Difference between add and addu
addition, instruction-set, mips
Solution
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.
Problem
I am confused about the difference between add and addu. The MIPS instruction reference says: - add (with overflow) - add unsigned (no overflow) 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)?