What is meant Auxiliary Carry Bit?

The Auxiliary Carry Bit (AC) is a flag in computer architecture, particularly in binary-coded decimal (BCD) arithmetic. It indicates whether a carry was generated from the lower nibble (the four least significant bits) during an arithmetic operation, such as addition or subtraction. This bit is useful for BCD operations, as it helps manage the carry between the two nibbles when processing decimal numbers. In many microprocessor architectures, the AC flag is used in conjunction with other status flags to provide accurate arithmetic results.