Intel's Centrino CPU (Pentium-M): Revolutionizing the Mobile World

Pentium III Execution Power

There's a difference between making a derivative microprocessor architecture and not re-inventing the wheel; we've already established that Banias is significantly different from the Pentium III, but the Israel design team didn't just synthesize the CPU from scratch in order to be genuinely different than the Pentium III.

Much like the Athlon 64 shares the same execution units as the Athlon XP, the Banias has the same execution unit layout as the Pentium III. This means that Banias features no more than 5 execution ports, significantly less than what we're used to seeing from the Pentium 4 for example.

By sticking with a relatively narrow execution layout, Intel continues to limit the power consumption of the Banias core; as our IPC comments from the previous section indicate however, the limited number of execution resources won't hold back performance. What the limited number of execution resources does mean however is that Hyper-Threading will not "work" on Banias.

Remember that Hyper-Threading works based on the idea that there are enough execution units in the Pentium 4 and enough pauses in the pipeline that there are execution units that remain idle during normal operation. With Banias' short pipeline, micro-ops fusion and relatively narrow architecture (read: lack of a large amount of execution units in parallel), Hyper-Threading won't find much use because of the lack of periods of idle execution.

So, no Hyper-Threading, big deal right? It would have been easy to shrug off if it hadn't been for Pat Gelsinger mentioning that "threading" was the future of Intel's microprocessor design efforts at dinner a year ago. By "threading" Pat was referring to being able to execute multiple threads on a single microprocessor, the most familiar way these days being through the use of Hyper-Threading, a technology that dispatches multiple instruction streams from different threads through the same pipeline and down to the same execution units.

We've already discussed why this won't work for Banias, so what will? Although it will be a while before we see this, the idea would be to have a multi-core die, with some sharing of execution units and/or caches in order to keep die size down to a minimal. We won't get too off track with talking about the future of Banias, but for now it's something to keep in the back of your mind for the years to come…