UGC NET CSE | December 2015 | Part 3 | Question: 5

Question

A DMA controller transfers 32-bit words to memory using cycle Stealing. The words are assembled from a device that transmits characters at a rate of 4800 characters per second. The CPU is fetching and executing instructions at an average rate of one million instructions per second. By how much will the CPU be slowed down because of the DMA transfer?

• A. 0.06%
• B. 0.12%
• C. 1.2%
• D. 2.5%

Answer 1

1 character=1 byte

32 bit word= 4byte

4800 bytes in 1 sec

so for 1 byte  it takes 1/4800 sec

so for 4 byte word it takes  1/1200 sec

so 1200 words are transfered in 1 sec through cycle stealing

and given 1 million instructions are fetching and executing in 1 sec out of which 1200 words are executing through cycle stealing so

%of slow down or cycle wasted in dma transfer = (1200/1000000)*100

=0.12%

option b)