Process state

In a multitasking computer system, processes may occupy a variety of states. These distinct states may not actually be recognized as such by the operating system kernel, however they are a useful abstraction for the understanding of processes.

Primary process states

The following typical process states are possible on computer systems of all kinds. In most of these states, processes are "stored" on main memory.

Created

(Also called New) When a process is first created, it occupies the "created" or "new" state. In this state, the process awaits admission to the "ready" state. This admission will be approved or delayed by a long-term, or admission, scheduler. Typically in most desktop computer systems, this admission will be approved automatically, however for real-time operating systems this admission may be delayed. In a real time system, admitting too many processes to the "ready" state may lead to oversaturation and overcontention for the systems resources, leading to an inability to meet process deadlines.

Ready or Running

(Also called waiting or runnable) A "ready" or "waiting" process has been loaded into main memory and is awaiting execution on a CPU (to be context switched onto the CPU by the dispatcher, or short-term scheduler). There may be many "ready" processes at any one point of the systems execution - for example, in a one processor system, only one process can be executing at any one time, and all other "concurrently executing" processes will be waiting for execution.

A ready queue is used in computer scheduling. Modern computers are capable of running many different programs or processes at the same time. However, the CPU is only capable of handling one process at a time. Processes that are ready for the CPU are kept in a queue for "ready" processes. Other processes that are waiting for an event to occur, such as loading information from a hard drive or waiting on an internet connection, are not in the ready queue.

Blocked

A process that is waiting for some event (such as I/O operation completion or a signal).

Terminated

A process may be terminated, either from the "running" state by completing its execution or by explicitly being killed. In either of these cases, the process moves to the "terminated" state. If a process is not removed from memory after entering this state, this state may also be called zombie.

Additional process states

Two additional states are available for processes in systems that support virtual memory. In both of these states, processes are "stored" on secondary memory (typically a hard disk).

Swapped out and waiting

(Also called suspended and waiting.) In systems that support virtual memory, a process may be swapped out, that is removed from main memory and placed in virtual memory by the mid-term scheduler. From here the process may be swapped back into the waiting state.

Swapped out and blocked

(Also called suspended and blocked.) Processes that are blocked may also be swapped out. In this event the process is both swapped out and blocked, and may be swapped back in again under the same circumstances as a swapped out and waiting process (although in this case, the process will move to the blocked state, and may still be waiting for a resource to become available).

References

Stallings, William (2005). Operating Systems: internals and design principles (5th edition). Prentice Hall. ISBN 0-13-127837-1.

Particularly chapter 3, section 3.2, "process states", including figure 3.9 "process state transition with suspend states"