Книга: Introduction to Microprocessors and Microcontrollers

How do we measure the speed of a microprocessor?

This is a lot more difficult than we think because the developers of microprocessors are in competition with each other so as soon as a method is suggested, they try to exploit the situation to present their microprocessor as faster than all the others.

Be wary when reading comparisons – which tests have they chosen, and why? While watching the last Olympic games, it occurred to me that I was probably faster that any of those competing in the 100 metres. Yes, I felt confident that I could build a working microprocessor-based system quicker than any of the athletes. You see, comparisons all depend on the test that we have decided to use. Anyone can be World champion. It’s only a matter of choosing the tests well enough. With that in mind, here are a few popular speed comparisons.

MIPS (millions of instructions per second)

This appears an easy measurement to take. It is simply a matter of multiplying the number of clock cycles in a second by the clock cycles taken to complete an instruction.

The current Athlon for example can run at 2 GHz or 2000 MHz. It can perform up to 9 instructions per clock cycle so its number of instructions per second is simply 2?10⁹?9=18 000 million cycles per second.

Life is never that simple. Some instructions are more time consuming than others as they take a different number of clock cycles to perform the task. Competitors will obviously choose instructions that give the most impressive results on their own microprocessor.

An extreme example occurred about ten years ago with the Intel 80836, we could ask it to perform some additions. By consulting the instruction set, we could see that they each take two clock cycles to complete. Now, if we take a clock frequency of 25 MHz, each clock cycle would last for 40 ns so an ‘add’ instruction would take 80 ns. This would equate to a speed of 12.5 MIPS. An unkind competitor may take ‘at random’ the divide instruction that takes 46 clock cycles, they could reduce its MIPS rating to a measly 0.54 MIPS. A really vindictive person could search through the instruction set with a magnifying glass and find that there is a really obscure instruction that takes 316 clock cycles. This would provide a speed of 0.08 MIPS – about the same as a four-bit microprocessor.

We cannot even say that we can make it fair by using the same instruction for each microprocessor since they don’t always have their talents in the same area. If we had two microprocessors, each with a ‘load’ instruction taking five clock cycles and they both ran on a 10 MHz clock, what would be their speed? 10/5 = 2 MIPS. (In working this out we can just ignore the fact that the clock cycle is in megahertz since the speed is measured in millions of instructions.) But can they do the job at the same speed? Possibly or possibly not. What if one was a 64-bit microprocessor and the other was an 8-bit microprocessor. The 64-bit one could shovel data at eight times the speed. For good reason MIPS have been referred to as ‘Meaningless Indication of Performance by Sales reps’.

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