I/O Processors

I/O PROCESSORS.

IO processor is

• A specialized processor

• Not only loads and stores into memory but also can execute instructions, which are among a set of I/O instructions

• The IOP interfaces to the system and devices

• The sequence of events involved in I/O transfers to move or operate the results of an I/O operation into the main memory (using a program for IOP, which is also in main memory)

•  Used to address the problem of direct transfer after executing the necessary format conversion or other instructions

•  In an IOP-based system, I/O devices can directly access the memory without intervention by the processor

IOP instructions

• Instructions help in format conversions─ byte from memory as packed decimals to the output

device for line-printer

• The I/O device data in different format can be transferred to main memory using an IOP

Sequence of events when using an I/O Processor

Sequence 1:

A DRQ (for IOP request) signal from an IOP device starts the IOP sequence, the IOP signals an interrupt on INTR line this requests attention from the processor

Sequence 2:

The processor responds by checking the device’s status via the memory-mapped control registers and issues a command telling the IOP to execute IOP instructions for the transfer to move the formatted data into the memory.

Sequence 3:

During each successive formatted byte(s) transfer, the device DMAC (DMA controller) logic inside the IOP uses a processor bushold request line, HOLD, distinct from INTR device interrupt request line

• The main processor sends to the device a signal from the processor called DACK (distinct from

INTA device-interrupt request-acknowledge line)

• The I/O device bus has access to the address and data buses of the memory bus when DACK is activated

•  It has no access when DACK is not activated when a HOLD request is not accepted by the processor when the processor is using the memory bus

•  Once the DMA logic start command has been issued to IOP, the main processor begins working on something else while the I/O device transfers the data into the memory

Sequence 4:

When the IOP’s DMA transfer as per instructions is complete, the I/O device signals another interrupt (using DRQ) . Lets the main processor know that the DMA is done and it may access the data