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. 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 idiosyncrasies 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.

• When imm cannot be represented as a 16-bit number

      li reg, imm
            

  is expanded to

      lui reg, imm[31:16]
      ori reg, reg, imm[15:0]

•     move reg1, reg2
            

  is expanded to

      or reg1, reg2, $0

•     la reg, label
            

  is expanded to

      li reg, labelAddress
            

•     load or store reg, label
            

  is expanded to

      la $at, label
      load or store reg, 0($at)

• op reg1, reg2, imm
            

  is expanded to

      li $at, imm
      op reg1, reg2, $at