For programmed I/O, Figure 7.5 indicates that the processor is stuck in a wait loop doing status checking of an I/O device. To increase efficiency, the I/O software could be written so that the processor periodically checks the status of the device. If the device is not ready, the processor can jump to other tasks. After some timed interval, the processor comes back to check status again.
a. Consider the above scheme for outputting data one character at a time to a printer that operates at 10 characters per second (cps).What will happen if its status is scanned every 200 ms?
b. Next consider a keyboard with a single character buffer. On average, characters are entered at a rate of 10 cps. However, the time interval between two consecutive key depressions can be as short as 60 ms. At what frequency should the keyboard be scanned by the I/O program?

7 6 5 4 3 2 1 0 516 Keyboard input data register 7 6 5 4 3 2 1 0 Keyboard input status and control register 517 1 ready 0 - busy Set to 1 to start read ADDRESS 200 INSTRUCTION OPERAND Load AC Store AC Load AC Branch if Sign -0 202 Load AC COMMENT Load accumulator Initiate keyboard read Get status byte Loop until ready Load data byte 517 517 202 516 (a) Memory-mapped vO ADDRESS INST RUCTION OPERAND COMMENT Initiate keyboard read Check for completion Loop until complete Load data byte 200 201 Load IO Test LVO Branch Not Ready In 201 (b) Isolated LO Figure 7.5 Memory-Mapped and Isolated LO