Overflow occurs when the result from an operation cannot be represented with the available hardware. When can overflow occur in addition and subtraction?
- When adding operands with different signs, overflow can not occur.
- When subtracting operands with the same sign, overflow can not occur.
- Overflow occurs when adding two positive numbers and the sum is negative, or vice versa.
- Overflow occurs in subtraction when we subtract a negative number from a positive number and get a negative result, or when we subtract a positive number from a negative number and get a positive result.
| Unsigned integers are commonly used for memory addresses where overflows are ignored. |
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- Add (
add), add immediate (addi), and subtract (sub) cause exceptions on overflow. - Add unsigned (
addu), add immediate unsigned (addiu), and subtract unsigned (subu) do not cause exceptions on overflow.
add |
addu (without overflow) |
|---|---|
.text li $a0, 2147483647 # largest signed word is # 0x7FFFFFFF = 231-1 # = 2,147,483,647 add $a0, $a0, 1 li $v0, 1 syscall li $v0, 10 syscall |
.text li $a0, 2147483647 # largest signed word is # 0x7FFFFFFF = 231-1 # = 2,147,483,647 addu $a0, $a0, 1 li $v0, 1 syscall li $v0, 10 syscall |
| Output | Output |
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