1.4 Process

A process is an instance of a computer program that is being executed. While a program itself is just a passive collection of instructions, a process is the actual execution of those instructions. Several processes may be associated with the same program - each would execute independently (multithreading - where each thread represents a process), either synchronously (sequentially) or asynchronously (in parallel). Modern computer systems allow multiple programs and processes to be loaded into memory at the same time and, through time-sharing (or multitasking), give an appearance that they are being executed at the same time (concurrently) even if there is just one processor. Similarly, using a multithreading OS and/or computer architecture, parallel processes of the same program may actually execute simultaneously (on different CPUs) on a multiple CPU machine or network. http://en.wikipedia.org/wiki/Process_%28computing%29

Is a process the same as a program? No, it is both more and less. (what is a program? the statements that a user writes, or a command he invokes)

• More - a program is only part of the state; several processes may be derived from the same program. If I type "ls", something different happens than if you type it.
• Less - one program may use several processes, e.g. cc runs other things behind your back.

Some systems allow only one process. They are called uniprogramming systems (not uniprocessing; that means only one processor). Easier to write some parts of OS, but many other things are hard to do. E.g. compile a program in background while you edit another file; answer your phone and take messages while you are busy hacking.

Overview

In general, a computer system process consists of the following resources:

• An image of the executable machine code associated with a program.
• Memory (typically some region of virtual memory); which includes the executable code, process-specific data (input and output), a call stack (to keep track of active subroutines and/or other events), and a heap to hold intermediate computation data generated during run time.
• Operating system descriptors of resources that are allocated to the process, such as file descriptors (Unix terminology) or handles (Windows), and data sources and sinks.
• Security attributes, such as the process owner and the process' set of permissions (allowable operations).
• Processor state (context), such as the content of registers, physical memory addressing, etc. The state is typically stored in computer registers when the process is executing, and in memory otherwise.

The operating system holds most of this information about active processes in data structures called process control blocks (PCB, part 3).

Any subset of resources, but typically at least the processor state, may be associated with each of the process' threads in operating systems that support threads or 'daughter' processes.

The operating system keeps its processes separated and allocates the resources they need so that they are less likely to interfere with each other and cause system failures (e.g., deadlock or thrashing). The operating system may also provide mechanisms for inter-process communication to enable processes to interact in safe and predictable ways.