15 213 Introduction to Computer Systems Program Translation and Execution II Processes Oct 1 1998 Topics User level view of processes Implementation of processes setjmp longjmp class12 ppt Processe A process is an instance of a running s program runs concurrently with other processes multitasking managed by a shared piece of OS code called the kernel kernel is not a separate process but rather runs as part of some user process each process has its own data space and process id pid data for each protected protected from other processes Process A Process B user code kernel code Time user code kernel code Just a stream of instructions user code class12 ppt 2 CS 213 F 98 int fork void Fork creates a new process child process that is identical to the calling process parent process returns 0 to the child process returns child s pid to the parent process if fork 0 printf hello from child n else printf hello from parent n class12 ppt 3 CS 213 F 98 Exit void exit int status exits a process atexit function registers functions to be executed on exit void cleanup void printf cleaning up n main atexit cleanup if fork 0 printf hello from child n else printf hello from parent n exit class12 ppt 4 CS 213 F 98 Wait int wait int child status waits for a child to terminate and returns status and pid main int child status if fork 0 printf hello from child n else printf hello from parent n wait child status printf child has terminated n exit class12 ppt 5 CS 213 F 98 Exec int execl char path char arg0 char arg1 loads and runs executable at path with args arg0 arg1 returns 1 if error otherwise doesn t return main if fork 0 execl usr bin cp cp foo bar 0 wait NULL printf copy completed n exit class12 ppt 6 CS 213 F 98 Example Concurrent network server void main master sockfd sl passivesock port create master socket while 1 slave sockfd sl acceptsock master sockfd await request switch fork case 0 child closes its master and manipulates slave close master sockfd code to read and write to from slave socket goes here exit 0 default parent closes its copy of slave and repeats close slave sockfd case 1 error fprintf fork error n exit 0 class12 ppt 7 CS 213 F 98 Process hierarchy 0 init 1 Daemon e g snmp shell child child grandchild class12 ppt child grandchild 8 CS 213 F 98 Unix startup 1 1 Pushing reset button loads the pc with the address of a small bootstrap program 2 Bootstrap program loads the boot block disk block 0 3 Boot block program loads kernel e g vmunix 4 Boot block program passes control to kernel 5 Kernel handcrafts the data structures for process 0 0 init 1 class12 ppt process 0 handcrafted kernel process process 1 user mode process fork and exec sbin init 9 CS 213 F 98 Unix startup 2 0 etc inittab Daemons e g snmp class12 ppt init 1 getty 10 init forks new processes as per the etc inittab file forks a getty get tty or get terminal for the console CS 213 F 98 Unix startup 3 0 init 1 login class12 ppt 11 getty execs a login program CS 213 F 98 Unix startup 4 0 init 1 login gets user s login and passw if OK it execs a shell if not OK it execs another getty tcsh class12 ppt 12 CS 213 F 98 Loading and running programs from a shell read command line until EOF while read stdin buffer numchars parse command line if command line contains amper 1 else amper 0 for commands not in the shell command language if fork 0 execl cmd cmd 0 if amper 0 retpid wait status class12 ppt 13 CS 213 F 98 Process memory image Reserved for kernel Alpha 0xffff ffff ffff ffff 0xffff fc00 0000 0000 0xffff fbff ffff ffff 0x0000 0400 0000 0000 0x0000 03ff ffff ffff 0x0000 03ff 8000 0000 0x0000 03ff 7fff ffff Not accessible Reserved for shared libraries and dynamic loader Available for heap Heap via malloc or sbrk Grows up Bss segment gp 0x0000 0001 2000 0000 0x0000 0000 1fff ffff sp 0x0000 0000 0001 0000 0x0000 0000 0000 ffff 0x0000 0000 0000 0000 class12 ppt Data segment Text segment Stack Grows down to zero Available for stack Not accessible by convention 64KB 14 CS 213 F 98 Kernel block diagram User programs libraries User level Kernel level system call interface file system process control buffer cache char block device drivers Kernel level Hw level class12 ppt hardware control interrupt and exception handlers hardware processor and devices 15 CS 213 F 98 User and kernel modes User mode Process can execute its own instructions and access its own data Process cannot execute kernel instructions or privileged instructions e g halt access kernel data or data from other processes Kernel mode Process can execute kernel instructions and privileged instructions access kernel and user addresses Processes transition from user to kernel mode via interrupts and exceptions system calls traps class12 ppt 16 CS 213 F 98 System call interface System calls traps are expected program events e g fork exec wait getpid User code call user level library function executes special syscall instruction e g syscall id switch from user mode to kernel mode transfer control to kernel system call interface System call interface find entry in syscall table corresponding to id determine number of parameters copy parameters from user member to kernel memory save current process context in case of abortive return invoke appropriate function in kernel class12 ppt 17 CS 213 F 98 Hardware control Interrupts and exceptions are unexpected hardware events Interrupts events external to the processor I O device asking for attention timer interrupt typically indicated by setting an external pin Exceptions events internal to processor as a result of executing an instruction divide by zero Same mechanism handles both Interrupt or exception triggers transfer of control from user code to interrupt handlers in the hardware control part of the kernel kernel services interrupt or exception If a timer interrupt kernel might decide to give control to a new process context switch class12 ppt 18 CS 213 F 98 Process control Context of a process The context of a process is the state that is necessary to restart the process if its interrupted Union of user level context register context system level context User level context text data and bss segments and user stack Register context PC general purpose integer and floating point regs IEEE rounding mode kernel stack pointer process table address System level context various OS tables process and memory tables kernel stack class12 ppt 19 CS 213 F 98 Process control Context switch The kernel can decide to pass control to another
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