Chapter 1: Introduction

1. Chapter 1: Introduction • What is the aim of the subject? • Why are OSes important? • What is an OS? • A bit of history • Some basic OS concepts • How does an OS work? • OS Structures Ceng 334 - Operating Systems

2. The Aim of the Subject WILL NOT TEACH YOU HOW TO USE AN OPERATING SYSTEM. • It will examine • the way in which an OS works • the algorithms and data structures inside an OS • the problems, solutions and trade offs in designing an OS TO ACHIEVE AN UNDERSTANDING OF HOW AN OPERATING SYSTEM WORKS. Ceng 334 - Operating Systems

3. Computer System Components • Application Software : Bank automation system, airline reservations, payroll etc. • System Software : OS, data base, compilers, editors etc. Ceng 334 - Operating Systems

4. What is SYSTEM SOFTWARE? • System software provides the environment and the tools to create the application software (sort of virtual machine) • It is also the interface between the hardware and the applications Ceng 334 - Operating Systems

5. Why is the OS Important? • The operating system is the foundation upon which all computing work is performed. • Knowledge of the internals of an OS is essential to achieve efficiency in • building software applications • deciding upon a computing platform Ceng 334 - Operating Systems

6. What is an Operating System? • A big, complex program (sometimes many) • It has two main purposes in life • An interface between the user and the hardware (provides a virtual machine) • Provide efficient, safe management of computing resources Ceng 334 - Operating Systems

7. Life without an OS • Every programmer would • have to know the hardware • be able to access the hardware • Every program • would contain code to do the same thing • probably do something wrong Ceng 334 - Operating Systems

8. Where does the OS Fit? System Calls Users and User Programs Operating System CPU & Memory Hardware I/O Devices Ceng 334 - Operating Systems

9. History :First Generation (1945-1955) • Vacuum tubes • No operating system • Programming is done by wiring a plug board • Applications are mostly numerical calculations (trajectory computations, computation of tables such as sine, cosine etc.) Ceng 334 - Operating Systems

10. History:Second Generation (1955-1965) • Transistors • Commercially produced computers • Very expensive and very slow computers compared with your old PC at home • Batch operation (collect jobs, run in one go, print all outputs) Ceng 334 - Operating Systems

11. Spooling (Simultaneous Peripheral Operation On-line) • off-line spooling • on-line spooling • Off-line spooling : replace slow I/O devices with I/O dedicated computers so that the main system sees these machines as its I/O devices Ceng 334 - Operating Systems

12. Early Batch Systems • bring cards to 1401 • read cards to tape • put tape on 7094 which does computing • put tape on 1401 which prints output Ceng 334 - Operating Systems

13. A Deck of Cards (Program) Ceng 334 - Operating Systems

14. Applications are mostly scientific and engineering calculations (eg., solution of partial differential equations) • High level languages such as FORTRAN and COBOL Ceng 334 - Operating Systems

15. History:Third Generation (1965-1980) • Integrated circuits (small scale) packed as chips • I/O processors (channels) which can work in parallel with CPU - Multiprogramming Ceng 334 - Operating Systems

16. Multiprogramming system - three jobs in memory Ceng 334 - Operating Systems

17. On-line spooling (using channels) • Time-sharing (TTY terminals and VDU’s) • Multics OS - original UNIX Minicomputers - Cheaper than mainframes but with limited hardware (eg. DEC PDPx) Ceng 334 - Operating Systems

18. History:Fourth Generation (1980-1990) • Large scale integration • Personal computers • CP/M, MS DOS, Unix operating systems • Networks Ceng 334 - Operating Systems

19. Now! • Client/Server computation • Clients : PCs, workstations running under Windows and UNIX operating systems • Servers : systems that run under UNIX and Windows NT • Internet and intranet networking (WWW) Ceng 334 - Operating Systems

20. Important Points • OS provides • a simpler, more powerful interface • higher level services • OS services only accessed via system calls • Users and programs can’t directly access the hardware Set of System Calls (APIs) is what programs think the operating system is. Ceng 334 - Operating Systems

21. Some OS Concepts • Kernel • The main OS program. Contains code for most services. Always in primary memory • Device Drivers • Programs that provide a simple, consistent interface to I/O devices • Typically part of the kernel Ceng 334 - Operating Systems

