The quirk of subf
PowerPC has no plain sub. To compute a - b it uses subf — subtract from — which computes rD = rB - rA. The operands are reversed:
subf r3, r4, r3 # r3 = r3 - r4 = a - b
blr
So subf r3, r4, r3 subtracts r4 from r3, leaving a - b in r3. Once you internalize that subf rD, rA, rB computes rB - rA, the disassembly stops looking backwards.
Your task
Write sub2, returning a - b.