Operating Systems Chapter 3

Process

A program that is being executed; Unit of work in a modern time sharing system

Processes that make up a system

Operating systems processes executing system code and user processes executing user code

What is the benefit of switching the CPU between processes

Doing so makes the computer more productive

Components of a process

Text section (code), data section(global variables), heap(memory), stack (temporary data)

States of a process

New (created) Running (instructions being executed) Waiting (Process is waiting for some event to occur) Ready (Waiting to be assigned to processor) Terminated (Finished execution)

Process Control Block (PCB) Contains

Process state (new, ready, etc...)
Program Counter (Address of next instruction)
CPU Registers
CPU Scheduling information (process priority, etc...)
Memory Management information ()
Accounting Information
I/O Status information

Process Scheduler

Selects process for execution on CPU

Job Queue

Holds All processes in system

Ready Queue

Holds processes in main memory that are waiting to be executed. Usually in linked list.

longterm/job scheduler

Selects processes and loads them into main memory where they await execution

short term scheduler/ cpu scheduler

Selects processes that are ready to execute and allocates cpu to one of them

I/O bound vs CPU bound

An I/O-bound process is one that spends more of its time doing I/O than it spends doing computations. A CPU-bound process, in contrast, generates I/O requests infrequently, using more of its time doing computations.

Swapping

Processes are swapped in and out of main memory to the disk

Context Switch

Switching the CPU to another process requires performing a state save of the current process and a state restore of a different process

fork()

System call that creates a new process

exec()

loads binary file into memory and starts its execution

wait()

Used by parent process to movie itself off ready queue until termination of child