Since each of these logic blobs has an input bit, carry in, normal addition that first carry in is a zero, but for subtraction we can make that carry in a 1 and invert the second operand to get a + b = a + (~b) + 1
#Condition zero carry plus#
Each are three bits in two bits out, the two operands in plus carry in and carry out and the result out. Same in binary, with two operands you can have 1 + 1 = 0 carry the one. We also know from grade school the concept of carrying. We also know from beginner programming classes that with twos complement to get the negative you invert and add one. And that is how logic does it we dont subtract we add the negative. We know from grade school that a - b = a + (-b). Not if you subtract from zero, it comes naturally that for any number, but zero itself, you'll always have the CF set, as the subtrahend has at least one bit set.įinally, some instructions let you change the sign bit without affecting the CF (see for example the logic operations or the behavior of neg). This is equivalent to an overflow condition, for signed numbers the equivalent flag is OF.įor an (unnecessary?) visual clue, in this 4-5 operation, it is the second borrow, the red one, that set the CF It is also used in multiple-precision arithmetic.ĭon't associate the CF with the sign bit, in a subtraction CF is set whenever the minuend, treated as unsigned, is less than the subtrahend, treated as unsigned. This flag indicates an overflow condition for Carry flag is carry or borrow out of the Most Significant bit (MSb):ĬF (bit 0) Carry flag - Set if an arithmetic operation generates a carry or a borrow out of the mostsignificant bit of the result cleared otherwise.
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