

8.7 Public and Private Keys

A clever intruder could always behave as an imposter - arranging it so that it appeared that a network request came from a trusted machine, when in fact it came from the intruder's machine. Moreover, one could easily make a device which collected all the information which was sent over a network and analyzed it to find out what was being said - and to whom.

To try to prevent such problems from occurring, we can use a system of data encryption (coding). The idea is to encode all data using a special key. Both the sender and the receiver need to know the key - the encryption and decryption algorithms are publicly known.

The problem is then to agree on a key. This can be achieved using public and private keys.

Two parties wish to communicate with one another in private, so they encrypt the data they send over the network. Each host has a private key which is a large number which is encrypted with the user's password and stored in a database. Each user also has a public key, which anyone can look up in a database.

In order to exchange information, both the sender and the receiver need to have the correct key. The ingeneous part is that, both parties combine their private keys with the others' public keys and end up with a conversation key which they both agree on. To decode a message they only need the conversation key and their own private key. The coding algorithm is based on some inspired mathematics of modulo arithmetic.

Party A knows that party B is who he claims to be because

  1. The message sent to A was encrypted using the conversation key.
  2. The only way that B could generate the conversation key would be by knowing A's public key and B's private key.
  3. To know B's private key, B's password is needed.

Because the key encryption is quite time consuming and difficult, it is only used to establish an initial connection and conversation key. Once the conversation key is known, normal crypt()type encryption is used for passing data. This key encryption scheme is the basis of secure communication links like SSL (Secure socket layer) and PGP (Pretty Good Privacy).

Where next?

There are many topics which have only been covered superficially in this introduction. A deeper understanding of networks and system administration can be found in
... quasi-multitasking 1.1
At the present time Apple are preparing a new operating system called NextStep or Rhapsody or Mac OS Server X which is based on BSD 4.4 Unix running on a Mach micro-kernel and will run old MacIntosh software under emulation
... system 1.2
In Manitoba, Canada, the telephone system is also called MTS. The telephone system is probably more advanced than the original MTS, and certainly faster!

... OS 1.3
This is not really true of course - if they did not wait for the OS, then they would not be able to continue executing at all!
...heap 1.4
This is described more fully in the next chapter
... keyboard 1.5
Actually only one keyboard is needed because it is only possible to write one thing at a time, but it is possible to read two things at the same time.
... filesystem 4.1
Experts might say that this has purely to do with filesystem semantics (see chapter 5) but given the filesystem, this becomes a synchronization problem.
... model 6.1
PC users often think that a server has to be a special kind of computer. This true on old PC networks because they run an operating system which cannot run several processes. Only a single task is required to run a server, but since PC networks before NT could not multitask, a whole machine was required to do this job. On modern systems, any machine acn be a server and a client simultaneously.

... requests 6.2
Note: this is not true multi-threading since all of the processes are heavyweight processes.


  • Assembler : An assembler is a program which converts assembly language into machine code. Assembly language is a mnemonic (symbolic) form of the numerical machine code. Each instruction corresponds to one machine code instruction.
  • Bits : Binary-digits. 1's and 0's.
  • Buffer : Waiting area for data. A buffer is used to synchronize the arrival of data from a device with the eventual reading of the data by a program.
  • Clock cycles : The system clock is an inmportant part of the hardware of a computer. The clock works like a pump, driving the CPU to execute intstructions. On earlt microprocessors, each instruction tool several cycles of the system clock. Newer RISC processors can execute a whole instruction per clock cycle, and some can even perform several instructions per clock cycle, by ingeneous design of the hardware.
  • Compiler : A program which converts a high level language into machine code.
  • Concurrent. : This is distinct from parallel. Processes which have the appreance of being executed simultaneously, because the system can perform time-sharing, are called concurrent processes.
  • CPU : Central processor unit. This is the chip which adds numbers together and moves data around in the memory. Parallel computers have several CPUs.
  • Fragmentation : Data are said to be fragmented when parts of the data exist in very different locations, joined together by pointers. Fragmentation occurs because the OS must find a free space whereever it can. Ideally, data would alway be stored in contiguous blocks, but in practice files may be deleted, leaving holes in the data which must then be filled up by the OS.
  • Handshaking : A system of signals between two processes/computers which allows them to tell each other when to start and stop sending information.
  • Host : A machine, computer.
  • ID : Identifier. A name or number which refers to something - often a process or a user.
  • I/O : Input/output.
  • IPC : Inter-process communication. A mechanism by which unix processes can exchange data with one another. See also RPC.
  • kernel : The core of a multitasking operating system which deals with the basic system resources. The kernel drives physical devices and handles I/O, memory management and process scheduling.
  • Loopback : The loopback device is a pseudo network device in the UNIX operating system which, rather than sending data out onto a physical network, sends packets straight back into the system. The protocols for talking to the loopback device are the same as those for the physical network, so programs employing interprocess communication have only to hold to a single standard, regardless of whether the processes are on the same machine, or on different machines.
  • Machine code : The basic numerical language of codes which the CPU understands. Compilers and assemblers convert programs into machine code.
  • Multiplex : To switch between several activities or devices.
  • Multi-user system : An operating system where several users can use the system simultaneously.
  • OS : Operating system.
  • Parallel : Parallel processes are not merely timeshared (concurrent) but actually run simultaneously on different CPUs.
  • Pixel : A single dot on the screen. This is one `grain' - or the object of maximum resolution.
  • Primary memory : RAM internal memory (see secondary memory).
  • Primitive : A very low level function or routine. A basic element in a library of functions.
  • RISC : Reduced instruction set chip. This is part of a new philosophy to make microprocessors faster by giving them fewer (less complicated) instructions which are optimized to run very fast. Usually each instruction completes in a single clock cycle.
  • RPC : Remote Procedure Call. This is a mechanism for executing tasks on remote machines across the network. The RPC protocol makes use of the XDR (external data representation) protocol for passing data. It is a relatively high level interface between networked machines.
  • Secondary memory : Disk or tape storage.
  • Semantics : This term is used to describe the `method of operation' of a particular system. The prescribed way in which a particular system is supposed to behave. The logic of operation of the system.
  • Single-user system : A system in which only one user can use the system at a time.
  • Starvation : A process is said to starve if it never gets a share of the CPU. This can occur is there are errors or deadlocks in scheduling.
  • Transparently : This word is often used to mean that something happens without anyone needing to know the details about how it happens. For example, the OS handles timesharing transparently - i.e. without users needing to know about how it happens.
  • Vector : An array of data or a segment of memory.
  • Virtual : Almost, a likeness of, simulated.


