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I'm reading through William Stalling's operating system intrnals and design principles book. Talking about interrupts, it gives the following examples when comparing the speed of a processor and a hard disk:

To give a specific example, consider a PC that operates at 1 GHz, which would allow roughly 10^9 instructions per second. A typical hard disk has a rotational speed of 7200 revolutions per minute for a half-track rotation time of 4 ms, which is 4 million times slower than the processor.

My question is how was the result calculated? How did we know that the hard disk is 4 million times slower?

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This is to some extent a comparison between apples and oranges. The assumption is that the basic operation on a CPU takes 1 ns (since the clock frequency is 1 GHz), whereas a basic operation on the hard disk takes 4 ms. The quoted factor is just 4 ms / 1 ns = 4×106.

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  • $\begingroup$ And the 4 ms is the average time for the disk to spin around to where the heads are located (7200 rpm / 60 seconds/minute=120 revolutions per second or 8ms per revolution) $\endgroup$
    – Brian Borchers
    Commented May 7, 2015 at 4:24

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