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Polling (computer science)
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Polling, or polled operation, in computer science, refers to actively sampling the status of
an external device by a client program as a synchronous activity. Polling is most often
used in terms of input/output (I/O), and is also referred to as polled I/O or software
driven I/O.
Polling is sometimes used synonymously with busy-wait polling (Busy waiting). In this
situation, when an I/O operation is required the computer does nothing other than check
the status of the I/O device until it is ready, at which point the device is accessed. In other
words the computer waits until the device is ready. Polling also refers to the situation
where a device is repeatedly checked for readiness, and if it is not the computer returns to
a different task. Although not as wasteful of CPU cycles as busy-wait, this is generally
not as efficient as the alternative to polling, interrupt driven I/O.
In a simple single-purpose system, even busy-wait is perfectly appropriate if no action is
possible until the I/O access, but more often than not this was traditionally a consequence
of simple hardware or non-multitasking operating systems.
This section may stray from the topic of the article. Please help improve this section
or discuss this issue on the talk page.
Polling is often intimately involved with very low level hardware. For example, polling a
parallel printer port to check whether it is ready for another character involves examining
as little as one bit of a byte. That bit represents, at the time of reading, whether a single
wire in the printer cable is at low or high voltage. The I/O instruction that reads this byte
directly transfers the voltage state of eight real world wires to the eight circuits (flip
flops) that make up one byte of a CPU register.
[edit] See also
Pull technology
Interrupt request
This computer science article is a stub. You can help by expanding it.
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This page was last modified on 29 April 2009, at 16:07 (UTC).
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