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CS 414 : Operating Systems
UNIVERSITY OF VIRGINIA
Department of Computer Science
Fall 2005
Topic 2: Processes
Context switching: saving the state (to where?) and restoring it
What gets saved? Everything that next process could damage: program counter, processor status word, registers. all of memory? all disks? all the stuff on tape? options for memory contents: a) trust next process b) move everything to disk (Alto sys- tem), c) memory protection
Why tricky?
All machines provide some special hardware support. Motorola 68000: hardware just moves PC and status word to the stack. OS then handles rest of state itself. Must be done carefully. Why? Intel 432: hardware does all state saving and restoring into process control block, and even dispatching. Desirable?
Ugly issue of performance: sometimes making dirty shortcuts.
Birth of a process: creating it from scratch: Allocate memory Load code and data into memory. Create (empty) call stack. Create and initialize process control block. Make process known to dispatcher.
Forking: copying existing process Make sure process to be copied isn’t running and has all state saved. Make a copy of code, data, stack. Copy PCB into new PCB. Make process known to dispatcher. What’s missing?
Independent process: neither affect nor affected by the rest of the universe. its state is not shared in any way with other processes deterministic reproducible scheduling does not affect the result There are many different ways in which a collection of independent processes might be exe-
cuted on a processor. How often are processes independent?
Cooperating processes: cooperating processes share state. May or may not actually be ‘‘cooperating’’. nondeterministic : depends on relative execution sequence. irreproducible. example: one process writes ‘‘ABC’’ to the monitor, another writes ‘‘CBA’’.
Why allow processes to cooperate? want to share resources: one computer, many users. one file of checking account records, many tellers. want to do things faster: read next block while processing current one. divide job into sub-jobs, execute in parallel. Basic assumption for cooperating process systems is that the order of some operations is ir-
relevant; certain operations are independent of certain other operations. Only a few things matter: example: A = 1; B = 2; has same result as B = 2; A = 1;
