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What is a Process ?

•The activity of program execution.

•Also called a task or job

•Has associated with it:CodeDataResourcesA State

•An executing set of machine instructions

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When is a Process Created?

Principal events that cause process creation

1. System initialization

2. Execution of a process creation system

3. User request to create a new process

4. Initiation of a batch job

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The resources ‘owned’ by the process include:the (virtual) Address Space to hold the process image

and may include control of other resources such as

Main memoryI/O channelsI/O devicesFiles

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What are the Operating System Responsibilities for Processes?

• Interleave the execution of several processes to maximize processor utilization while providing reasonable response time

• Allocate resources to processes

• Support interprocess communication and user creation of processes

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Conceptual model of 4 independent, sequential processes

• Multiprogramming of four programs

• Only one program active at any instant

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OS Handling Processes in a Multiprogrammed System

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Process States and Transitions between States

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Fields of a Process Control Block or Table

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Process Termination Conditions

1. Normal exit (voluntary) - job completed.

2. Error exit (voluntary) - occurs within execution of process such as division by zero but process may be just signaled and not terminated.

3. Fatal error (involuntary) - impossible for process to continue. Ex. Cc file.c process called but file.c does not exist.

4. Killed by another process (involuntary)

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Process Hierarchies

• Parent creates a child process, child processes can create its own process

• Forms a hierarchy– UNIX calls this a "process group”– when a signal is sent from the keyboard, each process in the

group receives the signal.– all processes belong to a single tree with the special process ‘init’

at the root. ‘init’ is in the boot.

• Windows has no concept of process hierarchy– all processes are created equal