Everything about Polling Computer Science totally explained
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
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. 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's 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 isn't 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.
Polling is often intimately involved with very low level hardware. For example, polling a parallel printer port to check whether it's 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.
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