MULTU - MIPS instruction reference

multuMultiply unsigned 
Multiplies $t1 by $t2 as unsigned 32-bit integers and leaves the 64-bit product in HI and LO.
Forms  multu ,   
Operands $t1 Any general-purpose register, $0 to $31, using its numeric or named spelling, such as $8 or $t0 ($zero is $0; $ra is $31). 
$t2 A general-purpose register. It may be the same register as $t1. 
The 64-bit product is split: upper 32 bits go into HI, lower 32 bits go into LO. Use 'mfhi'/'mflo' to retrieve the results.
 # Multiply 6 by 7. Inspect HI:LO: 42 (HI=0, LO=42).
li $t0, 6
li $t1, 7
multu $t0, $t1
mflo $t2
mfhi $t3