How the communication between CPU and IOP established?

The communication between CPU and IOP may take different forms depending on the particular computer considered. The sequence of operations during CPU and IOP communication is shown in figure.

• The CPU sends a test I/O instruction to IOP to test the IOP path.
• The responds by inserting a status Word in memory location.

· The CPU refers to the status Word in memory. If everything is in order, the CPU sends the start I/O instruction to start the I/O transfer.

• The IOP accesses memory for IOP program.

· The CPU can now continue with another program while the IOP is busy with the program. Both programs refer to memory by means of DMA transfer.

• When the IOP terminates the execution of its program, it sends an interrupt request to the CPU.
• The CPU then issues a read I/O instruction to read the status from the IOP.
• The IOP transfers the status Word to memory location.

· The status Word indicates whether the transfer has been completed satisfactorily or if any error has occurred during the transfer.