Lecture 12

1. Lecture 12 Operating Systems (S&G, §6.4) CS 100 - Lecture 12

2. Read S&G ch. 7,§§7.1 – 7.8(High-Level Languages)for next week CS 100 - Lecture 12

3. Operating Systems • Wait for user requests • Launch programs (e.g., assemblers, loaders) to service those requests • Provide many other services to the user… CS 100 - Lecture 12 (slide < C. Hundhausen)

4. User Interface • Operating system’s most important task: Wait for user requests and process them • Receptionist • Dispatcher • The user interface performs this task CS 100 - Lecture 12 (slide adapted < C. Hundhausen)

5. Text-OrientedUser Interface Traditional user interface:command line • e.g., DOS, Linux, UNIX • > cd mydocs/history • > ls • > ls –al > out.txt • > grep "http" *.html | more CS 100 - Lecture 12 (slide adapt. < C. Hundhausen)

6. GraphicalUser Interface(GUI) Modern alternative: Graphical User Interface • Pioneered by Apple Macintosh (1984) • Windows, Icons, Menus, Pointer (WIMP) • Directories represented by folder icons • Files represented by specialized icons CS 100 - Lecture 12 (slide adapt. < C. Hundhausen)

7. Security • In the 50s and 60s, physical security restricted access to computers and the files and programs they stored • In modern times, that responsibility has shifted to the operating system • Operating system controls access to computer by requiring users to log in with a username and password CS 100 - Lecture 12 (slide < C. Hundhausen)

8. Encrypted Passwords • OS encrypts passwords so that they can’t be stolen • Encryption an active research area in computer science • Need both encrypted text and algorithm for decoding it • Modern “public key” cryptography based on extremely large prime numbers CS 100 - Lecture 12 (slide < C. Hundhausen)

9. File Access Permissions • OS restricts users from accessing the files of others • Files can be set so that they are readable, writable, and executable by only a limited set of people, e.g.: • Only the owner • Only users in the owner’s group • Anyone • Authorization lists maintain this information; they are encrypted so that they cannot be modified by unauthorized users • OS prevents you from harming yourself CS 100 - Lecture 12 (slide adapt. < C. Hundhausen)

10. Hierarchy of File Access Permissions Delete • Change • Append • Read CS 100 - Lecture 12

11. Process Scheduling • Operating system ensures that the processor is being utilized efficiently • While waiting for I/O operations to complete, the computer can do useful work • This is done by maintaining a queue of processes (programs) to be run • Programs have three status levels • Running • Ready • Waiting CS 100 - Lecture 12 (slide < C. Hundhausen)

12. D reads a disk file A waits for keyboard input B writes to the printer; keystroke arrives for A Process Scheduling (2) waiting ready running event A, B, C D D B, C A A, D C B B, D C A Etc…. CS 100 - Lecture 12 (slide adapt. < UT Austin)

13. Deadlock • Operating system aims to prevent deadlock: a state in which no useful work can be done • How deadlock happens (example) • Programs A and B both want to print a file located on the CD-ROM • A requests CD-ROM, then printer • B requests printer, then CD-ROM • A obtains CD-ROM, but is told that printer is not available • B obtains printer, but is told that CD-ROM is not available • Deadlock! • Both are waiting for a resource allocated to the other • the only way to end the wait is for one of them to free the resource that the other needs CS 100 - Lecture 12 (slide < C. Hundhausen)

14. Deadlock Prevention • How to prevent deadlock • If all resources needed by a program are available • allocate the resources to the program • If not all resources needed by a program are available • do not allocate any resources to the program • require the program to make a new request for resources • This algorithm would prevent deadlock in the preceding example: • A and B both want to print a file located on CD-ROM • A requests CD-ROM and printer • A obtains CD-ROM and printer • B requests printer and CD-ROM. The request is denied • A finishes and releases CD-ROM and printer • B re-requests printer and CD-ROM • B obtains printer and CD-ROM • Deadlock is prevented! CS 100 - Lecture 12 (slide adapt. < C. Hundhausen)

15. First Generation (1945–55) • No operating systems • Assemblers and loaders only • Programmers themselves managed allocation of resources and execution of programs • They signed up for blocks of time • They brought in their punched card programs during their allocated block of time • They manually loaded punched cards into computers • They pressed buttons on console to initiate assembler translation process • They manually loaded computer program into memory and began its execution • This was a very tedious process! CS 100 - Lecture 12 (slide adapt. < C. Hundhausen)

