Magnitude comparison — determine comparator outputs for A and B. Given A = 1100₂ and B = 1001₂ applied to a 3-output magnitude comparator, what are the output levels for A > B, A < B, and A = B?

Difficulty: Easy

Correct Answer: A > B = 1, A < B = 0, A = B = 0

Explanation:


Introduction / Context:
Digital magnitude comparators (such as the 7485 family) produce three outputs that indicate whether one binary word is greater than, less than, or equal to another. Interpreting these outputs is a basic skill for designing decision logic, sorting networks, and address decoding hardware.


Given Data / Assumptions:

  • A = 1100₂ (binary).
  • B = 1001₂ (binary).
  • Outputs are independent indicator lines for A > B, A < B, A = B (active-HIGH).


Concept / Approach:
Convert each binary number to decimal or compare bit-by-bit from MSB to LSB. The first differing bit from the left determines the result: if A’s bit is 1 and B’s is 0, then A > B; if opposite, A < B; if all bits equal, A = B.


Step-by-Step Solution:

Interpret A: 1100₂ = 8 + 4 + 0 + 0 = 12.Interpret B: 1001₂ = 8 + 0 + 0 + 1 = 9.Since 12 > 9, the correct indications are A > B = 1, A < B = 0, A = B = 0.


Verification / Alternative check:
Bitwise MSB-first: compare bits at positions 2^3, 2^2, 2^1, 2^0. At 2^2 (the second bit), A has 1 while B has 0, and no higher bits differ; therefore A > B without needing to check lower bits.


Why Other Options Are Wrong:

  • Any option asserting A < B is false because A (12) clearly exceeds B (9).
  • Any option asserting A = B is false because they differ at multiple bit positions.


Common Pitfalls:

  • Mistaking LSB for MSB during comparison.
  • Adding bits incorrectly (e.g., miscounting the 4’s or 1’s places).


Final Answer:
A > B = 1, A < B = 0, A = B = 0

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