Two ABI rules in one body
The earlier lessons gave you the moves one at a time: a value that must survive a call goes into r31 (callee-saved), and a call's arguments are marshalled into r3, r4, … before the bl. A real wrapper does both at once — it keeps one value alive across the call while feeding the rest into the callee.
Consider hold_sub(w, x, y, z), which calls op3(x, y, z) and then subtracts the result from w:
stwu r1,-16(r1)
mflr r0
stw r0,20(r1)
stw r31,12(r1)
mr r31,r3 # w must outlive the call -> park it in r31
mr r3,r4 # marshal x into the 1st arg slot
mr r4,r5 # marshal y into the 2nd
mr r5,r6 # marshal z into the 3rd
bl op3 # op3(x, y, z); r3..r12 may be destroyed
lwz r0,20(r1)
subf r3,r3,r31 # r31 - result = w - op3(...)
mtlr r0
addi r1,r1,16
blr
Read it in two layers. The mr r31, r3 is the survival move — w arrives in r3 but r3 is about to be reused as the first outgoing argument, so w is saved into the callee-saved r31 first. The three mr instructions then slide x, y, z down from r4–r6 into r3–r5. After the call, subf combines the preserved value with the returned one.
The target assembly for accum has the same skeleton with fewer marshalled arguments and a different final operator. Find which parameter is stashed in r31, which ones are shuffled into the argument registers for the call, and how the post-call instruction folds the survivor into the result.
Your task
Write accum, which calls proc and combines a surviving parameter with the result, to reproduce the target assembly. proc is declared for you.