State (computer science)

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In information technology and computer science, a system is described as stateful if it is designed to remember preceding events or user interactions;[1] the remembered information is called the state of the system.

The set of states a system can occupy is known as its

digital logic circuits and components, automata and formal language, computer programs, and computers
.

The output of a digital circuit or deterministic computer program at any time is completely determined by its current inputs and its state.[2]

Digital logic circuit state

signals are dependent only on its present input signals, and sequential logic, whose outputs are a function of both the current inputs and the past history of inputs.[3] In sequential logic, information from past inputs is stored in electronic memory elements, such as flip-flops. The stored contents of these memory elements, at a given point in time, is collectively referred to as the circuit's state and contains all the information about the past to which the circuit has access.[4]

Since each binary memory element, such as a flip-flop, has only two possible states, one or zero, and there is a finite number of memory elements, a digital circuit has only a certain finite number of possible states. If N is the number of binary memory elements in the circuit, the maximum number of states a circuit can have is 2N.

Program state

Similarly, a computer program stores data in variables, which represent storage locations in the computer's memory. The contents of these memory locations, at any given point in the program's execution, are called the program's state.[5][6][7]

A more specialized definition of state is used for computer programs that operate serially or sequentially on

stateful protocol and the data carried over from the previous processing cycle is called the state. In others, the program has no information about the previous data stream and starts fresh with each data input; this is called a stateless protocol
.

visibility of the changes of state made by other parts of the program, known as side effects
.

In declarative programming languages, the program describes the desired results and doesn't specify changes to the state directly.

In

pure functional
subroutine only has visibility of changes of state represented by the state variables in its scope.

Finite state machines

The output of a sequential circuit or computer program at any time is completely determined by its current inputs and current state. Since each

finite state machine
, used to design both sequential digital circuits and computer programs.

Examples

An example of an everyday device that has a state is a television set. To change the channel of a TV, the user usually presses a channel up or channel down button on the remote control, which sends a coded message to the set. In order to calculate the new channel that the user desires, the digital tuner in the television must have stored in it the number of the current channel it is on. It then adds one or subtracts one from this number to get the number for the new channel, and adjusts the TV to receive that channel. This new number is then stored as the current channel. Similarly, the television also stores a number that controls the level of volume produced by the speaker. Pressing the volume up or volume down buttons increments or decrements this number, setting a new level of volume. Both the current channel and current volume numbers are part of the TV's state. They are stored in non-volatile memory, which preserves the information when the TV is turned off, so when it is turned on again the TV will return to its previous station and volume level.

As another example, the state of a

hard disk
, so it can be restored when the computer comes out of hibernation, and the processor can take up operations where it left off.

See also

  • Data (computing)

References

  1. ^ "What is stateless? - Definition from WhatIs.com". techtarget.com.
  2. ISBN 978-0123704979
    .
  3. ISBN 978-0521882675
    .
  4. ISBN 978-8120327856
    . Retrieved 7 December 2012. page 46
  5. ISBN 978-0849326912
    .
  6. ISBN 978-0387952062
    .
  7. ISBN 978-0132713603
    .
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