Nowadays UD0 UD1 UD2 are in the SDM and APM.
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.
I've always remembered FF FF as "??? DI", because that's what DEBUG's disassembler would render it as.
Couldn't find UDW in the intel docs - https://intel.github.io/SDM/sdm.html
But found it in wikipedia which is well structured and has detailed notes/comments on all instructions - https://en.wikipedia.org/wiki/List_of_x86_instructions
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.
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