16. Second Generation (1955–65) • 1st generation systems often sat idle, because programmers spent a lot of their allocated time thinking and troubleshooting • Led to development of batch operating systems • Computer programmers hand in programs to computer operator, who runs collections of programs in batches CS 100 - Lecture 12 (slide adapt. < C. Hundhausen)

17. Second Generation (2) • Command languages introduced • Enable programmers to specify to operating system what they want done • Usually a mix of programs, data, and commands • Role of operating systems as “receptionist” and “dispatcher” was born CS 100 - Lecture 12 (slide adapt. < C. Hundhausen)

18. Third Generation (1965-1985) • As computational speeds improved, it became unacceptable for computers to sit and wait for I/O operations • The goal: Minimize CPU idle time • Multiprogramming operating systems were born • Many user programs loaded simultaneously into memory • If program has to pause for I/O, another program begins executing • Substantial improvements in CPU utilization were realized CS 100 - Lecture 12 (slide adapt. < C. Hundhausen)

19. Third Generation (2) • Issue: With multiple programs in memory, programs run risk of corrupting other programs by writing in their memory space • Solution: • Keep track of beginning and ending address of each program’s memory space • If a program attempts to reference/write to memory outside of its boundaries: • report an error message • shut down the program • resume execution of another program CS 100 - Lecture 12 (slide adapt. < C. Hundhausen)

20. Third Generation (3) • Time sharing systems emerged • Took advantage of emerging network technologies • Like multi-programming OS • But programs do not have to be loaded into memory in advance • Rather, programs can be loaded dynamically by users sitting at remote terminals; illusion of single-user computer • Need for computer security emerged, because computer could now be accessed remotely • Since many users accessed the computer simultaneously, I/O events could no longer be the only event that triggered a switch to a new program • A given program is run for a designated time slice • When time slice is up or an I/O operation occurs, the CPU begins running another program • This proceeds in round-robin fashion CS 100 - Lecture 12 (slide < C. Hundhausen)

21. Fourth Generation (1985-present) • Personal computers became cheap and powerful enough that they could get work done more efficiently than a timeshare system • At the same time, computer peripherals remained expensive • This suggested that local computation was desirable, with remote access to more expensive peripherals • Return to single-user computer but with much better OS CS 100 - Lecture 12 (slide < C. Hundhausen)

22. Fourth Generation (2) • Network operating systems emerged • Manages resources of local computer (client) • Manages shared resources (servers) of a local area network (LAN) • Typical servers include file, mail, and print CS 100 - Lecture 12 (slide adapt. < C. Hundhausen)

23. Fourth Generation (3) Real-time operating systems • Manage resources of embedded systems that are placed inside equipment (e.g., automobiles, airplanes, ovens, watches) • Prioritizes requests, so that most critical ones are serviced first • E.g., request for collision avoidance in an airplane would be serviced before request to turn up the heat in the cabin CS 100 - Lecture 12 (slide < C. Hundhausen)

24. Future Generations • Multimedia, multimodal, and tangible user interfaces (speech, gesture, virtual reality, etc.) • Issue commands via speech, gesture, interaction with tangible objects • Parallel processing • Distributed operating systems • No boundaries between local area networks and global networks • Users no longer need to be aware of where a resource is coming from • Users can utilize global resources seamlessly – as though they are local resources • But, physical location is not irrelevant CS 100 - Lecture 12 (slide < C. Hundhausen)

25. Ad Hoc Networks • Wireless & mobile communication • No fixed network structure (pattern of interconnectivity) • Each node discovers & keeps track of which other nodes it can communicate with • Messages are routed in accordance with current configuration of nodes • Self-organize & adapt like social networks CS 100 - Lecture 12

26. “Smart Dust” • Currently available “motes”: • Bottle-cap size • $100–$200 each ($1 in 5 years) • Sense temperature, light, motion, energy use, GPS, gas, pressure, … • Set up ad hoc network • Battery lasts for years • 8K program memory, 512K RAM • coded in C, runs TinyOS • See Dust Inc. <http://www.dust-inc.com> • Privacy issues? CS 100 - Lecture 12

27. CS 100 - Lecture 12 (fig. < IEEE Computer)