Phones OFF Please Operating System Introduction Parminder Singh Kang Home:

1. Computer system consists of : Hardware (monitor, keyboard, case, motherboard, disk drives, etc ) System Software (operating systems, compilers, etc) Application Software (database, client, web browser) Translators (enable programs in languages humans can understand to be executed by the computer) Debuggers (assist in debugging programs ) IDE (a combined editor, compiler and run time environment ) and Profilers.

2 What is an Operating System? It is an interface between;  User and hardware, Application software and hardware.  Hides hardware complexity. It manages resources ;  Processors (CPU ’ s) - applications are allocated execution time on a CPU(s)  Memory - allocates real or virtual memory to an application as required  Peripherals - allocation of disk space, print spooling, etc. Coordinator of operations  Optimising utilisation, i.e. sharing CPU time and disk I/O between applications.  Synchronising activities, i.e. between applications and I/O

 Controlling access, i.e. ensuring applications and users only access resources they have rights too  Accounting for usage, i.e. to bill users for CPU usage and disk space used. Operating System Computer Hardware Compiler Assembler …..….………….. other applications System and Application Programs User-1 User-2 User-n ……...

3 Desirable features of an O/S features can be defined by using two main areas; by User view and System View. user view; OS is designed mostly to ease of use with some attention paid to performance but none paid to resource utilisation. System View; OS acts as a control program. A control program that manages execution of user programs and prevents errors and improper use of computer. Also, Features of operating system also vary according to system and application.

Main features of OS are: Efficiency  speed of execution – run applications as fast as possible  throughput – maximizes number of applications put unit time  resource utilisation e.g. share CPU and I/O between CPU hungry applications and I/O bound applications Compactness  memory – O/S uses as little memory as possible.  external storage – O/S does not fill the disks. Quality  reliability – little or no down time.  Maintainability – easy to fix problems.

4 Structure of an Operating System Most O/S are layered - reduces complexity, easier to change machine dependent bits User Level(User mode – unprivileged) =========================================== Kernel Level (Protected/privileged mode) Hardware Independent Hardware Dependent

For Example: Unix Unix Divides Operating System into three main Components:  The Kernel  The Shell  Directory Tree Kernel is Core Component of UNIX OS. It is the master program that manages all physical resources, including;  File System  Device Management  Process Management  Memory Management Shell is an interface between user and kernel. The primary role of shell is to accept the commands, interprets these commands and then executes them.

Kernel Shell Hardware; CPU, I/O, Memory and Storage Devices

5 Operating systems and Unix In the past (until the early 1990 ’ s)  each manufacturer had own O/S, e.g. IBM, DEC, PRIME, CDC, etc.  some manufacturers had several O/S, e.g. IBM  generally written in assembly language, i.e. machine dependent!!  most were very poor & difficult to use! In late 1960 ’ s programmers at Bell labs produced UNIX O/S  target: an interactive timesharing system for professional programmers  specified the low-level language C as a replacement for assembly language  O/S was written mainly in C  minimum (e.g. hardware dependent bits) in assembly language  Given away free to Universities, etc. – this lead to widespread use!  Ported to many different hardware configurations

Why Unix? Tree structured hierarchal file system Consistent structure, i.e. all I/O devices looked the same Users files are just a sequence of bytes – O/S imposed no structure on them Proper multi-tasking virtual memory O/S Networking built in at early stage, e.g. ftp, , etc. Simple system interface Powerful Command Line Interpreter (shell) – unprivileged not part of O/S Unix is well documented Free versions (with source) Linux Lots of free software for Unix Most O/S copy Unix ’ s ideas

6 Unix structure | App1 | App2 | | Application programs | | | Compiler | Editor | Shell |.... | System Programs | | | Standard Libraries : Open,.... | User-O/S interface |=================================| privileged Kernel | File System | Memory Mgt |.....| Machine Independent O/S | | | | | Process Cntrl | H/W Cntrl |.....| Machine Dependent O/S | |=================================| | Hardware |

7 O/S Process Management Most O/S are multi-tasking/multi-processing  Can run many processes (applications, system tasks) concurrently  CPU execution time shared among the processes Gives users virtual processors  Applications think they have their own CPU and memory Processes given a priority; Depending upon resuouce usage? O/S schedules processes  OS schedules processes to run on available CPUs using some scheduling algorithm, e.g. priority, round-robin, etc  O/S maintains status of every process  Running, suspended, waiting for I/O, etc

8 O/S File System Management Hides details of the physical organisation  the physical disk structure (tracks, cylinders, sectors)  users unaware of where files are stored,  e.g. local disks or remote network server Provides users with a logical interface  e.g. named files stored in a tree structure of directories (folders) Gives file system a uniform structure  e.g. no matter how many cylinders or what sector size the file system looks the same on all disks Elements referred to by name, rather than physical position on disk All I/O made to look the same data stored on disks, cameras, remote servers, etc.

9 O/S Memory Management O/S decides where programs stored in memory  applications can not access physical memory directly With Virtual Memory programs have illusion of (almost) infinite memory  program address space is broken in pages stored on disk  O/S moves pages to/from disk and memory as required  a memory management unit (under control of O/S) maps memory references by a program to physical memory  programs cannot access physical memory outside their own address space,  e.g. operating system memory or memory allocated to other processes

10 O/S User Interface Provides the mechanism for human interaction  i.e. sits in between the user and hardware resources The Unix shell (command line processor) is unprivileged.  i.e. not part of the UNIX O/S  several shells are available, e.g. bash, csh  the shell is an interpreted programming language GUI ’ s (windows based Graphical User Interfaces) are easier for novice  but less powerful than the shell  programs access O/S using system calls