In amultitaskingcomputer system,processes may occupy a variety ofstates. These distinct states may not be recognized as such by theoperating systemkernel. However, they are a useful abstraction for the understanding of processes.
The following typical process states are possible on computer systems of all kinds. In most of these states, processes are "stored" onmain memory.
When a process is first created, it occupies the "created" or "new" state. In this state, the process awaits admission to the "ready" state. Admission will be approved or delayed by a long-term, or admission,scheduler. Typically in mostdesktop computer systems, this admission will be approved automatically. However, forreal-time operating systems this admission may be delayed. In a realtime system, admitting too many processes to the "ready" state may lead to oversaturation andovercontention of the system's resources, leading to an inability to meet process deadlines.
A "ready" or "waiting" process has been loaded intomain memory and is awaiting execution on aCPU (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 system's 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.
Aready queue orrun queue is used incomputer 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 aqueue 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.
A process moves into the running state when it is chosen for execution. The process's instructions are executed by one of the CPUs (or cores) of the system. There is at most one running process per CPU or core. A process can run in either of the two modes, namelykernel mode oruser mode.[1][2]
A process transitions to ablocked state when it cannot carry on without an external change in state or event occurring. For example, a process may block on a call to an I/O device such as a printer, if the printer is not available. Processes also commonly block when they require user input, or require access to acritical section which must be executed atomically. Such critical sections are protected using a synchronization object such as a semaphore or mutex.
A process may beterminated, either from the "running" state by completing its execution or by explicitly beingkilled. In either of these cases, the process moves to the "terminated" state. The underlying program is no longer executing, but the process remains in theprocess table as azombie process until its parent process calls thewaitsystem call to read itsexit status, at which point the process is removed from the process table, ending the process's lifetime. If the parent fails to callwait, this continues to consume the process table entry (concretely theprocess identifier or PID), and causes aresource leak.
Two additional states are available for processes in systems that supportvirtual memory. In both of these states, processes are "stored" on secondary memory (typically ahard disk).
(Also calledsuspended and waiting.) In systems that support virtual memory, a process may be swapped out, that is, removed from main memory and placed on external storage by the scheduler. From here the process may be swapped back into the waiting state.
(Also calledsuspended 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).