By Tom R. Halfhill

It's hard enough explaining the differences between multitasking, multiprocessing, multithreading, Hyper-Threading, single-core processors, multicore processors, and multiprocessor systems. Now Intel comes along and throws us another curve ball by announcing multicore processors that are actually multichip modules.

Intel recently announced a bunch of multicore processors with typically obscure code-names. Smithfield, officially christened Pentium D, has two Pentium 4-class processor cores on a single die. It’s a true multicore processor. But when Intel moves to the next-generation 65-nanometer chip-fabrication process, the company will introduce a new desktop processor code-named Presler that encloses two die in one chip package. Presler will look like a multicore processor to the operating system, but inside, it will actually have only one core per die. That's not quite a true multicore processor.

There's a common term for a processor like Presler: multichip module (MCM). An MCM is simply a chip package containing two or more die. Old-timers might recall that Intel's Pentium Pro, introduced in 1995, was also an MCM. It enclosed a processor chip and an SRAM chip in a single package. The SRAM was an external L2 cache, because in those days it was too expensive to integrate a large L2 cache on the same die with the processor. But today, it's obviously not too expensive to integrate two processor cores on a single die. So why is Presler seemingly taking a step backward?

First answer: economy. The 65nm process shrink will dramatically reduce Intel's manufacturing cost for a single-core chip. MCMs are more expensive to produce, but Intel has made some advances in this technology since the Pentium Pro. Therefore, it will be cheaper for Intel to stuff two Presler-class dies into a single package than to make a dual-core Presler-class die, even with the additional cost of the MCM package.

Second answer: flexibility. By designing Presler with one core per die, Intel can make a lower-priced Celeron version of Presler with only one die per package. This would be even better for Intel, because the company wouldn't have to disable part of Presler's L2 cache to make a lower-end processor (as Intel does today with Celeron), and the single-die package would eliminate the extra cost of the MCM.

So Intel's multicore strategy certainly makes sense. And most people will call Presler a multicore processor without ever knowing the difference.

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