One multiply, no room to spare
The ARM7TDMI has a hardware multiplier, and Thumb gives it exactly one instruction: mul rD, rM, meaning rD = rD * rM.
That is the whole encoding. There is no three-operand form to write the product somewhere else, and no immediate form to multiply by a constant. Both absences show up in real listings constantly — the first as copies, the second as the strength-reduced chains coming up in the next few lessons.
Here is x * y * z across the first three arguments:
0 mul r0, r1
2 mul r0, r2
4 bx lr
The product accumulates in r0, each mul overwriting it. Now the same three factors written in the opposite order, z * y * x:
0 mul r1, r2
2 mul r0, r1
4 bx lr
z * y is computed in r1 first, and only then is x multiplied in. As with the addition chains earlier, agbcc has preserved the association: the register written first held the innermost pair.
A single multiply is different, though. x * y and y * x compile to the same mul r0, r1, because there is no encoding that could record the swap — with no three-operand form the operand order simply has nowhere to go. So a lone mul tells you nothing about which factor came first in the source, while a chain of them tells you everything.
When the leftmost factor is not already in r0, the copy has to happen before the multiply can start, and you get a bare mov r0, rN at the top. Two rules constrain that copy: the destination must hold the left operand, and on this core mul rD, rD is not allowed at all — the two register fields have to differ, which is why squaring a value always costs a second register.
Read your target's first instruction as setup, then follow the accumulator.
Your task
Write func_080532a0 to reproduce the target assembly.