Differencing Between Myths and Facts? This time with examples...

In our example performance rating (PR) model, you can get an idea of what you can truly expect, performance-wise, from a processor by combining two important factors—clock cycles and IPC.

Athlon XP 2400+:               9 IPC, 2000MHz
Intel Pentium 4 2.4GHz:     6 IPC, 2400MHz

If you multiply IPC by clock rate, you can arrive at a generalized PR and conclude that the Athlon XP 2400+ has a PR of 18,000 (also number of instructions executed per total-cycle) compared to the 14,400 PR of the Intel Pentium 4 2.4GHz processor. As a result, you can expect the Athlon XP 2400+ models to perform somewhat better in general usage. However, media applications have tended to favor the Pentium’s longer-pipelined architecture and thus, the PR rating cannot be effectively used to reach an appropriate decision.

Another flaw of our example — which is only used for generalized purposes — is that it doesn’t take into account the possibility of differing cores with different architectures, and thus, you can expect a noticeable difference in the performance rating of each.

You must never look only at the combined GHz and MHz ratings of a particular microprocessor, but instead, should compare the number of clock cycles per second (GHz/MHz) with the IPC and architecture in order to make a decision of purchase. As of now, you have several types of modern processors to choose from: Pentium 4, Celeron, Celeron D, Pentium M, Athlon XP, Athlon 64, Athlon FX, and Sempron. The Intel Pentiums (with the exception of Pentium M) and Celerons tend to have more stages in their pipelines than the Athlons and Semprons, thus depending more on their clock speed to "keep up" with lower clocked Athlons and Semprons. The Pentium M has a similar amount of pipeline stages as the Athlons and Semprons. The decision on which processor to use will probably be founded upon the main application you intend to use it for. Just remember, gigahertz and megahertz aren’t everything!