EPIC failure (to cancel the project when it was first failing)

Posted Dec 1, 2023 12:20 UTC (Fri) by sammythesnake (guest, #17693)
In reply to: EPIC failure (to cancel the project when it was first failing) by farnz
Parent article: The push to save Itanium

> Panic-implement x86, but 64-bit. This is basically what AMD did for AMD64, because they needed a 64-bit CPU, but didn't have the money to do a "clean-sheet" redesign

Although I'm sure the cost of a from-scratch design would have been prohibitive in itself for AMD, I think the decision probably had at least as much to do with a very pragmatic desire for backward compatibility with the already near-universal x86 ISA.

History seems to suggest that (through wisdom or luck) that was the right call, even with technical debt going back to the 4004 ISA which is now over half a century old(!) (https://en.m.wikipedia.org/wiki/Intel_4004)

EPIC failure (to cancel the project when it was first failing)

Posted Dec 1, 2023 13:17 UTC (Fri) by farnz (subscriber, #17727) [Link]

There's a lot about AMD64 that is done purely to reuse existing x86 decoders, rather than because they're trying to have backwards compatibility with x86 at the assembly level. They don't have backwards compatibility at the machine code level between AMD64 and x86, and they could have re-encoded AMD64 in a new format, while having the same instructions as they chose to implement.

That's what I mean by "not having the money"; if they wanted assembly-level backwards compatibility, but weren't short on cash to implement the new design, they could have changed instruction encodings so that (e.g.) we didn't retain special encodings for "move to/from AL" (which exist for ease of porting to the 8086 from the 8085). Instead AMD reused the existing x86 encoding, with some small tweaks.