Why inc and dec Do Not Affect the Carry Flag in assembly language?

Answer 1

The increment and decrement circuitry is dedicated circuitry that simply adds or subtracts one from an operand. Both operations are handled by the same circuitry. Being the most common operations by far, the circuitry is typically placed as close to the registers and the address lines as possible. The circuitry primarily consists of a sequence of half-adders each of which takes an input carry bit and outputs a carry bit to the next half-adder. Since this would result in a "carry ripple" effect, the half-adders are combined such that multiple carry bits can be handled more efficiently. This makes the circuitry more complex, but is more than compensated by the reduced effect of carry ripple. To speed things up further, the final carry bit is simply ignored (leaving the carry flag unaffected), it being superfluous to the vast majority of increment and decrement operations.

One of the primary roles of the increment circuitry is to advance the instruction pointer to the next instruction. This operation must execute at least once per instruction, so the faster it achieves this the better. The circuitry is also used to both increment or decrement the stack pointer. The carry flag is superfluous to these and the majority of all other increment or decrement operations, so they have no effect upon it, thus maintaining as high a degree of efficiency as possible.