Access control lists
Address binding
Address resolution
AmigaDOS , [*]
Andrew file system
Application layer
Asymmetric multiprocessing
Asynchronous I/O
Back door
Be Box
Black boxes
Block allocation
British Telecom
Broadcast address , [*]
BSD unix
Busy waiting
C-SCAN algorithm
C-shell resources
Client server model
Clock cycles
Command language
Connection oriented socket
Connectionless socket
Context switching , [*]
CPU burst
Critical sections
Cyclic redundancy check
Cylinder groups
Data links layer
Data types
Deadlock prevention
Defect list
Demand paging
Device driver
Device ID's
Direct memory access
Disk blocks
Disk scheduling
Disk surface
Distributed file system (DFS)
Distributed file systems
DNS , [*]
Domain name service , [*]
Encapsulation of data
Entry points to OS code
FCFS , [*] , [*]
FIFO , [*] , [*] , [*]
File locks
File permissions
File types
Filename extensions
Filesystem , [*]
First come first served
First in first out
Floppy disks
Formatting , [*]
Fragments (network)
Frame table
Hewlett Packard
Hierarchical filesystems
High level
I/O burst
I/O sharing
Index register
Internet protocol
Interprocess communication
Interrupt vector , [*]
Interrupts , [*] , [*]
IP address
Last in first out
Lazy evaluation
Least recently used algorithm
Lightweight process
Lightweight processes
Logical device
Logical memory
LOOK algorithm
Low level
MacIntosh , [*]
Memory management unit
Memory map
Memory mapped I/O
MMU , [*]
Monitor mode
Multi tasking OS
Multi user mode
Multiple processors
Multitasking system
Mutual exclusion
Name server
Network filesystem
Network information service
Network layer
Object orientation
Operating system
Page fault
Page table
Paging algorithms
Paging memory banks
Paging to disk
Permissions and access on files
Physical layer
Physical memory
Port number
POSIX threads
Preemptive multitasking
Presentation layer
Primary memory
Private key
Privileged user
Process , [*]
Process control block
Process creation
Process hierarchy
Process states
Protocol , [*]
Protocols , [*]
Public key
Queue scheduling
Relative addressing
Reliable protocol
Resolving addresses
Response time
Round robin scheduling
Router , [*]
RPC , [*] , [*] , [*]
SCAN algorithm
Scheduing criterea
Scheduling , [*]
Second chance algorithm
Secondary memory
Secondary storage
Segmentation , [*]
Segmentation fault
Serialization , [*]
Session layer
Setuid programs
Shared data
Shared libraries
Shortest job first
Shortest seek time first algorithm
Single task system
Single user mode
Spin lock
Stack frame
Stack overflow
Stack pointer
Status register
Sun Microsystems , [*]
Super user
Supervisor mode
Symmetric multiprocessing
Synchronous I/O
System 5/System V
System calls
System overhead
Telnet , [*]
Tetris algorithm
Thread levels
Time sharing
Transport layer
Trojan horse
Two mode operation
UFS filesystem
Unreliable protocol
User ID
User mode
Virtual memory
Well known ports
Window system
Windows , [*]
Word size
X windows

About this document ...

A short introduction to operating systems

This document was generated using thetranslator Version 99.2beta6 (1.42)

Copyright © 1993, 1994, 1995, 1996,, Computer Based Learning Unit, University of Leeds.
Copyright © 1997, 1998, 1999,, Mathematics Department, Macquarie University, Sydney.

The command line arguments were:
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The translation was initiated by Mark Burgess on 2001-10-03

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