The simplest strategy for handling communication between the CPU and an I/O module is programmed I/O. Using this strategy, the CPU is responsible for all communication with I/O modules, by executing instructions which control the attached devices, or transfer data.
For example, if the CPU wanted to send data to a device using programmed I/O, it would first issue an instruction to the appropriate I/O module to tell it to expect data. The CPU must then wait until the module responds before sending the data. If the module is slower than the CPU, then the CPU may also have to wait until the transfer is complete. This can be very inefficient.
Another problem exists if the CPU must read data from a device such as a keyboard. Every so often the CPU must issue an instruction to the appropriate I/O module to see if any keys have been pressed. This is also extremely inefficient. Consequently this strategy is only used in very small microprocessor controlled devices.
NEXT STOP: I/O Concepts: Implementation Alternative: Interrupt Driven
