Communications of the ACM
Microprocessor Architectures Follow Markets and Silicon
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The viewpoint "The Value of Microprocessor Designs" by Ana Aizcorbe et al. (Feb. 2013) aimed to analyze the value of microarchitectures in isolation, as though they could be mixed-and-matched with various silicon implementation technologies over the years. This is a nonsensical proposition. For example, a Pentium III microarchitecture could not have been realized in 0.8u technology due to insufficient transistors. A 486 microarchitecture could be instantiated on, say, 90nm technology but would have been too slow to be competitive. And the design trade-offs baked into the 486 pipeline, appropriate for the silicon of the 1980s, would not leverage the much larger transistor budgets of the 1990s and later.
These microarchitectures were not independent of one another, as Aizcorbe et al. implicitly assumed. The Pentium III was the same microarchitecture as the Pentium II but with SSE instructions added. Moreover, both came from the original Pentium Pro P6 microarchitecture. The Pentium-M was also a descendant of the P6. The Pentium 4 microarchitecture was substantially different, but, as chief architect of both P6 and Pentium 4, I can testify that the Pentium 4 was not unrelated to P6.
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