The diagram below illustrates the three microinstructions that make up the load register immediate (LDR) instruction, with explanations on some of the important fields. The first microinstruction computes the address: the indicated fields instruct the ALU to add or subtract the 12-bit offset value from the instruction, and put the value on the address bus. The ALU control logic uses the U (up/down) and P (pre/posts) bits in the instruction to determine if the offset should be added or subtracted or ignored. This illustrates that the microinstruction only partially defines the instruction; the hardcoded control logic also makes decisions based on the instruction. The microinstruction also specifies that the sequencer should move to the next microinstruction.
The next microinstruction instructs the ALU to update the offset register. As before, the ALU control logic determines if the update requires an add or subtract. The register control logic determines if the register should be updated. The microinstruction also indicates that the fetched data should be read in.
The final microinstruction stores the fetched result in a register. It specifies Rd as the destination register and indicates a register write. The microinstruction tells the sequencer this is the end of the instruction.