All MIPS instructions are 32 bits, or 4 bytes, long. The fixed length is almost universal in RISC processors. The 32-bit size is currently the most common size. This could change in a few years.

When MIPS instructions are classified according to coding format, they fall into four categories: R-type, I-type, J-type, and coprocessor. These formats are closely related to the execution activities in the processor circuitry.

The coprocessor coding format is not described here. Although the use of a coprocessor is advantageous for an embedded processor, desktop and server processors have not used them for about 20 years.

Many MAL instructions get expanded into 2 or 3 MIPS instructions. These expansions are often used to deal with immediate operands and idiosyncracies of the MIPS instruction set. Some of these expansions use $at as a temporary register.

MIPS assemblers normally make the following expansions. For some instructions more than one expansion may be applied. There are other expansions that are not described here.

Instruction execution involves the following activities, which are approximately in order. The program counter (PC) hold the address of the next instruction. The arithmetic-logic unit (ALU) performs arithmetic and logical operations such as adds and subtracts. The ALU is designed to combine two source operands to produce a result.