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The Process

Ami Tri Cahyadi

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Many definitions have been given for theterm

process, including

A program in execution An instance of a program running on a computer

The entity that can be assigned to and executedon a processor

A unit of activity characterized by a single

sequential thread of execution, a current state,and an associated set of system resources

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Component of Process

An executable program

The associated data needed by theprogram (variables,work space, buffers,

etc.) The execution context of the program

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An address space

A program

A execution engine (program

counter, ...)

Data

Resources

Process control block (PCB) withprocess identifier (PID)

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The execution context, or processstate, is the internal data by whichthe OS is able to supervise and control

the process. This internal information is separatedfrom the process, because the OS hasinformation not permitted to the

process.

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Classic Process (one

thread)

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Address Space

Each process views its memory as acontiguous set of logical memoryaddresses. The operating system

manages the translation of logicaladdresses to the physical memoryaddresses of the computer.

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Process Control Block

(PCB)

The process control block is a data structurefor each process holding:

Register values

Logical state Page map table mapping logical addresses tophysical addresses

Type & location of resources it holds List of resources it needs Parent process identification Security keys etc.

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Process Creation

At system boot time, one user levelprocess is created. In Unix, this processis called init.

In Windows, it is the System IdleProcess. This process is the parent orgrand-parent of all other processes.

New Child Processes are created byanother process (the Parent Process).

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Windows Process Creation

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Unix Process Creation

Unix fork() creates a process

Creates a new address space

Copies text, data, & stack into new adressspace

Provides child with access to open files

Unix exec() allows a child to run a newprogram

Unix wait() allows a parent to wait for achild to terminate

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int pidValue;

...

pidValue = fork(); /* Creates a child process */

if(pidValue == 0) {

/* pidValue is 0 for child, nonzero for parent */

/* The child executes this code concurrently with parent */ childsPlay(...); /* A procedure linked into a.out */

exit(0);

}

/* The parent executes this code concurrently with child */

parentsWork(...);

wait(...);

...

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Multi-threaded Process

The simple model of a process containingonly one thread, is referred to as a classicprocess. Modern operating systems alsosupport processes with multiple threads.

Each thread of a single process can use theprocess global data with synchronizationresources to easily communicate with otherthreads. Multi-threaded programs run moreefficiently and use less resources than a

program that creates multiple processes toaccomplish the same task.

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The Process Managers Job

The responsibilities of the process managerinclude:

Process creation and termination

Thread creation and termination Process/thread synchronization Resource allocation Resource protection Cooperation with the device manager toimplement I/O

Implementation of the address space Process Scheduling

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Process Finite State

Diagrams

Simple FSM

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Unix FSM

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Windows FSM

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Process Resource

Allocation

If a process requests a resource that is already in use, the process is held in a

queue until the resource becomes available.