# CO-Grand Test 1-Q14

A computer consists of a processor and an I/O device D connected to main memory M via a shared bus with a data bus width of one word. The processor can execute a maximum of $10^7$ instructions per second. An average instruction requires five machine cycles, three of which use the memory bus. A memory read or write operation uses one machine cycle. Suppose that the processor is continuously executing “background” programs that require 99% of its instruction execution rate but not any I/O instructions. Assume that one processor cycle equals one bus cycle. Now suppose the I/O device is to be used to transfer very large blocks of data between M and D. If programmed I/O is used and each one-word I/O transfer requires the processor to execute two instructions, estimate the maximum I/O data-transfer rate, in words per second, possible through D.

reshown Jun 26, 2019

+1 vote

The processor can execute maximum = 10^7 instructions/sec

background programs require execution rate but not I/O = 99%

so I/O program requires = 1%

so,

10000000 X 0.01 = 100000 instructions/sec

but each word requires the processor to execute 2 instructions

1 word $\longrightarrow$ 2 instructions

1 instructions $\longrightarrow$ 0.5 instructions

Hence, 100000X 0.5 = 50000 words/sec

answered Jul 2, 2019 by (90,770 points)

The processor can execute maximum = $10^6$ instructions/sec

background programs require execution rate but not I/O = 99%

so I/O program requires = 1%

so,

1000000 X 0.01 = 100000 instructions/sec

but each word requires the processor to execute 2 instructions

1 word $\longrightarrow$ 2 instructions

1 instructions $\longrightarrow$ 0.5 instructions

Hence, 100000X 0.5 = 50000 words/sec

answered Jun 26, 2019 by (7,230 points)
edited Jul 2, 2019