There is no subtract-immediate
MIPS has no subiu. When C subtracts a constant, the compiler reaches for the instruction you already know — addiu — and simply hands it a negative immediate. That signed 16-bit field works in both directions:
addiu v0, a0, -7 # v0 = a0 + (-7) …i.e. a0 - 7
jr ra
nop
Some disassemblers invent a friendlier subi alias to paper over this. Ours doesn't, and honestly that's better: the listing shows you the truth, an addiu carrying a negative number. Once you know the idiom, reading it is nothing — find the immediate, flip its sign, and that's the amount the C subtracts.
Your task
Write subConst, taking an s32 a, to reproduce the target addiu.