The term arithmetic overflow or simply overflow has the
following meanings. In a computer, the condition that occurs when a calculation
produces a result that is greater in magnitude than that which a given register
or storage location can store or represent. In a computer, the amount by which
a calculated value is greater in magnitude than that which a given register or
storage location can store or represent. Note that the overflow may be placed
at another location.
Most computers distinguish between two kinds of overflow
conditions. 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. Therefore, it is useful to check the overflow flag after
adding or subtracting numbers that are represented in two's complement form.
