Two's-complement subtraction rule: When subtracting 6 from 9 using a 2's-complement adder, the ________ is 2's-complemented before the addition.

Introduction / Context:
Digital subtractors typically reuse adder hardware by representing subtraction A − B as A + 2's-complement(B). This technique works for any word size and unifies the implementation of add and subtract.

Given Data / Assumptions:

• We wish to compute 9 − 6.
• Arithmetic is performed using a 2's-complement adder.

Concept / Approach:
Form the 2's-complement of the subtrahend (6) and add it to the minuend (9). Equivalently, invert the bits of 6 and add 1, then add to 9, propagating carries as usual.

Step-by-Step Solution:

Verification / Alternative check:
Numerically, 9 + (−6) = 3, matching the intended subtraction.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing one's-complement (~B) with two's-complement (~B + 1).

Final Answer:
six