22. Some OS Concepts • Program • A static file of machine code on a disk • Process • A program in execution. • The collection of OS data structures and resources owned by a program while it is running. Ceng 334 - Operating Systems

23. Producing an Executable Source Code Object File Executable Compile Link Libraries and other Object files Ceng 334 - Operating Systems

24. A Simple Program to print a directory • #include <sys/types.h> • #include <dirent.h> • #include "ourhdr.h" • int main(int argc, char *argv[]) • { • DIR *dp; • struct dirent *dirp; • if (argc != 2) • err_quit("a single argument (the directory name) is required"); • if ( (dp = opendir(argv[1])) == NULL) • err_sys("can't open %s", argv[1]); • while ( (dirp = readdir(dp)) != NULL) • printf("%s\n", dirp->d_name); • closedir(dp); • exit(0); • } Functions supplied by system libraries. These functions will contain a trap instruction. Ceng 334 - Operating Systems

25. User Program #2 User Mode RAM 1. Program performs trap 2. OS determines service number 3. Service is located and executed. 4. Control returns to user program. User Program #1 trap 002 4 1 3 Based on a diagram from “Modern Operating Systems” by Andrew Tanenbaum. 2 Kernel System/Kernel Mode Ceng 334 - Operating Systems

26. Steps in Making a System Call There are 11 steps in making the system call : read (fd, buffer, nbytes) Ceng 334 - Operating Systems

27. Some System Calls For Process Management Ceng 334 - Operating Systems

28. Some System Calls For File Management Ceng 334 - Operating Systems

29. Some System Calls For Directory Management Ceng 334 - Operating Systems

30. Some System Calls For Miscellaneous Tasks Ceng 334 - Operating Systems

31. A System Call Example • A stripped down shell: while (TRUE) { /* repeat forever */ type_prompt( ); /* display prompt */ read_command (command, parameters) /* input from terminal */ if (fork() != 0) { /* fork off child process */ /* Parent code */ waitpid( -1, &status, 0); /* wait for child to exit */ } else { /* Child code */ execve (command, parameters, 0); /* execute command */ } } Ceng 334 - Operating Systems

32. OS Structures • Monolithic systems • Hierarchy of layers • Virtual machines • Micro-kernel (client/server) model Ceng 334 - Operating Systems

33. SVC Program Interrupt Handler Service routine Monolithic System • OS has no structure but is a collection of procedures with well defined calling interfaces • Program - OS interface is via supervisor calls (SVC) Ceng 334 - Operating Systems

34. Simple structuring model for a monolithic system Ceng 334 - Operating Systems

35. Monolithic System (Cont.) • OS code is a binded object program and its source code may be logically divided into • OS main program • System call service routines • Utility procedures which help service routines Ceng 334 - Operating Systems

36. Layered System • Structure of “THE” OS (a batch OS) Ceng 334 - Operating Systems

37. Layered System (Cont.) 0. Process switching, multi programming, CPU scheduling 1. Memory and swap space (disk) management (“segment controller”) 2. Message interpretation, job control (JCL) functions 3. I/O management (virtual peripherals) 4. User programs 5. Operator Ceng 334 - Operating Systems

38. Layered System (Cont.) • Synchronisation between layers : Hardware and software (semaphores) interrupts • Each layer provides some sort of a “virtual machine” Ceng 334 - Operating Systems

39. User Programs OS1 OS2 OS3 OS4 Virtual Machine PhysicalHardware Virtual Machines • VM provides “n” duplicates of physical hardware using software. So different Oses can work in the same machine at the same time Ceng 334 - Operating Systems

40. What is wrong so far? • OS is one large program that provides all the required services. • Anytime you add a new device you must • get a device driver for the device • recompile the kernel with the new device driver • reboot the machine so the new kernel will be used Ceng 334 - Operating Systems

41. Micro-Kernel (Client/Server) Model • OS is a minimal core known as Kernel. Ceng 334 - Operating Systems

42. The kernel contains the minimum of function • memory management • basic CPU management • inter-process communication (messages) • I/O support • Other functionality provided by user level processes Ceng 334 - Operating Systems

43. Characteristics of Kernel • Makes use of message passing • Easy to replace server processes • Easier to write and port OS • Design is perfect for distributed systems • Less performance Ceng 334 - Operating Systems