Programmed input and output is a way for the processor itself to move each byte to or from a disk controller. The processor writes a value, waits until the controller is ready, and writes the next. Nothing else runs on that processor while it is in the loop. Later methods let the controller move the block and interrupt the processor when the block is done.
The mode was used by Integrated Drive Electronics (IDE) disks. The processor did the copying. A later mode on the same disks let the drive write memory directly and freed the processor from the loop.
What the Processor Does in the Loop
- Wait. It reads a status bit until the controller can take another value.
- Move one value. It copies a byte or a word, then waits again.
- Nothing else. The loop holds the processor until the block is finished.
Processor Loop and Direct Memory Transfer
| Compared | Processor Loop | Direct Memory Transfer |
| Who copies the bytes | The processor | The controller |
| Other work meanwhile | Stalls | Continues |
| When the processor is told | It already knows. It is in the loop | An interrupt at the end of the block |