addu $s1,$s0,40000
Translates into:
li $at,40000 # use pseudoinstruction li addu $s1,$s0,$at # perform 32-bit addition
The
li
in the above is itself a pseudoinstruction.
The extended assembler will translate it, also, into basic instructions.
There is also (with the extended assembler)
a subu d,s,xaddu d,s,-x
The negu d,s$s and puts it in register $d.
negu d,s # d = -s
# (pseudoinstruction)