Processes States

The three states of a process

Now that we have some idea of what a process is (though we will continue to refine this notion), and (roughly) how it is created, let us talk about the different states a process can be in at a given time. The notion that a process can be in one of these states arose in early computer systems1. “Programming Semantics for Multiprogrammed Computations” by Jack B. Dennis, Earl C. Van Horn. Communications of the ACM, Volume 9, Number 3, March 1966. This paper defined many of the early terms and concepts around building multiprogrammed systems. 2. “Structure of the Multics Supervisor” by V.A. Vyssotsky, F. J. Corbato, R. M. Graham. Fall Joint Computer Conference, 1965. An early paper on Multics, which described many of the basic ideas and terms that we find in modern systems. Some of the vision behind computing as a utility are finally being realized in modern cloud systems.. In a simplified view, a process can be in one of three states:

• Running: In the running state, a process is running on a processor. This means it is executing instructions.

• Ready: In the ready state, a process is ready to run but for some reason, the OS has chosen not to run it at this given moment.

• Blocked: In the blocked state, a process has performed some kind of operation that makes it not ready to run until some other event takes place. A common example: when a process initiates an I/O request to a disk, it becomes blocked and thus some other process can use the processor.

Transitioning from one state to another

If we were to map these states to a graph, we would arrive at the diagram as shown below.