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GATE CSE 2021 Set 1 Q65: Memory location 3010 after a loop of array updates

Question

Consider the following instruction sequence where registers R1, R2 and R3 are general purpose and MEMORY[X] denotes the content at the memory location X.

Assume that the content of the memory location 5000 is 10, and the content of the register R3 is 3000. The content of each of the memory locations from 3000 to 3010 is 50. The instruction sequence starts from the memory location 1000. All the numbers are in decimal format. Assume that the memory is byte addressable.

After the execution of the program, the content of memory location 3010 is ________.

InstructionSemanticsInstruction Size (bytes)
MOV R1, (5000)R1 ← MEMORY[5000]4
MOV R2, (R3)R2 ← MEMORY[R3]4
ADD R2, R1R2 ← R1+R22
MOV (R3), R2MEMORY[R3] ← R24
INC R3R3 ← R3+12
DEC R1R1 ← R1-12
BNZ 1004Branch if not zero to the given absolute address2
HALTStop1

Try it: numerical answer: type the value

Numerical answer. No negative marking.
Show answer and explanation

Answer: 50 (official GATE 2021 final answer key)

Concept used: Trace the loop: address 1004 is the second instruction, and R1 counts the iterations down from 10.

First step: The loop body runs 10 times with R3 = 3000, 3001, …, 3009: is 3010 ever written?

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Source: official GATE 2021 Set 1 CS question paper and final answer key, organised by IIT Bombay. Question paper · Answer key

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