Programmed I/O

Programmed I/O

• I/O operations will mean a data transfer between an I/O device and memory or between an I/O device and the processor.

• If in any computer system I/O operations are completely controlled by the processor, then that system is said to be using 'programmed I/O'.

• When such a technique is used, processor executes programs that initiate, direct and terminate the I/O operations, including sensing device status, sending a read or write command and transferring the data.

• It is the responsibility of the processor to periodically check the status of the I/O system until it finds that the operation is complete.

Example

• The processor's software checks each of the I/O devices every so often. During this check, the processor tests to see if any device needs servicing. Fig. 12.2.1 shows the flowchart for this.

• This is a simple program which services I/O ports A, B and C. The routine checks bag the status of I/O ports in proper sequence.

• If the request bit is set, the corresponding service routine is called to complete the I/O request.

• This test and service procedure continues until all the I/O port status registers are tested and all the I/O ports requesting service are serviced. Once this is done, the processor continues to execute the normal programs.

• The routine assigns priorities to the different I/O devices. Once the routine is started, the service request bit at port A is always checked first. Once port A is checked, port B is checked, and then port C. However, the order can be changed by simply changing the routine and thus the priorities.

Review Questions

1. What do you mean by programmed I/O ?

2. Write short notes on polling.