In a multitasking

computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations ( computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These prog ...

system,

processes A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management *Business process, activities that produce a specific se ...

may occupy a variety of states. These distinct states may not be recognized as such by the

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learn ...

. 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 Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a comput ...

Created

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 most

desktop computer A desktop computer (often abbreviated desktop) is a personal computer designed for regular use at a single location on or near a desk due to its size and power requirements. The most common configuration has a case that houses the power supply ...

systems, this admission will be approved automatically. However, for

real-time operating system A real-time operating system (RTOS) is an operating system (OS) for real-time applications that processes data and events that have critically defined time constraints. An RTOS is distinct from a time-sharing operating system, such as Unix, which m ...

s this admission may be delayed. In a realtime system, admitting too many processes to the "ready" state may lead to oversaturation and overcontention of the system's resources, leading to an inability to meet process deadlines.

Ready

A "ready" or "waiting" process has been loaded into

and is awaiting execution on a

CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, a ...

(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. A ''ready queue'' or run 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.

Running

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

or core. A process can run in either of the two modes, namely ''kernel mode'' or ''user mode''.

Kernel mode

* Processes in kernel mode can access both: kernel and user addresses. * Kernel mode allows unrestricted access to hardware including execution of ''privileged'' instructions. * Various instructions (such as I/O instructions and halt instructions) are ''privileged'' and can be executed only in kernel mode. * A

system call In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...

from a user program leads to a switch to kernel mode.

User mode

* Processes in user mode can access their own instructions and data but not kernel instructions and data (or those of other processes). * When the computer system is executing on behalf of a user application, the system is in user mode. However, when a user application requests a service from the

(via a

), the system must transition from user to kernel mode to fulfill the request. * User mode avoids various catastrophic failures: ** There is an isolated

virtual address space In computing, a virtual address space (VAS) or address space is the set of ranges of virtual addresses that an operating system makes available to a process. The range of virtual addresses usually starts at a low address and can extend to the hig ...

for each process in user mode. ** User mode ensures isolated execution of each process so that it does not affect other processes as such. ** No direct access to any hardware device is allowed.

Blocked

A process transitions to a blocked 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 a

critical section In concurrent programming, concurrent accesses to shared resources can lead to unexpected or erroneous behavior, so parts of the program where the shared resource is accessed need to be protected in ways that avoid the concurrent access. One way to ...

which must be executed atomically. Such critical sections are protected using a synchronization object such as a semaphore or mutex.

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. The underlying program is no longer executing, but the process remains in the process table as a ''

zombie process On Unix and Unix-like computer operating systems, a zombie process or defunct process is a process that has completed execution (via the exit system call) but still has an entry in the process table: it is a process in the " Terminated state". ...

'' until its parent process calls the

wait 

Wait or WAIT may refer to:

  Music 
* Wait (musician), British town pipers
 Albums and EPs
*  ''Wait'' (The Polyphonic Spree EP), by The Polyphonic Spree
*  ''Wait'' (Emanuel Nice EP), a 2002 EP released by the band Emanuel Nice
*  ''Wait'' (Stee ...

to read its exit status, at which point the process is removed from the process table, finally ending the process's lifetime. If the parent fails to call wait, this continues to consume the process table entry (concretely the process identifier or PID), and causes a

resource leak In computer science, a resource leak is a particular type of resource consumption by a computer program where the program does not release resources it has acquired. This condition is normally the result of a bug in a program. Typical resource lea ...

Additional process states

Two additional states are available for processes in systems that support

virtual memory In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...

. In both of these states, processes are "stored" on secondary memory (typically a

hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magn ...

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 on external storage by the 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

* :Particularly chapter 3, section 3.2, "process states", including figure 3.9 "process state transition with suspend states" {{DEFAULTSORT:Process States Process (computing)