In MIPS assembly, what is the benefit of using addiu
over addi
? Isn't addiu
unsigned (and will ruin our calculations?)
and will ruin our calculations
No, MIPS uses two's complement, hence the same instruction for addition/subtraction can be used for both signed and unsigned operations. There's no difference in the result.
That's also true for bitwise instructions, non-widening multiplication and many other operations. See
The only difference between them is that addi
generates a trap when overflow while addiu
doesn't. So addi
and its overflow family (add
, sub
...) is often useless. In fact it's so rarely used that addi
was removed in MIPSr6 to release valuable opcode space to other instructions
Here the instruction name is extremely misleading, because it's not actually an "unsigned" addition. The immediate is still sign extended instead of zero extended. So addiu $1, $2, 0xFFFF
will actually subtract 1 from $2
instead of adding 65535 to it.
Despite its name, add immediate unsigned (
addiu
) is used to add constants to signed integers when we don't care about overflow. MIPS has no subtract immediate instruction, and negative numbers need sign extension, so the MIPS architects decided to sign-extend the immediate field.
Read more Difference between add and addu