Need for I/O processor (IOP)

An input-output processor (IOP) is a processor with its own instruction set and control unit that handles all I/O transfers, so the CPU is free for computation. It is needed because:

• With programmed or interrupt-driven I/O the CPU is busy for every transfer or interrupt, and the system slows down when there are many devices.
• A single DMA controller handles only simple block transfers. An IOP can do more: format conversion, error checking, code conversion, and chained transfers described by an I/O program.
• Devices differ in speed, word format and control. The IOP hides these differences and gives the CPU one simple interface.
• The IOP works in parallel with the CPU, so the overall throughput and the CPU utilisation rise.
• Many peripherals can be attached to one IOP (a channel), reducing the load on the CPU and the system bus.

Block diagram

        +---------+          +-------------------+
        | Memory  |<-------->|   Memory bus      |
        |  unit   |          +----+---------+----+
        +---------+               |         |
                              +---v---+  +--v----+
                              |  CPU  |  |  IOP  |
                              +-------+  +--+----+
                                            | I/O bus
                          +---------+-------+---------+
                          v         v                 v
                      Keyboard    Printer           Disk
                                  (peripherals)

The CPU and the IOP share the memory through the memory bus. The IOP has a separate I/O bus to the peripherals. The CPU starts an I/O program and goes on; the IOP fetches the I/O instructions from memory, transfers data directly to or from memory (DMA) and interrupts the CPU at the end.