Difference between add and addu

add mips isa

collimarco · May 19, 2013 · Viewed 36.1k times · Source

I am confused about the difference between add and addu.

The MIPS instruction reference says:

My understanding is to use add with signed operands and addu with unsigned operands.

But let's consider this example (with only 6bit):

overflow
|
V
1 | 1 1 1  <- carry
  | 1 1 1 1 0 1 +
  | 1 1 1 1 1 0 =
-----------------
  | 1 1 1 0 1 1

And this is my reasoning:

if I consider the first and second operand signed numbers (two's complement), then the result is correct (-3 + -2 = -5) and I don't want an overflow exception. So I would use addu to avoid this exception, but, although the result is the same, the name suggests to use addu is for unsigned numbers!
if I consider the first and second operand unsigned numbers, then I want an exception to be raised (because 61 + 62 is not equal to 59). So I would use add to raise the exception, and not addu, as the name would suggest to do.

Now my questions are:

assuming that operands are signed (negative in the example above) numbers, should I use addu (as my reasoning suggests) or I should use add (as the name suggests)?
assuming that operands are unsigned (positive) numbers, should I use add (as my reasoning suggests) or addu (as the name suggests)?

Answer

The instruction names are misleading. Use addu for both signed and unsigned operands, if you do not want a trap on overflow.

Use add if you need a trap on overflow for some reason. Most languages do not want a trap on signed overflow, so add is rarely useful.