4-bit parallel adder behavior If the carry-in (Cin) of a 4-bit parallel adder is tied to logic HIGH, what is the effect on the final sum output?

Introduction / Context:
Carry-in is used to cascade adders or to implement two’s-complement subtraction. Understanding its effect on the sum is crucial in ALU and arithmetic block designs.

Given Data / Assumptions:

• A standard 4-bit parallel adder with inputs A[3..0], B[3..0], and Cin.
• The carry-in Cin is tied HIGH (logic 1).

Concept / Approach:
The adder computes S = A + B + Cin. Thus, Cin = 1 simply adds an extra 1 to the least significant stage, which propagates as needed, making the overall sum equal to A + B + 1.

Step-by-Step Solution:
General formula: Sum = A + B + Cin.With Cin = 1: Sum = A + B + 1.This is sometimes used to add the +1 term in two’s-complement subtraction (A + B' + 1).

Verification / Alternative check:
Example: A=0101 (5), B=0011 (3): normal sum 1000 (8). With Cin=1 the sum is 1001 (9), confirming the +1 effect.

Why Other Options Are Wrong:
Cin is not ignored; it directly enters the LSB adder cell.Carry-out depends on operands; it is not always HIGH.Output forcing (like MSB LOW) is unrelated to Cin behavior.

Common Pitfalls:
Confusing Cin with Cout or assuming it only matters when cascading. It always influences the computed sum.

Final Answer:
a one will be added to the final result