Do you know if NOP is really executed as `XCHG AX, AX` (as would be implied from its position in the opcode map and its cycle count) or if it is special-cased in some way?

I've always been curious about the XLAT instruction. It performs an operation like `MOV AL, [BX + AL]` (if that were a valid instruction). I assume this instruction was designed to do EBCDIC <-> ASCII conversions using a simple loop consisting of `LODSB / XLAT / STOSB` which would explain why the input and output are both hardcoded to AL. But it seems like it would have been fairly easy to allow the programmer to specify different register(s) and the instruction probably would have been a lot more useful generally if that was allowed.

It is the only place in the ISA that I know of where an 8-bit value is zero-extended to 16 bits before being added to another value. Short jumps use a sign-extended 8-bit value (and are a special case anyway since they deal with the instruction pointer). Other additions are always 8+8 or 16+16. Is there some special case in the processor to allow just AL to be zero-extended in this way? If so, that would go some way toward explaining why this instruction isn't more flexible. I realize this may be very difficult to determine working from a die photo, at least without completely reverse engineering the entire microcode ROM.