memcpy used to be a rep movsb on 8086 DOS compilers. I don't remember if rep movsb stops if cx=0 on entry, or decrements first and wraps around, copying 64K of data.
The specification does not explicitly say that, but the clear intention is that REP with CX=0 should be no-op (you get exactly that situation when REP gets interrupted during the last iteration, in that case CX is zero and IP points to the REP, not the following instruction).
I know at least MSVC's memcpy on x86_64 still results in a rep movsb if the cpuid flag that says rep movsb is fast is set, which it should be on all x86 chips from about 2011/2012 and onward ;)
