CSC 501Lecture 2: Processes

Von Neumann Model

• Both program and data reside in memory

• Execution stages in CPU:• Fetch instruction• Decode instruction• Execute instruction• Write back result

Process

• Process is• a running program• a program in execution• an “instantiation” of a program

• We want to have multiple running programs• However, we only have a few CPUs

• So, limit the number of programs you are running

Time to invent CPU virtualization• OS virtualizes the CPU• By time sharing it

• Mechanisms <- this lecture• Policies <- next lecture

A Process

• A running program includes:• Memory• Registers• I/O, opened files

• APIs: create, destroy, wait, control, status

CodeStatic data

Heap

Stack

Process Creation

• Loading: code and static data• Eagerly vs lazily

• Allocate memory for stack, heap• Initialize file descriptors• Jump to the main

Process states

• As a process executes, it changes state• new: The process is being created• running: Instructions are being executed• blocked: The process is waiting for some event to occur• ready: The process is waiting to be assigned to a

processor• terminated: The process has finished execution

AdmittedAdmitted ExitExit

I/O: doneI/O: done

ScheduledScheduled

I/O: initiateI/O: initiate

DescheduledDescheduled

Process Lifecycle New

Ready Running

Terminated

Blocked

Data Structures

• Process list• Common elements in process structure• Process state• Program counter• CPU registers• CPU scheduling information• Memory-management information• Accounting information• I/O status information

Example PCB in XINU

Process API

• On Unix:• fork, exec, wait, …

• Why:• they are essential in building a Unix shell

Process Creation

• UNIX examples• fork system call creates new process• exec system call used after a fork to replace new

process’ memory space with a new program

Exmapleint main(){ pid_t pid;

/* fork another process */pid = fork();if (pid < 0) { /* error occurred */fprintf(stderr, "Fork Failed");exit(-1);}else if (pid == 0) { /* child process */execlp("/bin/ls", "ls", NULL);}else { /* parent process *//* parent will wait for the child to complete */wait (NULL);printf ("Child Complete");exit(0);}

}

Process Termination

• Possible scenarios for process termination• Exit (by itself)• Abort (by parent)• Kill (by sysadmin)

Process Termination

• Exit (by itself)• Process executes last statement and asks operating

system to delete • Abort (by parent)• Child has exceeded allocated resources• Task assigned to child is no longer required• If parent is exiting

• Some operating system do not allow child to continue if its parent terminates

• All children terminated - cascading termination

• Kill (by sysadmin)• Administration purpose

Process Suspension

• Temporarily ‘‘stop’’ a process• Prohibit from using the CPU

• Why?• What should be done?• Change its state in PCB• Save its machine states for later resumption

• Process table entry retained• Complete state saved

Limited Direct Execution

• Time sharing the CPU:• Run one process for a little while, then run another one,

and so forth

• Performance and control

• The “direct execution” part of the idea is simple:• Just run the program directly on the CPU.

Protocol without limits

• Restricted Operations• How to take over control

Restricted Operations

• The need to perform restricted operations• Let processes do whatever they want• Two modes: user mode and kernel mode

• How to perform restricted operations• System calls, and they are procedure calls

How to execute system call

• Trap instruction• The program executes trap, jump into kernel and raise

privilege• Kernel does the work• Kernel calls return-from-trap, return into the calling user

program and reducing the privilege level

• Save caller’s registers• On x86, PC, flags, and a few others saved to a per-process

kernel stack

Which code to run

• Let the calling process specify the address• Set up a trap table at boot time• The hardware remembers the locations of trap handlers

Switching Between Processes• Since OS is not running, how can it do anything?

• A cooperative approach• Used in early version of the Macintosh OS• Wait for systems calls which include yield• Wait if illegal actions which generate trap

• What if a process gets stuck in an infinite loop

A Non-Cooperative Approach:The OS Takes Control• Timer interrupt• Boot time:• Set interrupt handler• Start timer

Context Switch

• When CPU switches to another process• System must • Save the state of the old process (suspend) and• Load the saved state for the new process (resume)

• Context-switch time is overhead• System does no useful work while switching

• Time dependent on hardware support

Saving and Restoring Context• Save state of currently executing process• Copy all “live” registers to process control block

• Restore state of process to run next• Copy values of live registers from process control block

to registers

• How to get into and out of the context switching code?

Next

• Process scheduling• CPU scheduling• Multi-level feedback• Lottery scheduling