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Introduction
• Teaching by lectures, laboratories and tutorials. – the material is mainly theoretical
– the analysis is mainly informal
– reinforced by practical labs under LINUX
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Module Assessment
• The module will be assessed by– written exam: 2 hours (75%)
– coursework (25%)• programming assignment
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Recommended Texts
• Recommended module texts– 1. A. Tanenbaum “Modern Operating Systems”
Prentice-Hall, 2001, ISBN 0-13-031358-0
– 2. G. Nutt “Operating Systems- A modern Perspective”, Addition-Wesley, 1997, ISBN 0-8053-12951.
– 3. C. Ritchie, “Operating Systems incorporating UNIX and Windows” 3rd edition, Letts Education Publication, 1997, ISBN 1-85805-3021.
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Syllabus
Introduction tooperating system
Processes andProcesses
Scheduling
Inter processesCommunication
File Concept andFile
ImplementationMemory
Management
VirtualMemory
What is an Operating System?
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Operating Systems
• In the early days of computing (1950’s and 60’s) the operator had to know how the system worked– the users programmed the machine code via paper tape
– the computers performed one program at a time
– all resources were available to the program• any resource not in use was idle (i.e. wasted)
– e.g. the CPU was idle while the tape was being read
– system was idle while the user programmed a new task
• The process is wasteful, inefficient, and impossible to manage as computers got bigger and better
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user
operatingenvironment
application programs
operating system
hardware
What is an Operating System?
• An operating system is a suite of programs that relieves the user from the tedious chores of having to deal with the hardware directly
• An operating system has three main functions– to manage hardware– to support applications– to establish a user interface
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Two Views of an Operating System
• Operating systems can be thought of in two ways– virtual machine viewpoint
• the operating system deals with the hardware devices directly and provides the user with a set of device independent, simple and logical set of services
– the resulting ‘virtual machine’ is easier to program
– resource manager viewpoint• a computer consists of many resources such as processor(s),
memory, disks, screens, printers and interface cards, etc
• the operating system shares these resources (fairly) between different users or different programs that are running
– imagine two programs trying to print at the same time without an operating system making sure they don’t interfere with each other!
A Brief Historyof Operating Systems
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The First Generation
• Early computers were specially built machines that filled entire rooms– dedicated users would sign up for blocks of time
• They were programmed one step at a time by switches, paper tape or, later, punched cards– tape/card had to be loaded into the computer, run and
when finished the operator could inspect the output
• As more complex hardware, such as printers, came along they had to be programmed each time– software libraries of useful functions were developed– these could be loaded from tapes or cards if required
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The Second Generation
• The introduction of the transistor in mid-1950’s meant commercial computer systems to do work– big corporations, government agencies or universities
• To run a job (one or more programs)– program written out on cards, cards submitted,
program read in, data read in, job run, collect output– special employees who were practiced operators
• High level languages such as COBOL and FORTRAN were developed to make writing programs easier– the compiler must be loaded from tape prior to job– jobs batched up (submitted in groups) to save time
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The Third Generation
• With automated job control it was realised that much CPU time was wasted ‘waiting’ for peripherals (input / output devices)– one solution was to allow several programs to be read
into main memory and to switch jobs automatically• multiprogramming
– another solution was to read input or write output onto hard disk (which was quicker) until the input / output device was free, so that the job could continue
• spooling: simultaneous peripheral operation on-line
• These required more complex operating systems
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The Fourth Generation
• As technology improved (more and more rapidly) and prices tumbled the idea of personal computers became more popular– each user has her/his complete computer system with
input / output devices• keyboard, mouse, display screen, printer
• With plentiful resources and only one user, do we still need operating systems?– Yes!
• increased demand for ‘user-friendliness’: i.e. abstraction
• demand for flexibility: i.e. job control, virtual memory
Operating System Concepts
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What is a Process?
• A process is an instance of a program that is being executed– a program which is loaded into memory and is running
• A process is an active entity– a program is passive: instructions that may be run
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A Process Tree• Processes may create other processes
– the creator is referred to as the parent process– the created is referred to as the child process
A
D E F
B C
• Process A created two child processes: B and C– process B created three child processes: D, E and F
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Files
• A file is a collection of related information– all modern operating systems provide services to allow
flexible files of variable size– allows data to be stored together, while hiding the
details of the actual hardware device from the user
• All operating systems now support the concept of the directory or folder– a file of special significance to the operating system
• holds the names of other files, to group them together
• Directories can be placed in other directories to form a tree structure (which may be very ‘deep’)
Operating System Structures
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System Calls
• System calls provide the interface between a process and the operating system– the virtual machine layer visible to the user– the control mechanism that connects a user process
(user mode) to the operating system (privileged mode)
• They typically provide services such as– process control: create, terminate, wait
– file manipulation: create, delete, open, close, read, write
– device manipulation: allocate, deallocate, read, write
– information maintenance: set/get date/time, user info
– communications: create/delete connection, send, receive
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The Unstructured Approach• The ‘big mess’
– the structure is that there is no structure!– MS-DOS is a ‘good’ example of this approach
• application programs (users) have direct access to devices
device drivers
MS-DOS device drivers
resident system program
application program
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Layers
• A structured approach is found in the layer model– operating system comprises a number of layers, each
of which can only invoke operations on lower layers
• A typical (abstract) layer structure may be
layer 0: hardware
layer 1: CPU scheduling
layer 2: memory management
layer 3: device drivers
layer 4: buffering for input / output devices
layer 5: user programs
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Summary
• Module Overview
– Syllabus
– assessment outline
• What is an Operating System?
– purpose and functions
• A Brief History
– five generations of computing
