1. An I/O device whose maximum bandwidth is 100,000 bits/sec is connected to
a computer system. The I/O controller interrupts the CPU whenever it receives
a byte. The interrupt handling routine consists of 100 instructions. The CPU
executes 10 million instructions per second on the average. What percentage
of the CPU's time is used to serve the I/O device, asuming that the device
transmits data with the maximum speed?
(a)1.25% (b)2.5% (c)12.5% (d)25%
答案是(C)
100,000 bytes
我的算法是 100,000 bits/sec = ------------- --> 8 * 10^(-5) sec/byte
8 sec
100 instructions
--------------------------- = 10^(-5) sec
10 * 10^6 instructions/sec
10^(-5) 1 1
所以答案是 --------------------- = --------- = --- = 0.111
10^(-5) + 8*10^(-5) 1 + 8 9
可是答案看起來好像是 1/8 = 0.125
也就是說傳輸的時候 interrupt handling routine 也同時在進行嗎@@?
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