To execute an I/O-related instruction, the processor issues an address, specifying the particular I/O module and external device, and an I/O command. There are four types of I/O commands that an I/O module will receive when it is addressed by a processor –
- Control : Used to activate a peripheral device and instruct it what to do. For example, a magnetic tape unit may be instructed to rewind or to move forward one record. These commands are specific to a particular type of peripheral device.
- Test : Used to test various status conditions associated with an I/O module and its peripherals. The processor will want to know if the most recent I/O operation is completed or any error has occurred.
- Read : Causes the I/O module to obtain an item of data from the peripheral and place it in the internal buffer.
- Write : Causes the I/O module to take an item of data ( byte or word ) from the data bus and subsequently transmit the data item to the peripheral.