8/16/2019 The Process Control Block

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The process control block (PCB)

When a process gives up the processor when an interrupt occurs, the OS saves or maintains the

 process details or information within the system in a data structure called a PCB so that later,

execution would resume from exactly the same place when process execution was interrupted.

PCB contains the following information

•

Process I !uni"ue process identifier#• Current process state !ready, running, $loc%ed !suspended##

• &o$ priority or other scheduling information

• Program counter 

• 'egister save area, where the current contents of all registers are saved $efore they are ta%en

over $y the next process

• ( pointer to the processor)s allocated memory area

• Pointers to other allocated resources !open files, dis%, printer etc.#

• CP* time used so far 

• +stimated time to completion

• in%s to other PCBs

-hreads

• ( thread is a lightweight su$process

• ( thread is a path of execution within a process

• ( process can have one or more processes

• +ach thread has a scheduling state !ready, running or $loc%ed#

• (ll threads in a process share most of their environment

• -hreads from the same process have the same virtual address space for instructions and glo$al

varia$les, $ut they have their own local varia$les, program counter, registers and stac% pointer 

• +ach thread has its own thread control $loc% !-CB#

• -he PCB holds references to one or more -CBs

• -his means less main memory is used and it re"uires less wor% to create or delete threads than

separate processes $ecause threads share virtual address space

• ( good programmer must design the program in such a way that all threads can run

concurrently without interfering with each other