How Much Faster Is the P6?

Intel says the P6 is about twice as fast as the Pentium. But Intel is comparing the estimated performance of a 133-MHz P6 (200 SPECint92) to that of a 100-MHz Pentium (112.7 SPECint92). Is that fair?

"What you have to compare is given the same process technology, how one microarchitecture technology compares to the other one," says Lew Paceley, P6 marketing director. "A 100-MHz Pentium processor compares to a 133-MHz P6. Why? Because the P6 is superpipelined, so its clock rate is higher. On any given process technology, the P6 will run faster."

The P6's pipeline has up to 14 stages, compared to the Pentium's five stages. Yet the P6 can move instructions through its pipeline faster because the stages have shorter latencies. Of course, the P6 has many other advantages as well, such as three-way pipelines, a closely coupled secondary cache, a transactional I/O bus, more execution units, and so on. These are exactly the kind of features that distinguish one microarchitecture from another.

Intel acknowledges it is possible to fabricate a 133-MHz Pentium on the same process technology used for the P6 (0.6-micron, four-layer metal BiCMOS). Last year, in fact, Intel presented a paper at the International Solid-State Circuits Conference that described a 150-MHz Pentium on this process. Whether such a chip ever becomes a commercial product is another question, but Intel is known to have a 120-MHz Pentium that may show up in products soon and is rumored to have a 133-MHz version.

In any scientific experiment, the goal is to control all the variables except the one you're testing. If you compare two microprocessors that implement the same general architecture on the same process technology and both chips are running the same benchmark program at the same clock speed, any difference in performance must be due to the relative efficiencies of their microarchitectures.

The estimated performance of a 133-MHz Pentium is 150 SPECint92. Therefore, a P6 running at the same clock speed is about one-third faster, not twice as fast. Nevertheless, Intel is correct in saying that on any given process technology it's possible to make a faster P6. So if Intel ever does produce a 133- or 150-MHz Pentium on its 0.6-micron process, the P6 could be clocked at about 200 MHz--and it would be about twice as fast as the 133- or 150-MHz Pentium.