A fun thing is that e.g. "cmp ax, 0x4231" differs from "cmp eax, 0x87654321" only in the presence of the data16 prefix, and thus the longer immediate; and it's the only significant case (I think?) of a prefix changing the total instruction size, and thus, for some such instructions, the 16-bit version, sometimes (but not always!) is significantly slower. "but not always" as in, if you try to microbenchmark a loop of such, sometimes you can have entire microseconds of it consistently running at 0.25 cycles/instr avg, and sometimes that same exact code (in the same process!) will measure it at 3 cycles/instr (tested on Haswell, but uops.info indicates this happens on all non-atom Intel since Ivy Bridge).