It's worse than this for the 6502! DEY takes 2 cycles, and BNE (when taken) takes 3. LDA abs,Y takes 4-5 cycles (the extra one happens if a page boundary is cro

It's worse than this for the 6502! DEY takes 2 cycles, and BNE (when taken) takes 3. LDA abs,Y takes 4-5 cycles (the extra one happens if a page boundary is crossed when performing the address calculation - in practice you'd try to arrange for this not to happen), and STA abs,Y takes 5 cycles. So out of the 24 cycles per iteration, there's 2 useful ones: 1 read, and 1 write. You need to unroll this sort of loop a few times to get anything near 9 cycles per byte, and ~80% of the cycles are still going to be kind of wasted. (Still annoys me to this day if I find myself writing code to copy stuff around. Why isn't it in the right place already?!)