# Carry flag (C flag)

What is the carry flag (C flag)?
A carry flag (C flag) in computer science works with the arithmetic-logic unit (ALU) of a computer central unit in order to handle arithmetic and bit-wise logic operations on binary numbers. The carry flag is used when an operation changes the left bit of the binary system. Some call this the most significant or 'leftmost' bit.

The way the carry flag works is based on how addition and subtraction is done with binary numbers. Changes to the leftmost bit indicate some type of conversion of a binary number set. For example, when a binary sequence of 1111 is added to 0001 and becomes 0000, the carry flag is turned on. If 0000 is subtracted from 0001, the result is 1111 and the carry flag is turned on.

Problems with carry flags and other types of flags such as overflow flags can contribute to overflow errors and other errors in your code. The role of these flags and indicators is related to the different ways in which binary arithmetic works compared to the addition and subtraction that is taught in school in relation to integers.

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