A computer is required to accept characters from 20 video terminals. The main memory
area to be used for storing data for each terminal is pointed to by a pointer PNTR
n, where n=1 through 20. Input data must be collected from the terminals while another
program PROG is being executed. This may be accomplished in one of two ways:
(a) Every T seconds, program PROG calls a polling subroutine POLL. This subroutine
checks the status of each of the 20 terminals in sequence and transfers any input
characters to the memory. Then it returns to PROG.
(b) Whenever a character is ready in any of the interface buffers of the terminals, an
interrupt request is generated. This causes the interrupt routine INTERRUPT to
be executed. After polling the status registers, INTERRUPT transfers the input
character and then returns to PROG.
Write the routines POLL and INTERRUPT using either pseudo code or the assembler
language of the processor of your choice. Let the maximum character rate for any
terminal be c characters per second, with an average rate equal to rc, where r ≤
1. In method (a), what is the maximum value of T for which it is still possible to guarantee that no input characters will be lost? What is the equivalent value for method (b)? Estimate, on the average, the percentage of time spent in servicing the terminals for
methods (a) and (b), for c=100 characters per second and r =0.01, 0.1, 0.5, and 1.
Assume that POLL takes 800 ns to poll all 20 devices and that an interrupt from a
device requires 200 ns to process.
In instructor manual Value for T is given as 100 times persecond for solution a but i am not able to figure it out why.!! Can anyone explain me the solution.Thanks in advance.
