I/O Techniques

(1) Programmed I/O When a processor encounters an I/O instruction, it issues a command to the appropriate I/O module. The I/O module sets the appropriate bits in the I/O status register but does not alert the processor. The processor will need to check for the I/O completion periodically after the I/O instruction is executed. The processor is also responsible to transfer the data from the hardware buffer to memory. Processor has various I/O instruction to control the device (e.g. unwind a tape drive), test status and transfer data.

(2) Interrupt driven I/O The I/O module will interrupt the processor when the I/O completes. However, the processor is still required to transfer the data to memory. There are 2 drawbacks: the I/O transfer rate is limited by the speed the processor can test and service a device. The processor is also tied up in managing I/O transfer

(3) Direct Memory Access Interrupted driven I/O is not efficient when a large amount of data are to be transferred. DMA is performed by a separate module on the system bus. Processor issues a command to the DMA module with information such as operation (READ/WRITE) required, address of the I/O device, starting address of the memory location and number of words to be moved. The DMA module will transfer the data directly, one word at a time. When the transfer is completed, DMA module alerts the processor. As the DMA module needs to take control of the bus to transfer data, it may contend with the processor for the use. The processor will wait for one bus cycle when DMA is using the bus. However, no context switch is incurred. Overall, DMA is more efficient when transfer multi-words I/O.