addu $s1,$s0,40000  ==  

li   $at,40000    # use pseudoinstruction li
addu $s1,$s0,$at  # perform 32-bit addition

Subtraction

The li in the above is itself a pseudoinstruction. It will be further translated into a basic instruction.

There is also (with the extended assembler) a  subu d,s,x  instruction where the last operand can be a register, a 16-bit immediate, or a 32-bit immediate. This instruction translates into the pseudoinstruction  addu d,s,-x  (which is then translated into basic instructions).

The  negu d,s  pseudoinstruction calculates the two's complement negation of register $s and puts it in register $d.

negu d,s   # d <-- -s
           # (pseudoinstruction)

negu $v2,$s3  ==  sub  ____,$0,____