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GATE CSE 2025 Set 2 · Q32

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The following two signed 2's complement numbers (multiplicand M and multiplier Q) are being multiplied using Booth's algorithm: M: 1100 1101 1110 1101 and Q: 1010 0100 1010 1010 The total number of addition and subtraction operations to be performed is __________. (Answer in integer)

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Solution

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Official answer: 13

ALU Design

In Booth's multiplication algorithm, the multiplier QQ is scanned from right to left (LSB to MSB) along with an appended bit Q−1=0Q_{-1} = 0.

The operations performed for each pair of adjacent bits (Qi,Qi−1)(Q_i, Q_{i-1}) are:

  • 0101: Addition (+M+M)

  • 1010: Subtraction (−M-M)

  • 0000 or 1111: No addition or subtraction (only shift)

Given the 16-bit multiplier QQ and Q−1=0Q_{-1} = 0:

Q=q15q14…q0=1010 0100 1010 1010,Q−1=0Q = q_{15} q_{14} \dots q_0 = 1010\ 0100\ 1010\ 1010, \quad Q_{-1} = 0

Let us evaluate each bit pair (qi,qi−1)(q_i, q_{i-1}) starting from i=0i = 0 to 1515:

  1. (q0,Q−1)=(0,0)→(q_0, Q_{-1}) = (0, 0) \rightarrow No operation

  2. (q1,q0)=(1,0)→(q_1, q_0) = (1, 0) \rightarrow Subtraction

  3. (q2,q1)=(0,1)→(q_2, q_1) = (0, 1) \rightarrow Addition

  4. (q3,q2)=(1,0)→(q_3, q_2) = (1, 0) \rightarrow Subtraction

  5. (q4,q3)=(0,1)→(q_4, q_3) = (0, 1) \rightarrow Addition

  6. (q5,q4)=(1,0)→(q_5, q_4) = (1, 0) \rightarrow Subtraction

  7. (q6,q5)=(0,1)→(q_6, q_5) = (0, 1) \rightarrow Addition

  8. (q7,q6)=(1,0)→(q_7, q_6) = (1, 0) \rightarrow Subtraction

  9. (q8,q7)=(0,1)→(q_8, q_7) = (0, 1) \rightarrow Addition

  10. (q9,q8)=(0,0)→(q_9, q_8) = (0, 0) \rightarrow No operation

  11. (q10,q9)=(1,0)→(q_{10}, q_9) = (1, 0) \rightarrow Subtraction

  12. (q11,q10)=(0,1)→(q_{11}, q_{10}) = (0, 1) \rightarrow Addition

  13. (q12,q11)=(0,0)→(q_{12}, q_{11}) = (0, 0) \rightarrow No operation

  14. (q13,q12)=(1,0)→(q_{13}, q_{12}) = (1, 0) \rightarrow Subtraction

  15. (q14,q13)=(0,1)→(q_{14}, q_{13}) = (0, 1) \rightarrow Addition

  16. (q15,q14)=(1,0)→(q_{15}, q_{14}) = (1, 0) \rightarrow Subtraction

Counting all addition and subtraction operations:

  • Number of subtractions (1010 transitions) = 77

  • Number of additions (0101 transitions) = 66

Total number of operations=7+6=13\text{Total number of operations} = 7 + 6 = 13

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This ALU Design previous year question is Q32 of the GATE CSE 2025 Set 2 paper. Find more GATE CSE PYQs on ALU Design in Computer Organization and Architecture, or browse all GATE CSE PYQs.

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ALU Design has 4 GATE CSE PYQs across 4 papers from 2016 to 2025. This is the only 1-mark NAT question among them, and it is the only one in GATE CSE 2025 Set 2.

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GateAI solution cardGATE CSE 2025 Set 2 · Q32 · NAT

The following two signed 2's complement numbers (multiplicand M and multiplier Q) are being multiplied using Booth's algorithm: M: 1100 1101 1110 1101 and Q: 1010 0100 1010 1010 The total number of addition and subtraction operations to be performed is … (Answer in integer)

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