"...despite the money and effort Intel put behind creating and marketing
its 64-bit Itanium chip, it was working on another project designed to
add extensions to its 32-bit processors."
So I guess most of that work Intel supposedly did on extensions to its
32-bit processors was thrown out for AMD's?
Probably not, they were likely ~90% identical. AMD64 is a very
natural extension to the x86 instruction set. If you wanted to make a
64-bit processor out of an 32-bit x86 chip, chances are that you would
end up with something like AMD64 regardless of who designed it. There
are definitely some minor differences that one could make, a few
tweaks here and there that might not be 100% obvious, but the bulk of
it is pretty straightforward.
That's part of the reason why so many people like the AMD64
instruction set, it just sort of makes sense (or at least as much
sense as x86 made in the first place, which admittedly isn't all that
much :> ).