Carry Bit

To make programming easier, a carry bit is provided by the 4004 to reflect the results of data operations. The descriptions of individual instructions specify which instructions affect the carry bit and whether the execution of the instruction is dependent in any way on the prior status of the carry bit.

The carry bit is “set” if 1 and “reset” if 0.

Certain data operations can cause an overflow out of the high-order 3-bit. For example, addition of two hexadecimal digits can give rise to an answer that does not fit in one digit:

            3 2 1 0     Bit number

    A       1 0 1 0
+   7       0 1 1 1
            -------
    1       0 0 0 1
carry

An operation that results in a carry out of bit 3 will set the carry bit.
An operation that could have resulted·in a carry out of bit 3 but did not will reset the carry bit.