Megahertz Myth

The megahertz myth, or less commonly the gigahertz myth, refers to the misconception of only using clock rate to compare the performance of different microprocessors. While clock rates are a valid way of comparing the performance of different speeds of the same model and type of processor, other factors such as pipeline depth and instruction sets can greatly affect the performance when considering different processors. For example, one processor may take two clock cycles to add two numbers and another clock cycle to multiply by a third number, whereas another processor may do the same calculation in two clock cycles. Comparisons between different types of processors are difficult because performance varies depending on the type of task. A benchmark is a more thorough way of measuring and comparing computer performance.

The myth started around 1984 when comparing the Apple II with the IBM PC. The argument was that the PC was 5x times faster than the Apple, as its Intel 8088 processor had a clock speed roughly 5x the clock speed of the Rockwell 6502 used in the Apple. However, what really matters is not how finely divided a machine's instructions are, but how long it takes to complete a given task. Consider the LDA # (Load Accumulator Immediate) instruction. On the 6502, which runs at 1 MHz, that instruction requires 2 clock cycles, or 2μs. Although the 4.77 MHz 8088's clock cycles are shorter, the LDA # needs 25 of them, so it takes 25/(4.77 x 106) = 5.24μs. For that instruction the Intel machine runs 2.62 times slower than the Rockwell.