Earlier today on Twitter, AMD engineer Phil Park identified a curious nugget of PC architectural history from, of all places, a year-old Quora answer posted by former Intel engineer Robert Colwell. The nugget indicates that Intel could have beaten AMD to the x86-64 punch if the former wasn't dead-set on the x64-only Itanium line of CPUs. The historical context of this story and the comments within are fascinating, especially considering today's desktop CPU architectures, where "64-bit" is immediately understood to mean x86-64 — because who wants a computer that can't natively run most applications made for a PC?
I was looking up some history behind the x86-64 transition, particularly around the Pentium 4 time frame, and I found out that Bob Colwell (Pentium Pro chief architect) has been posting on Quora.Pentium 4 had a version of x86-64 that was fused off. pic.twitter.com/eYWLpD0qoWOctober 18, 2024
To Intel's credit, it isn't that Intel Itanium processors completely did away with x86 software and architecture. However, they were intended to in the long run, especially as a measure of definitively striking down AMD in the marketplace. Unfortunately, this meant that the pure 64-bit architecture of Intel Itanium did not allow 32-bit (x86) applications to run natively, and the emulation solutions performed poorly. As a result, Itanium landed with a thud in the market despite being among the first to the 64-bit punch. Backward compatibility matters a great deal for big software and hardware purchases of any kind, but especially in the PC market. This fact was especially true in the server and enterprise markets where Intel targeted the Itanium CPUs.
As Intel's former chief x86 architect, Colwell knew this to be true, and thus designed the earliest internal versions of Pentium 4 CPUs to be x86-64 instead of mere x86 chips. Unfortunately, this design path was axed by the higher-ups at Intel, who clearly feared it would eat into or harm Itanium (much in the same way that AMD's debut x86-64 chips would end up doing anyway). In the face of repeated firing threats, the knowledgeable Colwell left the gates for 64-bit in Pentium 4, but fused off the functionality to make the inevitable return to x86 easier for Intel when the time came.
However, the story of Intel Itanium doesn't quite end at the launch of AMD64/x86-64 in 2003 despite the obvious chilling effect. Itanium still kept support and was iterated on until February 2017, and shipments didn't completely halt until July 2021. For some fringe scenarios, Itanium was very performant and even ideal.
But as Colwell knew, even back then, backward compatibility is king, especially on the PC, and especially in the enterprise. And don't even get me started on what consumers are willing to do for some good backward compatibility.
