Question

QUESTION 1. [14 marks] a) What is the main difference between programmed I/O and interrupt I/O? [4] b) During the initialization of programmed I/O, a test 1/0 command is issued by CPU. Explain the function of test I/O command. [4] c) What is the advantage of direct memory access (DMA) compared to programmed I/O? [4] d) When a DMA module takes control of a bus, and while it retains control of the bus, what does the processor do? [2]

Answer 1

(1)

Programmed I/O

On a basic level Programmed I/O is a method of transferring data between the Processor and other peripheral devices connected, where instruction running in the CPU performs an operation of data transferring from and/or to an I/O device. In this case the Processor issues a command and then waits for it to execute. Then it waits for the I/O device to complete the request and respond. Processor needs to keep check the status of the I/O device from time to time (this is called Polling). The important thing to consider here is that processor is way more faster than the I/O module and thus have to wait for a long time to respond.

Programmed I/O is the simplest I/O technique for exchanging data between processor and other external device.

In Programmed I/O processor executes a program that gives the direct control of I/O operation. Processor issue a command to the I/O module and wait until the operation is complete. Processor will periodically check the status of I/O module until it find that the operation is complete. If the processor is faster than I/O module , the processor time is waste .

It module is treated as slow module.

It used only in some low-end microcomputers.
* It has single input and single output instruction.
* Each instructions selects one I/O device (by number) and transfers a single character (byte)
* Four registers: input status and character, output status and character.

Interrupt-driven I/O-

In case of interrupt-driven I/O, processor after requesting the I/O module proceeds with its normal work. After I/O device has completed the request, it notifies the processor using an interrupt. This relieves the processor from checking again and again whether I/O device has completed the request. But, still this is not very effective technique, especially when data needs to be transferred in large quantity. In this case processor has to transfer data word by word between I/O module and memory. This leads us to another approach called Direct Memory Access (DMA).

Interrupt I/O is something similar to Programmed I/O technique.

At this technique the processor do not wait for until the I/O operation is complete. Rather processor normally do the other task. When I/O is complete the I/O module interrupt into processor . Interrupt means that the operation is completed.

It module is faster than programmed i/o module.

Primary disadvantage of programmed I/O is that CPU spends most of its time in a tight loop waiting for the device to become ready. This is called busy waiting.
* With interrupt-driven I/O, the CPU starts the device and tells it to generate an interrupt when it is finished.
* Done by setting interrupt-enable bit in status register.
* Still requires an interrupt for every character read or written.
* Interrupting a running process is an expensive business (requires saving context).
* Requires extra hardware (DMA controller chip).

(2)test  I/O is an alternative scheme dealing with I/O. Interrupt I/O is a way of controlling input/output activity whereby a peripheral or terminal that needs to make or receive a data transfer sends a signal. This will cause a program interrupt to be set. At a time appropriate to the priority level of the I/O interrupt. Relative to the total interrupt system, the processors enter an interrupt service routine. The function of the routine will depend upon the system of interrupt levels and priorities that is implemented in the processor.

(3)

DMA saves lots of CPU time so that CPU can have more time to execute CPU-bound tasks.

In a simple computer architecture, CPU and IO devices are linked with a bus. if CPU wants to get data from IO device, it will send a message to IO device via bus and wait for its response. During this time, CPU can do nothing except waiting for data. This method seems to waste CPU resource a lot and one solution of this problem is DMA.

In a computer architecture with DMA, CPU, DMA and memory are linked with a bus. And DMA will have an exclusive IO bus connected with IO devices. In order to get data from IO device, CPU just need to send a message to DMA and do any other task instead of waiting. After DMA receives the message, it first fetchs data from IO device and then put data in memory. Finally, it sends an interrupt to CPU for telling him the IO operation is finished, and CPU gets that IO data from memory after receiving the interrupt.

Writing data to IO device is similar to reading, and DMA can reduce the CPU waiting time of both IO operations.

(4)

The processor either doesn't need to use the bus at the time, or it is forced to suspend operation temporarily. The processor deals with other things, while the bus operation is left to DMA.

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