We also got UDB (D6), the one-byte variant that arrived with x86-64 for 64-bit mode.
And we have always had UDW (FF FF), aka group #5 (1st FF) with a modrm byte of mod=11b r/m=111b (/7) reg=111b (2nd FF) -- that one matters for memory with all bits set to 1, or for buses terminated to all 1 when no device claims an access.
Sorry y'all. In a thread about encodings whose only spec is that they mean nothing (UD0/UD1/UD2, FF /7), "349ru3h4f03" is the most thematically correct username possible, it really does "check out" dang it. It even looks like hex right up until it #UDs on the r, u, and h. Invalid encoding; faults on interpretation. Tough crowd tonite.-
There is also UD2A and UD2B which just seem to be synonyms for UD2 and UD1 respectively? Google tells me that they are legacy tool specific mnemonics used in older GNU GCC/Binutils.
We also got UDB (D6), the one-byte variant that arrived with x86-64 for 64-bit mode.
And we have always had UDW (FF FF), aka group #5 (1st FF) with a modrm byte of mod=11b r/m=111b (/7) reg=111b (2nd FF) -- that one matters for memory with all bits set to 1, or for buses terminated to all 1 when no device claims an access.