Arithmetic overflow

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The term arithmetic overflow or simply overflow has the following meanings.

1. In a digital computer, the condition that occurs when a calculation produces a result that is greater in magnitude than what a given register or storage location can store or represent.
2. In a digital computer, the amount by which a calculated value is greater than that which a given register or storage location can store or represent. Note that the overflow may be placed at another locations.

Most computers distinguish between two kinds of overflow condition. A carry occurs when the result of an addition or subtraction, considering the operands and result as unsigned numbers, does not fit in the result. Therefore, it is useful to check the carry flag after adding or subtracting numbers that are interpreted as unsigned values. An overflow proper occurs when the result does not have the sign that one would predict from the signs of the operands (e.g. a negative result when adding two positive numbers). Therefore, it is useful to check the overflow flag after adding or subtracting numbers that are represented in two's complement form (i.e. they are considered signed numbers).

There are several methods of handling overflow:

1. Design: by selecting correct data types, both length and signed/unsigned.
2. Avoidance: by carefully ordering operations and checking operands in advance, it is possible to ensure that the result will never be larger than can be stored.
3. Handling: If it is anticipated that overflow may occur and when it happens detected and other processing done. Example: it is possible to add two numbers each two bytes wide using just a byte addition in steps: first add the low bytes then add the high bytes, but if it is necessary to carry out of the low bytes this is arithmetic overflow of the byte addition and it necessary to detect and increment the sum of the high bytes. CPUs generally have a way of detecting this to support addition of numbers larger than their register size, typically using a status bit.
4. Propagation: if a value is too large to be stored it can be assigned a special value indicating that overflow has occurred and then have all successive operation return this flag value. This is useful so that the problem can be checked for once at the end of a long calculation rather than after each step. This is often supported in Floating Point Hardware called FPUs.
5. Ignoring: This is the most common approach, but it gives incorrect results and can compromise a program's security.

Division by zero is not a form of arithmetic overflow. Mathematically division by zero is explicitly undefined; it is not that the value is too large but rather that it has no value.de:Arithmetischer Überlauf it:Overflow nl:Overflow