Modes of Transfer in Computer Architecture

Data transfer between the central computer and I/O devices may be handled in a variety of modes. three possible modes:

1. Programmed I/O: The operations are the result of I/O instructions written in the computer program. Each data item transfer is initiated by an instruction in the program. The CPU stays in a program loop until the I/O unit indicates that it is ready for data transfer. This is a time-consuming process since it keeps the processor busy needlessly.

2. Interrupt-initiated I/O: It can be avoided by using an interrupt facility and special commands to inform the interface to issue an interrupt request signal when the data are available from the device. In the meantime, the CPU can proceed to execute another program.

3. Direct memory access (DMA): The interface transfers data into and out of the memory unit through the memory bus. The CPU initiates the transfer by supplying the interface with the starting address and the number of words needed to be transferred and then proceeds to execute other tasks.