The Adjust flag (AF) is a CPU flag in the FLAGS register of all x86-compatible CPUs, and the preceding 8080-family; it is also called the Auxiliary flag and the Auxiliary Carry flag (AC, though this may be confused with the Alignment Check register). The flag bit is located at position 4 in the CPU flag register. It indicates when an arithmetic carry or borrow has been generated out of the four least significant bits, or lower nibble. It is primarily used to support binary-coded decimal (BCD) arithmetic.