input/output (I/O)

A computer that only had a CPU and memory would be a brain in a sealed jar: it could think, but it could not see, hear, speak, or remember anything past the moment the power went off. Input/output, or I/O, is everything that connects the machine to the world beyond the processor and main memory — the keyboard and mouse that bring data in, the screen and speakers that send results out, the network that talks to other machines, and the disks that hold data when the lights go off. I/O is the machine's senses and its long-term memory.

Concretely, an I/O operation moves data between the CPU or memory and some device. The CPU does not poke at a disk platter or a network cable directly; instead each device sits behind a small chunk of hardware called a controller, and the CPU reads and writes that controller through a few special locations called device registers (see device-controller and memory-mapped-io). A typical exchange is: tell the controller what to do (read this block, send this packet), wait for the device to finish, then collect the result. Because real devices are slow compared with the CPU, the interesting questions are about how the processor spends the waiting time — by polling, by interrupts, or by handing the whole transfer to DMA.

The crucial mental shift for a beginner is that I/O is judged on three axes, not just raw speed. There is throughput (how many megabytes per second, or how many operations per second, called IOPS); there is latency or response time (how long one request takes from start to finish); and there is dependability (does the data come back correct, and does it survive a crash or a failed disk). A faster disk that silently loses your files is worse than a slower one that never does. And honestly, I/O is very often the real bottleneck: a program that waits on disk or network can leave a multi-gigahertz CPU mostly idle.