x86 has a lot of special cases and it's becoming harder and harder to find non-code material with all instructions in a nice tabular format. But overall the encoding isn't that complicated. What's complex in the architecture is the amount of legacy stuff that still lives in privileged code.
> x86-64 also has some, in that many instructions can't encode the upper 8 bits of a 16-bit register (AH, BH, DH, CH) if a REX prefix is used.
You can't use both the high registers and the extended low registers in the same instruction, indeed. But in practice nobody is using the high registers anymore so it's a relatively rare limitation.
> x86-64 also has some, in that many instructions can't encode the upper 8 bits of a 16-bit register (AH, BH, DH, CH) if a REX prefix is used.
You can't use both the high registers and the extended low registers in the same instruction, indeed. But in practice nobody is using the high registers anymore so it's a relatively rare limitation.