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.

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