PROCESS DESCRIPTION AND

CONTROL

Processes requirements

The OS must interleave the execution ofmultiple processes, to maximize processorutilization while providing reasonableresponse time.

The OS must allocate resources toprocesses in conformance with a specificpolicy (e.g., certain functions orapplications are of higher priority) while atthe same time avoiding deadlock.

The OS may be required to supportinterprocess communication and usercreation of processes, both of which mayaid in the structuring of applications

WHAT IS A PROCESS?

A program in execution

An instance of a program running on a

computer

The entity that can be assigned to and

executed on a processor

A unit of activity characterized by the

execution of a sequence of instructions, a

current state, and an associated set of

system resources

Process Control

Process Control Block (PCB) Identifier: A unique identifier associated with this process, to

distinguish it from all other processes.

State: If the process is currently executing, it is in the running state.

Priority: Priority level relative to other processes.

Program counter: The address of the next instruction in theprogram to be executed.

Memory pointers: Includes pointers to the program code anddata associated with this process, plus any memory blocks sharedwith other processes.

Context data: These are data that are present in registers inthe processor while the process is executing.

I/O status information: Includes outstanding I/O requests, I/Odevices (e.g., tape drives) assigned to this process, a list of files inuse by the process, and so on.

Accounting information: May include the amount of processortime and clock time used, time limits, account numbers, and soon.

PROCESS STATES A very simple

example. A small dispatcher program that switches the processor from one process to another. A process to continue execution for a maximum of six instruction cycles

A Two-State Process Model

Reason for proses creation

A Five-State Model

Description

PROCESS DESCRIPTION

The OS controls events within the computer system. It

schedules and dispatches processes for execution by the

processor, allocates resources to processes, and responds to

requests by user processes for basic services. Fundamentally,

we can think of the OS as that entity that manages the use of

system resources by processes.

PROCESS CONTROL

UNIX SVR4 PROCESS MANAGEMENT

Process States

• UNIX employs two Running states to indicate whether theprocess is executing in user mode or kernel mode.

• A distinction is made between the two states: (Ready to Run, in Memory) and(Preempted).These are essentially the same state, as indicated by the dotted line joining them.The distinction is made to emphasize the way in which the preempted state is entered.When a process is running in kernel mode (as a result of a supervisor call, clock interrupt,or I/O interrupt), there will come a time when the kernel