ARM > Introduction to ARM > Organisation

by David Thomas on —

Organisation

ARM has a three-address format:

  • Rd — destination register
  • Rn — source register
  • Rm — source register

e.g. ADD R0,R1,R2.

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.

Organisation diagram.

The two operands are processed by the ALU and the result written to Rd.

Remarks

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.