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?