Operating Systems

1. Operating Systems: Intro Operating Systems • Concepts and Principles • monolithic and micro kernels • processes and threads • their management and synchronisation • interprocess communication • interrupts and signals • virtual memory - paging and segmentation • Implementation Techniques • resource allocation • time management - process scheduling • memory management - usage models and page allocation • file systems • case studies - Kops, Linux, NT etc.

2. Operating Systems: Intro Coursework • To Be Announced • probably involving some programming • deadline - mid-term • Essay - in-depth comparison of PDA Operating Systems • structure, scheduling, memory management, security etc. • deadline - end of term Tutorials only when needed

3. Operating Systems: Intro Textbooks • William Stallings, Operating Systems, Internals & Design Principles, 4th edition, Prentice-Hall, 2001. • Abraham Silberschatz & Peter Galvin, Operating System Concepts, 5th edition, Addison-Wesley, 1998. • Gary Nutt, Operating Systems, A Modern Perspective, 2nd edition, Addison-Wesley, 2000. • D.A.Solomon & M.E.Russinovitch, Inside Windows 2000, 3rd edition, MicroSoft Press, 2000. • D.Boling, Programming MicroSoft Windows CE, MicroSoft Press, 1998. • Michael Beck et al., Linux Kernel Internals, 2nd edition, Addison-Wesley, 1997. • John O’Gorman, Operating Systems with Linux, Palgrave, 2001.

4. Operating Systems: Intro Motivation • An Automated Teller Machine (ATM) process • process to deposit an amount into an account: • deposit (account, amount) { • read ( account, balance ); // read balance from database • balance = balance + amount; // add deposit amount • write (account, balance ); // update database • } • process to withdraw an amount from an account: • withdraw ( account, amount ) { • read (account, balance); // read balance from database • balance = balance - amount; // subtract withdrawal amount • write ( account, balance); // update database • } • concurrent processes?

5. Operating Systems: Intro • To sum the elements of a matrix • in row order: sum = 0; for (row=0; row<row_max; row++) { for (col=0; col<col_max; col++) { sum = sum + array[row,col]; } } cout << “Array Sum =“ << sum << endl; • in column order: sum = 0; for (col=0; col<cp;_max; col++) { for (row=0; row<row_max; row++) { sum = sum + array[row,col]; } } cout << “Array Sum =“ << sum << endl; • any difference?

6. Operating Systems: Intro Operating Systems • Main purpose is to facilitate the execution of user application programs • bare hardware is extremely messy and difficult for users to program • processors • memory • peripheral devices • concurrency • interrupts • files • networks

7. Operating Systems: Intro • Modern operating systems structured around concept of a process process = “program in execution” • a process is not the same as a program • programs are passive, processes are active • a process consists of an executable program, associated data and its execution context • A process runs in a framework which provides a Virtual Machine for it • a Virtual Machine is a simplified machine with: • user-level processor • virtual memory • high-level facilities • a machine with one user

8. Operating Systems: Intro • An OS supports execution of many concurrent processes • many co-existing virtual machines • each run alternately - pseudo-concurrently • OS issues revolve around process management • how and when to create & destroy processes • how to avoid interference between processes • how to achieve cooperation between processes

9. Operating Systems: Intro • An OS manages resource requirements of processes • time • memory • files • I/O device access • processors • An OS aims to be efficient • for user • for system manager

10. Operating Systems: Intro • A process can be in one of several states • newly created • running • blocked • waiting for some event to occur • in main memory • moved out to disc • ready to run • terminated

11. Operating Systems: Intro • Overheads in swapping execution between processes • saving and restoring contexts • loss of cache contents • Places limits on how often execution should be swapped • performance will plummet if too frequent

12. Operating Systems: Intro • A relatively new mechanism to improve overheads is the Thread • a lightweight process • several threads within one process or virtual machine • much smaller context to preserve • must cooperate • must not compete • can be separately scheduled • can run concurrently on multiprocessor systems • often also a very convenient programming paradigm

13. Operating Systems: Intro Interfaces user user user user user application application application application application process process process process process Operating System Hardware

14. Operating Systems: Intro • Communications between OS and processes • from process to OS - system calls • from OS to process - signals

15. Operating Systems: Intro • Communications between OS and Hardware • from OS to hardware - register & status settings • from hardware to OS - interrupts and exceptions

