polling

Polling is the act of repeatedly asking 'are you ready yet?' Imagine a child in the back seat asking 'are we there yet?' every ten seconds for the whole trip. In computing, polling means the CPU keeps reading a device's status register in a loop, checking the same flag over and over, until the device says it is done. It is the waiting strategy used by programmed I/O.

Concretely, a polling loop looks like: read the status register; if the ready bit is not set, loop back and read it again; once it is set, do the transfer. There is a small handshake protocol behind this — the controller has a busy bit it sets while working and a done/ready bit it sets when finished, and the CPU watches those bits. A single read of a status register is cheap (often just a few instructions). The cost is doing that read thousands or millions of times while a slow device crawls toward done, which is called busy-waiting: the CPU is busy, but accomplishing nothing.

Why it matters: polling is dirt simple, has very low latency for a device that responds almost instantly, and avoids the overhead of setting up interrupts — so it is genuinely the right choice for very fast devices or when you will wait only a handful of cycles. But for slow devices it wastes enormous CPU time, and it cannot easily do anything else meanwhile. That trade-off is the whole reason interrupts were invented: instead of the CPU asking again and again, the device taps the CPU on the shoulder only when it is actually ready.