All MIPS instructions are 32 bits, or 4 bytes, long. The fixed length is almost universal in RISC processors.

When MIPS instructions are classified according to coding format, they fall into four categories: R-type, I-type, J-type, and coprocessor. To a large extent,these formats determine the execution activities in processor circuitry. Later, these instruction categories will be refined into instruction types that take into account the different ways that instructions make use of the fields of the instruction.

Limitations

This web page does not attempt to describe all MIPS instructions. The following types of instructions are not dealt with.

• Coprocessor instructions including floating point instructions.
• Integer multiply and divide instructions.
• Some branch instructions.

In order to deal with shifts and rotates it is assumed that the ALU contains some additional circuitry (typically a barrel shifter) for these operations and has control signals that can select these operations along with add, subtract and bitwise operations.