Processes & ECF CS 351: Systems Programming Michael Saelee - - PowerPoint PPT Presentation

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Processes & ECF CS 351: Systems Programming Michael Saelee - - PowerPoint PPT Presentation

Sep 28, 2023 255 likes •783 views

Processes & ECF CS 351: Systems Programming Michael Saelee <lee@iit.edu> Computer Science Science Agenda - Definition & OS responsibilities - Exceptional control flow - synch vs. asynch exceptions - exception handling

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Processes & ECF

CS 351: Systems Programming Michael Saelee <lee@iit.edu>

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Agenda

  • Definition & OS responsibilities
  • Exceptional control flow
  • synch vs. asynch exceptions
  • exception handling procedure
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§Definition & OS responsibilities

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a process is a program in execution

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programs describe what we want done, processes carry out what we want done

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a process comprises ... { code (program) + runtime data (global, local, 
 dynamic) + PC, SP , FP & other registers }

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essential to program execution is predictable, logical control flow which requires that nothing disrupt the program mid-execution

main() { fnA();
 }
 
 fnA() {
 fnB();
 } fnB() { loop { } }

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easiest way to guarantee this is for a process to “own” the CPU for its entire duration ... downsides?

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1.No multitasking! 2.A malicious (or badly written) program can “take over” the CPU forever 3.An idle process (e.g., waiting for input) will underutilize the CPU

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the operating system presents each process with a simulated, seamless logical control flow many of which can be taking place concurrently on one or more CPUs

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Logical control flow

Time Process A Process B Process C

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Physical flow (1 CPU)

Time Process A Process B Process C

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to do this, we need (1) a hardware mechanism to periodically interrupt the current process to load the OS , (2) an OS procedure that decides which processes to run, in what order , and (3) a routine for seamlessly transitioning between processes

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(1) is the periodic clock interrupt; 
 (2) is the OS scheduler;
 (3) is the context switch

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Context switches

Process A Process B User code Kernel code User code Kernel code User code Time Context switch Context switch read Disk interrupt Return from read

Need new diagram that shows context switches triggered by the clock interrupt.

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to implement scheduling and carry out context switches, the OS must maintain a wealth of per-process metadata

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a process comprises ... { code (program) + runtime data (global, local, 
 dynamic) + PC, SP , FP & other registers + “process control block” (OS metadata) }

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a process comprises ... { code (program) + runtime data (global, local, 
 dynamic) + PC, SP , FP & other registers + e.g., PID, mem/CPU usage, pending syscalls }

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context switches are external to a process’s logical control flow (dictated by user program) — part of exceptional control flow

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§Exceptional Control Flow

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int main() { while (1) { printf("hello world!\n"); } return 0; }

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int main() { while (1) { printf("hello world!\n"); } return 0; }

logical c.f.

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int main() { while (1) { printf("hello world!\n"); } return 0; }

logical c.f. exception!

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int main() { while (1) { printf("hello world!\n"); } return 0; }

logical c.f. exception!

?

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int main() { while (1) { printf("hello world!\n"); } return 0; }

logical c.f. exception!

?

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Two classes of exceptions:

  • I. synchronous

II.asynchronous

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  • I. synchronous exceptions are caused

by the currently executing instruction

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3 subclasses of synchronous exceptions:

  • 1. traps
  • 2. faults
  • 3. aborts
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  • 1. traps

traps are intentionally triggered by a process e.g., to invoke a system call

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char *str = "hello world"; int len = strlen(str); write(1, str, len); mov edx, len mov ecx, str mov ebx, 1 mov eax, 4 ; syscall #4 int 0x80 ; trap to OS

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return from trap (if it happens) resumes execution at the next logical instruction

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  • 2. faults

faults are usually unintentional, and may be recoverable or irrecoverable e.g., segmentation fault, protection fault, page fault, div-by-zero

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  • ften, return from fault will result in

retrying the faulting instruction — esp. if the handler “fixes” the problem

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  • 3. aborts

aborts are unintentional and irrecoverable i.e., abort = program/OS termination e.g., memory ECC error

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  • II. asynchronous exceptions are caused

by events external to the current instruction

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int main() { while (1) { printf("hello world!\n"); } return 0; } hello world! hello world! hello world! hello world! ^C $

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hardware initiated asynchronous exceptions are known as interrupts

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e.g., ctrl-C, ctrl-alt-del, power switch

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interrupts are associated with specific processor (hardware) pins

  • checked after every CPU cycle
  • associated with interrupt handlers
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1 2 . . . int # interrupt vector

  • int. handler 0 code

(system) memory

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interrupt procedure (typical)

  • save context (e.g., user process)
  • load OS context
  • execute handler
  • load context (for …?)
  • return
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important: after switching context to the OS (for exception handling), there is no guarantee if/when a process will be
 switched back in!

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P0 P1 P2 P3 P4 OS (kernel)

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P0 P1 P2 P3 P4 OS (kernel)

trap

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P0 P1 P2 P3 P4 OS (kernel)

handler trap

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P0 P1 P2 P3 P4 OS (kernel)

handler

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P0 P1 P2 P3 P4 OS (kernel)

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P0 P1 P2 P3 P4 OS (kernel)

handler trap

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P0 P1 P2 P3 P4 OS (kernel)

handler

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switching context to the kernel is potentially very expensive — but the only way to invoke system calls and access I/O

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moral (to be reinforced ad nauseum): use system calls (traps) sparingly!