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carry flag

The flag that records an unsigned carry out of the top bit when adding, or a borrow when subtracting.

After an addition, CF (x86-64) and C (ARM64) hold the carry out of bit n-1, 1 exactly when the unsigned result wrapped; after a subtraction, x86-64 sets CF on a borrow (a < b unsigned) while ARM64 sets C when there is no borrow (a >= b unsigned).

Examples

  • In 8 bits, 0xFF + 0x01 gives 0x00 with CF = 1: the unsigned sum didn't fit.

See also

status flags carry add with carry zeroing idiom
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