[floofy222]

I don't think the OP literally meant that porting code was impossible, but rather, that it was very hard to do so and have the result run efficiently.

[Arnt]

(OP speaking for himself:) When I ported Z80 code to the 6502 literally, the result didn't use the zero page to its full effect, because the zero page was so much bigger than the Z80's extra registers. When I ported 6502 to Z80 literally (I mean code that used the zero page well), too much of the zero-page work had to be replaced with memory work and not enough with the nice fast registers.