3. Carry Flag

• When numbers are added and subtracted, carry flag CF represents

  • 9th bit, if 8-bit numbers added

  • 17th bit, if 16-bit numbers added

  • 33rd bit, if 32-bit numbers added

  • and so on.

• With addition, the carry flag CF records a carry out of the high order bit. For example,

          mov al, -1
          add al, 1    ; AL = 0, ZF and CF flags are set to 1
  

• When a larger number is subtracted from the smaller one, the carry flag CF indicates a borrow. For example,

          mov al, 6
          sub al, 9    ; AL = -3, SF and CF flags are set to 1
  

  The result is -3, represented internally as 0FDh  (binary 11111101).