On: The x86 architecture is the weirdo (2004)

[gpderetta]

It is very inconsistent between major microarchitectural revisions, sometimes even between me minor steppings. Sometimes firmware updates can change the decoding (usually to fix bugs).

[Jasper_ → gpderetta]

The firmware can only do this by turning on an extremely slow fallback path. It's not common and it's seen as an avenue of last resort.