Everything this chapter taught, in one function
Time to put the whole kit together. One function, every idiom: a frame, homed arguments reloaded around calls, a local born from a call result, forwarding into a second call, and arithmetic on the far side.
Here's audit(base, n), which computes check(check(n, base), base) - n:
addiu sp, sp, -24
sw ra, 20(sp)
sw a0, 24(sp) # home base — needed after (and inside) the calls
sw a1, 28(sp) # home n — needed as an argument, and at the very end
lw a0, 28(sp) # first call: a0 = n (arguments swapped!)
jal check
lw a1, 24(sp) # …and a1 = base, in the delay slot
or a0, v0, zero # second call: a0 = first result
jal check
lw a1, 24(sp) # a1 = base again, reloaded from its home
lw t6, 28(sp) # n, one last time
lw ra, 20(sp)
subu v0, v0, t6 # second result − n
jr ra
addiu sp, sp, 24
Notice how much you can now reject on sight. The two homes at 24/28 say both parameters outlive their registers. The lw a0, 28(sp) / lw a1, 24(sp) pair says the first call receives them crossed — slot 28 (the second parameter) feeds a0, slot 24 (the first) feeds a1. The or a0, v0, zero chains call one into call two. And the closing subu reaches back to slot 28 for the raw parameter, not any call result.
The target is built from the same parts in a different arrangement: watch which parameter gets homed, what the second call receives, whether a call result becomes a local, and what the final arithmetic combines. Read the offsets, trust the idioms, and write the two-line body they describe.
Your task
extern s32 rate(s32 x, s32 y); is declared for you. Write func_80210330 to reproduce the target assembly.