# Carry bit

The **carry bit** ( Engl. *Carry bit* ) is a term used in computer science . It denotes a bit that contains the carryover of an addition or subtraction of bits to the next most significant bit .

The values provided for an elementary calculation in a processor mostly have a fixed memory size, for example *n* bits with the digits a _{0} to a _{n-1} . These values are usually processed bit by bit (using 1-bit ALUs ). If a result with two digits arises when processing a digit a _{i} , the higher digit of the result in the place value system is passed on as a carry bit to the next (also the higher digit) digit a _{i + 1 of} the values to be calculated so that this digit is calculated correctly can.

The last carry bit of the calculation at the most significant place a _{n-1} is often stored by processors in a status register as a carry flag, for example in order to process numbers that are larger than the bit width of the ALU . Using the same recursive principle as one calculates n-bit wide numbers from 1-bit additions, one can also calculate m-bit (m = k * n) wide numbers.

The carry flag basically represents the carry over of the position a _{n-1} , while the overflow bit depends on the interpretation of the numbers and signals that the valid range of numbers has been exceeded.