ARM has a three-address format:
Rd— destination register
Rn— source register
Rm— source register
Rn is used directly but
Rm is passed through the barrel shifter; a functional unit which can rotate and shift values. The result of this is called Operand2.
The two operands are processed by the ALU and the result written to
Although it’s a register, we show the CPSR separately on the diagram to indicate that it may be used in both the barrel shifter and the ALU stages.