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Operating Systems - Introduction

Operating Systems - Introduction. S H Srinivasan shs@cs.ucsd.edu. What is an OS?. Collection of software that makes the computer usable processes sharing useful to to many applications abstraction Between the application and the hardware. Layers.

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Operating Systems - Introduction

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  1. Operating Systems - Introduction S H Srinivasan shs@cs.ucsd.edu

  2. What is an OS? • Collection of software • that makes the computer usable • processes • sharing • useful to to many applications • abstraction • Between the application and the hardware

  3. Layers Banking Airline Games APPLICATIONS Compilers Editors Shell SYSTEM --------------------------------- system calls kernel PROGRAMS --------------------------------- device drivers Machine language Microprogramming HARDWARE Physical devices

  4. Why an OS? • Functionality (ease of use) • timesharing • networking • Abstractions (ease of programming) • sockets, pipes, ... • Performance (efficiency) • manage resources efficiently

  5. OS as an idealized machine • Physical machine • single processor • limited memory • complex devices • limited security • physical machine + OS = idealized machine

  6. Idealized Machine • As many processors as the number of programs • Unlimited memory • Clean and uniform device interfaces • Access control • Efficient resource use

  7. OS in action % ls Mail work games % Behind the scenes operations: (shell) print prompt WAIT for user input (user) type “ls” (device driver) read the keyboard inform the kernel (kernel) send it to the shell (shell) look for executable code “ls” make “system calls” (kernel) determine the type of file system cached, on-disk, cdrom, network

  8. OS in action (contd) (shell&kernel): get the contents of directory (shell): format the output (device driver): display on CRT Other operations: file permissions (can’t execute a text file) directory access (ls /secret) quota (ls > listing)

  9. OS in action (bigger picture) init init init init getty getty getty login /bin/sh /bin/sh /bin/date

  10. Is the complexity worth? • Abstraction • process • file (harddisk, cdrom, network, terminal) • Sharing • cpu, memory, devices • Control • access (file, memory, device) • Performance

  11. Layers • Shell - user • cd • System calls - system programmer • chdir • Software architecture - designer • kernel, file system, networking, ... • Implementation

  12. Concepts • Principles behind architecture and implementation • well-known algorithms • well-known problems

  13. Well-known OSs • Multics • Unix • Linux • BSD • MS Windows • Realtime OS

  14. History of OS • Late 50’s - Early 60’s • batch processing • Late 60’s - 70’s • Multiprogramming • time sharing • new abstractions

  15. History of OS (cont’d) • 80s - 90s • GUI • network support • network transparency

  16. Multiprogramming • Goal: keep CPU busy • Fact: I/O times are large • When one program is waiting for I/O, run another program • => Multiple programs resident in memory • Scheduling: non-preemptive

  17. Time sharing • Goal: allow access to multiple users at the same time • Fact: People’s response time is large • Schedule the programs fast • Scheduling: preemptive

  18. Major Unix Flavors • First Edition: Bell Labs, 1969 • 1BSD: UC, Berkeley, 1977 • System V: 1983 • POSIX standard

  19. Linux (open source) • Solaris (Sun) • AIX (IBM) • FreeBSD (open source)

  20. OS organization Libraries process process process (user) --------------------------------------------------------- Kernel Device Drivers (supervisor) Processor(s) Main Memory Devices

  21. Kernel • Process & resource manager • Memory manager • File manager • Device manager

  22. Processor Modes • Mode bit: supervisor or user • Supervisor mode • can execute all machine instructions • can reference all memory locations • User mode • subset of instructions • subset of memory locations

  23. Kernel • Trusted, management software • executes in supervisor mode • can enter user mode • How does a user program invoke a kernel operation?

  24. Trap instruction • Trap is like a function call • mode is set to supervisor • address of function is looked up from a table • the function body is executed • Direct invocation of the function is not permitted

  25. Other OS organizations • Object-oriented • object = data + associated procedures • Microkernel • most of the services implemented at the user level (servers) • kernel provides only the communication between servers

  26. Realtime OS • Performance guarantees

  27. Administration • Class webpage • www.cs.ucsd.edu/classes/sp00/cse120_A/ • Bulletin board • send mail to qxin@cs.ucsd.edu • Section: Fri 12:20-1:10 , CSB 001 • Text: Modern Operating Systems, Tanenbaum

  28. Evaluation • Four group (of 3) assignments • programming • report • 12.5% each • Midterm (20%) • Final (30%)

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