16. Operating Systems: Intro • Communications between processes • signals • message passing • via buffers • shared virtual memory • pipes • sockets • Communications between users and processes • keyboard, mouse, touch-screen • bit-mapped text and graphics display screens with windows • printers, plotters

17. Operating Systems: Intro Interrupts • An interruption in the normal execution flow of a processor • a mechanism for causing the processor to suspend its current computation and take up some new task • old context must be preserved • control can be returned to the original task at some later time • new context started • Reasons: • control of asynchronous I/O devices • exceptional conditions arising from execution

18. Operating Systems: Intro • OS sets up an Interrupt Vector with service routine entry points • one entry per I/O device or I/O channel • a dormant context set up for each routine which can be activated on demand • further interrupts usually switched off during initial interrupt servicing • Example: Intel x86 interrupt vector entries: 0 : divide error . . . 14: page fault 32: timer 33: keyboard . . . 36: serial port 1 37: parallel port 2 38: floppy controller . . . 46: hard disc

19. Operating Systems: Intro • An OS can be viewed as an event-driven system • just reacts to events as they occur • Interrupts need to be serviced carefully and quickly by the OS • cost similar to a process switch • too many interrupts will kill performance • Alternative to interrupts is Polling • process (or OS) continually polls I.e. inspects, device status registers awaiting some condition e.g. transfer completed • wastes time looping until condition occurs • much simpler to program than using interrupts but inefficient

20. Operating Systems: Intro Privilege • Processors run at various levels of privilege • user-level • only the types of instruction needed by applications programs • only access to permitted areas of virtual memory • supervisor or kernel level • all types of instruction including I/O instructions • access to system registers • virtual memory registers • interrupt vectors • access to all areas of virtual memory • ability to switch interrupts on and off • may be intermediate levels on some architectures • each level may have its own processor registers to speed context switching

21. Operating Systems: Intro What’s part of an Operating System? • Process management • Interrupt handling • Device drivers • File system • Networking • Applications? • Web browser? • Email? • Windows? • Command interpreters?

22. Operating Systems: Intro Kinds of Operating System • For single-user workstation • For multiple-user server • file,compute, mail, web servers etc. • For mainframe systems • transaction processing • database systems • For real-time systems • time-critical applications • industrial process control • hard rather than soft deadlines

23. Operating Systems: Intro • All built with the same concurrent multi-process organisation • Main difference is in process scheduling • single user system needs only to meet one user’s expectations • overall efficiency less important • multiple user system needs to be equitable between users • efficiency important • transaction processing system needs to give good response • database queries • real-time system may need to pre-allocate processor time to guarantee meeting deadlines • overall efficiency may need to suffer

24. Operating Systems: Intro • Only exception to multiple-process organisation is embedded systems • dedicated single-purpose processors • multi-media, telecoms • MPEG decoders, GSM phones, portable web browsers • washing machine controllers! • avionics • FADECs, GPS, FMS • absolute reliability required • very conservatively programmed • System Level Integrated circuits • usually have a processing core which requires a real-time OS • often multiprocessors with a general-purpose CPU plus a DSP processor • ARM + OAK

25. Operating Systems: Intro Operating System Structure • How to partition OS functions? • process management and scheduling, memory management, interrupt handling, device drivers etc. • Monolithic OS • each function coded as a separate procedure • linked into one executable code object • event-driven core which calls appropriate procedures when required • driven by interrupts and system calls from processes • Linux • modules can be dynamically linked and unlinked as required

26. Operating Systems: Intro • Micro-kernel OS • each function coded as a separate process • system processes • only the minimum possible function in the core kernel • virtual memory organising • interrupt handling • process dispatching • appropriate system process activated as soon as possible to deal with all other functions • system processes will have higher privilege and usually higher priority than user processes • Windows NT • Intermediate flavours • some system processes but not a minimal micro-kernel

27. Operating Systems: Intro Windows NT Structure Hardware

28. Operating Systems: Intro • Advantages and disadvantages • monolithic: • faster switching to kernel functions • simple interfaces between functions i.e. procedure calls • easier access to data structures shared between functions • larger memory resident core • may become too large to maintain easily • micro-kernel • better partitioning - should be easier to implement and maintain • smaller memory resident core • slower switching to system processes • inter-system-process communications may be slower