Swap Two Bytes in Memory

Two bytes are in memory. Exchange them, so that each ends up where the other was.

This is the first problem where the obvious instruction does not exist. You cannot write MOV M, M, and even if you could it would not help: the moment you copy the second byte over the first, the first is gone and there is nothing left to copy back. Every exchange, in every language, on every processor, is really three moves through a temporary place — the difference on the 8085 is that the temporary place is a register you have to choose yourself.

The second half of the problem is about where the two bytes are. Neighbours are easy: one pointer reaches both. Two addresses far apart need two pointers, and that turns out to be the pattern behind every copy, compare and sort you will write later.