Polling (computer science)

This article is about the computer science term. For other uses, see Polling (disambiguation).

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 waiting, 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 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.

Polling has the disadvantage that if there are too many devices to check, the time required to poll them can exceed the time available to service the I/O device. Polling can be described in following steps:

1. The host repeatedly reads the busy bit of the controller until it becomes clear.
2. When clear, the host writes in the command register and writes a byte into the data-out register.
3. The host sets the command-ready bit (set to 1).
4. When the controller senses command-ready bit is set, it sets busy bit.
5. The controller reads the command register and since write bit is set, it performs necessary I/O operations on the device. If the read bit is set to one instead of write bit, data from device is loaded into data-in register, which is further read by the host.
6. The controller clears the command-ready bit once everything is over, it clears error bit to show successful operation and reset busy bit (0).

See also

• Pull technology
• Interrupt request
• Select (Unix)
• Kqueue