Bit #0 - carry flag

The carry flag (CF) is mostly used for the detection of carry/borrow in arithmetic operations and is set if the bit width result of the last such operation (such as addition and subtraction) exceeds the width of the ALU. For example, the addition of two 8-bit values, 255 and 1, would result in 256, which requires at least nine bits to be stored. In such a case, bit eight (the ninth bit) is placed into the CF, thus, letting us and the processor know that the last operation ended with carry.