A DMA controller transfers 32-bit words to memory using cycle Stealing. The…
2015
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%
Attempted by 125 students.
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Correct answer: B
Answer: 0.12%
Compute how many 32-bit words are formed per second: 4800 characters/s ÷ 4 characters per 32-bit word = 1200 words/s.
Each word transfer steals one bus cycle (one CPU instruction time) under cycle stealing, so DMA steals 1200 CPU instruction-times per second.
CPU executes 1,000,000 instructions per second, so the slowdown fraction = 1200 ÷ 1,000,000 = 0.0012 = 0.12%.
Note: If the DMA required more than one bus cycle per word, multiply the 1200 stolen cycles by that number and recompute the percentage accordingly.
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