exec Function calls Used to begin a processes execution. Accomplished by overwriting process imaged of caller with that of called. Several flavors, use the one most suited to needs.

exec Function calls int execv( char *path, char *argv[]) ; path: directory path to executable image. Can be your program, or system program. argv: Array of pointers to null terminated strings. These are the arguments to the program being executed.

exec Function calls two conventions with argv: 1) argv[0] should hold the name of the program to be executed. 2) The last element must be NULL.

main (int argc, *argv[]) { int pid ; char *args[2] ; pid = fork() ; if (pid ==0) { args[0] = “./a.out” ;All executed by args[1] = NULL ;child process i = execv(“./aout”, args) ; printf(“OOOpppssss.\n”) ; } else printf(“Not a problem!\n”) ; Executed by parent }

int pid ; char *args[2] ; pid = fork() ;

int pid ; char *args[2] ; pid = ? fork Parent int pid ; char *args[2] ; pid = 0 Child

int pid ; char *args[2] ; pid = fork() if (pid == 0) {args[0] = “./a.out” ; args[1] = NULL ; i = execv(“./aout”, args) ; printf(“OOOpppssss.\n”) ; } else printf(“Not ….”);

Parent int pid ; char *args[2] ; pid = fork() if (pid == 0) {args[0] = “./a.out” ; args[1] = NULL ; i = execv(“./aout”, args) ; printf(“OOOpppssss.\n”) ; } else printf(“Not ….”);

Parent int pid ; char *args[2] ; pid = fork() if (pid == 0) {args[0] = “./a.out” ; args[1] = NULL ; i = execv(“./aout”, args) ; printf(“OOOpppssss.\n”) ; } else printf(“Not ….”); Child int pid ; char *args[2] ; pid = fork() if (pid == 0) {args[0] = “./a.out” ; args[1] = NULL ; i = execv(“./aout”, args) ; printf(“OOOpppssss.\n”) ; } else printf(“Not ….”);

Parent int pid ; char *args[2] ; pid = fork() if (pid == 0) {args[0] = “./a.out” ; args[1] = NULL ; i = execv(“./aout”, args) ; printf(“OOOpppssss.\n”) ; } else printf(“Not ….”); a.out

Processes: program + execution state Pseudoparallelism Multiprogramming Many processes active at once With switching: process execution is not repeatable processes should make no assumptions about timing Process consists of: Process’ core image: program, data, run-time stack Program counter, registers, stack pointer OS bookkeeping information

Process State As a process executes, it changes state new: The process is being created. running: Instructions are being executed. waiting: The process is waiting for some event to occur. ready: The process is waiting to be assigned to a process. terminated: The process has finished execution.

Diagram of Process State

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Process-Centered Operating System One way to view system: a scheduler, with processes running “on top”: Some of the processes are system processes Scheduler processprocess processprocess processprocess processprocess processprocess

Process Table One entry per process. PCB Pr 0 Pr 1 Pr 2 Pr 4 …..Pr N

PCB: Process Management registers, program counter, program status word, stack pointer process state time process started, CPU time used, children’s CPU time used alarm clock setting pending signal bits pid message queue pointers, other flag bits

PCB: Memory-Related Information pointers to text Data stack segments Pointer to page table

PCB: File-Related Information root, working directory file descriptors User ID Group ID

Process Control Block (PCB)

Context Switch (or Process Switch) Currently executing process looses control of CPU Its “context” must be saved (if not terminated) into PCS. New process is chosen for execution. New process context is restored. New process is given control of the CPU.

CPU Switch From Process to Process

Process Scheduling Queues Job queue – set of all processes in the system. Ready queue – set of all processes residing in main memory, ready and waiting to execute. Device queues – set of processes waiting for an I/O device. Process migration between the various queues.

Ready Queue And Various I/O Device Queues

Representation of Process Scheduling

Low Level Interrupt Processing Each HW device has slot in interrupt vector (IV). On interrupt, HW pushes PC, PSW, one or more registers. Loads in new PC from IV. END OF HW Assembly code to save registers and info on stack (to PCB). Assembly sets up new stack for handling process. Calls C interrupt handling routing to complete interrupt processing. Scheduler is called and determines next process to run. Assembly language restores registers and other necessary items (e.g., memory map) of selected process and begins its execution.

Schedulers Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue. Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU.

Schedulers (Cont.) Short-term scheduler is invoked very frequently (milliseconds)  (must be fast). Long-term scheduler is invoked very infrequently (seconds, minutes)  (may be slow). The long-term scheduler controls the degree of multiprogramming.

Schedulers (Cont.) Processes can be described as either: I/O-bound process – spends more time doing I/O than computations, many short CPU bursts. CPU-bound process – spends more time doing computations; few very long CPU bursts.

Context Switch When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process. Context-switch time is overhead; the system does no useful work while switching. Time dependent on hardware support.

#include main() { char *ptr ; char input[1024] ; char *tmp, *tmp1 ; int i ; while(1) { i = read(0, input, 512) ; printf("Ret %d\n", i) ; ptr = input ; while (*ptr != '\n') { if (*ptr == ' ') printf("Space!\n") ; else printf("Read %c\n", *ptr) ; ptr++ ; }