In this part, you will be using the reverse subtract operation (RSB) to perform r0= (r1*4) - r2

RSB rd , op1, op2 ;performs rd = op2 op1

where op1 is a register and op2 is a register, shifted register, or an immediate value

The reverse subtract operation is important because we have a barrel shifter on only one of the buses going into the ALU, so if you want to shift the minuend when subtracting, you will use the reverse subtract operation instead of the subtract operation. With the reverse subtract, the second operand can also be a constant, so you could subtract a register value from a constant.