Processor performance can determine easily whether a computer product is a commercial success or a failure, and yet it is frustratingly hard to measure. Imagine the performance of several cars must be rated, giving each one a single number as a measure of its performance. Top speed could be used as a measure of performance, but consumers would likely complain that they don't often have the opportunity to drive at over a 100 miles per hour (mph). The lowest time accelerating from 0 to 60 mph might be a more meaningful measure of speed drivers would care about. Alternatively all the cars could be driven through an obstacle course and their times compared to see which was the fastest under more realistic conditions. Which car rated as the fastest could easily turn out differently depending on which measure was chosen, and the manufacturers of the other cars would inevitably complain that this rating system was unfair. Measuring processor performance has the same types of problems.

One of the earliest metrics of computer performance was determined by measuring millions of instructions per second (MIPS). The MIPS rating was simple and easy to understand. Unfortunately not all instructions require the same amount of computation. A million adds in one second are very different from a million branches. When DEC launched the VAX-11/780 computer in 1977, it was sold as a "1 MIPS" computer because it had similar performance to an IBM computer that was being marketed as a "1...