Half-carry flag

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A half-carry flag (also known as an auxiliary flag) is a condition flag bit in the status register of many CPU families, such as the Intel 8080, Zilog Z80, the x86,^[1] and the Atmel AVR series, among others. It indicates when a carry or borrow has been generated out of the least significant four bits of the accumulator register following the execution of an arithmetic instruction. It is primarily used in decimal (BCD) arithmetic instructions.