Maybe most people know this, but I tried using ESP as a memory operand *4 for a table lookup, and I get a FASM error... Apparently you can't use ESP in this manner, thought I'd point out that funny little thing in my day.

Yes, ESP cannot be used as an index register. It doesn't make sense to use your stack pointer as an index offest anyway so why do you want it?

Yes, the SIB byte encodings that would otherwise index ESP, xx100xxx, instead mean 'no index'. You can encode R12 as an index, however because the processor combines the REX byte with the SIB byte to determine the index register. Encoding of R13 as a base is somewhat awkward because of what happens to its little brother RBP. As a consequence of their exceptional encodings, I simply avoid using RSP, RBP, R12, and R13 for addressing.