cmp/pz r6 ! Jump if we had at least two bytes.
bt/s 1f
clrt
+ add #2,r6 ! r6 was < 2. Deal with it.
bra 4f
- add #2,r6 ! r6 was < 2. Deal with it.
+ mov r6,r2
3: ! Handle different src and dest alignments.
! This is not common, so simple byte by byte copy will do.