The ALU is designed to combine two source operands to produce a result. The source operand fetch activity fetches the possible source operands for the ALU. One source operand is always the register specified by the rs instruction field. The other is selected from the following as part of the ALU Operation activity.

• The register specified by the rt instruction field
• The sign-extended immediate instruction field
• The zero-extended immediate instruction field

The MIPS single-cycle implementation diagram does not include signal handling for the zero-extended immediate instruction field. It is only used in a few unsigned integer instructions.