# Suppose the processor scans the keyboard every 0.5 second. How many times will the keyboard be checked in a 1-hour interval?

system is controlled by an operator through commands entered from a keyboard. The average number of commands entered in a 1-hour interval is 80.

1. Suppose the processor scans the keyboard every 0.5 second. How many times will the keyboard be checked in a 1-hour interval?

1. By what fraction would be the number of processor visits to the keyboard be reduced if interrupt-driven I/O were used? Calculation of the fraction: if the interrupt-driven I/O requires x number of visits, while the processor requires y number of visits in (a), the fraction is calculated: 1 – (x/y).

# Problem 2. Interrupt-driven I/O ()

Consider a system employing interrupt-driven I/O for a particular device that transfers data at an average of 2000 bytes/sec on a continuous basis.

1. Assume that interrupt processing takes about 200 usec (i.e., the time to jump to the interrupt service routine (ISR), execute it, and return to the main program). Determine what fraction of processor time is consumed by this I/O device if it interrupts for every byte.

1. Now assume that the device has two 8-byte buffers and interrupts the processor when one of the buffers is full. Naturally, interrupt processing takes longer because the ISR must transfer 8 bytes. While executing the ISR, the processor takes about 10 usec for the transfer of each byte. Determine what fraction of processor time is consumed by this I/O device in this case.

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