MIPS Multiply Unit

The multiply unit of MIPS contains two 32-bit registers called hi and lo. These are not general purpose registers. When two 32-bit operands are multiplied, hi and lo hold the 64 bits of the result. Bits 32 through 63 are in hi and bits 0 through 31 are in lo.

Here are the instructions that do this. The operands are contained in general-purpose registers.

mult    s,t        # hilo <-- $s * $t.  two's comp operands

multu   s,t        # hilo <-- $s * $t.  unsigned operands

There is a multiply instruction for unsigned operands, and a multiply instruction for signed operands (two's complement operands). Integer multiplication never causes a trap.

Note: with add and addu, the operation carried out is the same with both instructions. The "u" means "don't trap overflow". With mult and multu, different operations are carried out. Neither instruction every causes a trap.