Process State Diagram Process Control Block (PCB)

Process State Diagram

Process Control Block (PCB)

• Each process is represented on the OS by a PCB
• It is the data structure used by the OS
• Process state – new, running, waiting etc
• Program counter – address of next instruction to

be executed for this process

• CPU registers – indicates general purpose register, index registers and accumulators etc.
• Memory management information – include such information as the base and limit register, page tables or segment tables in memory
• I/O status information – I/O requests, I/O devices allocated , a list of open files
• Accounting information – CPU used, clock time elapsed since start, time limits

Threads

• Is a lightweight process
• Basic unit of CPU utilisation
• Consists of :
• Program counter – keeps track of which instruction to execute next
• Registers – holds its current working variable
• Stack – execution history
• Thread States
• bornState :A thread is just created
• readyState :The thread is waiting for CPU
• Running :System assigns the processor to the thread
• Sleep: :A sleeping thread becomes ready after the designated sleep time expires
• Dead :The execution of the thread is finished

Process vs Thead

• Process
• Is heavy weight or resource intensive
• process takes more time to create
• Execution is very slow
• Cant share the same memory area
• Communication between two processes is difficult
• Thread
• Is light weight, taking lesser resources
• Takes less time to create
• Execution is very fast
• Threads can share same memory area
• Communication between two threads is easy

Multithreading

• Is when a number of thread execute at the same time within a process
• In a multithreaded program, even when some portion of it is blocked, the whole program is not blocked
• The rest of the program continues running if multiple CPUs are available
• Multithreading therefore gives the best performance
• Multithreading can simplify code and increase efficiency
• Kernels are generally multithreaded