64-bit : When Intel followed AMD's lead, or Worse is Better

Also see: Betamax versus VHS

We all know how the x86 was such a success.

x86 is 32-bit.  There was a 16-bit in the early 90s, but that was quickly moved along to 32-bit by the time the Web became a thing that made home computers REALLY popular.

So it was almost a fait acompli that we'd soon move on to 64-bit.

But that transition did not go so smoothly.

Intel had traditionally been the company that established the ISA (Instruction Set Architecture) standards which the competitors (well, AMD) then followed.

Intel tried to do that with IA-64, code named Itanium, in the late 90s.  But they made it way too complicated for the engineers that would have to implement it, and not backwards compatible with lots of 32-bit stuff.  Meanwhile, AMD made a lazy modification of the x86 instruction sets, that could use 64-bit.  It was dubbed AMD64.  This was in 1999.  By 2004, Intel was actually using AMD's ISA and putting its Itanium only on a few specialized chips.  The AMD version was used:

all later Celerons, all newer models of Xeon, all newer models of Pentium Dual-Core processors , the Atom, and all versions of the Pentium D, Pentium Extreme Edition, Core 2, Core i7, Core i5, and Core i3 processors.