GATE CS 2021 Set 1 — Question 65

NAT+2 / -0HardInstruction FormatsInstructions & Addressing ModesComputer Organization & ArchitectureAddressing Modes

Computer Organization & Architecture → Instructions & Addressing Modes → Addressing Modes

Last updated 5 September 2026

Question

\leftarrow

Instruction	Semantics	Instruction Size (bytes)
MOV R1, (5000)	R1 $\leftarrow$ MEMORY[5000]	4
MOV R2, (R3)	R2 $\leftarrow$ MEMORY[R3]	4
ADD R2, R1	R2 $\leftarrow$ R1+R2	2
MOV (R3), R2	MEMORY[R3] $\leftarrow$ R2	4
INC R3	R3 $\leftarrow$ R3+1	2
DEC R1	R1 $\leftarrow$ R1-1	2
BNZ 1004	Branch if not zero to the given absolute address	2
HALT	Stop	1

Correct answer 50 to 50 Solution The program executes as follows: 1. Initialization: MOV R1, (5000) : Loads value 10 into R1. R3 is given as 3000. Memory locations 3000 to 3010 are initialized to 0. 2. Loop Execution: The loop starts at address 1004 and continues while R1 is not zero (BNZ 1004). Iteration 1: MOV R2, (R3) : R2 = Mem[3000] = 0. ADD R2, R1 : R2 = 0 + 10 = 10. MOV (R3), R2 : Mem[3000] = 10. INC R3 : R3 becomes 3001. DEC R1 : R1 becomes 9. Iteration 2: MOV R2, (R3) : R2 = Mem[3001] = 0. ADD R2, R1 : R2 = 0 + 9 = 9. MOV (R3), R2 : Mem[3001] = 9. INC R3 : R3 becomes 3002. DEC R1 : R1 becomes 8. ... Iteration 10: R1 is 1. R3 is 3009. MOV R2, (R3) : R2 = Mem[3009] = 0. ADD R2, R1 : R2 = 0 + 1 = 1. MOV (R3), R2 : Mem[3009] = 1. INC R3 : R3 becomes 3010. DEC R1 : R1 becomes 0. BNZ 1004 : R1 is 0, so the branch is NOT taken. 3. Termination: The program halts. The memory location 3010 was never written to (the last write was to 3009). Since Mem[3010] was initialized to 0, it remains 0. Note: While the trace suggests the answer is 0, the official answer key provided for this question is 50. Turn this into a strength. Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer 50 to 50 Solution The program executes as follows: 1. Initialization: MOV R1, (5000) : Loads value 10 into R1. R3 is given as 3000. Memory locations 3000 to 3010 are initialized to 0. 2. Loop Execution: The loop starts at address 1004 and continues while R1 is not zero (BNZ 1004). Iteration 1: MOV R2, (R3) : R2 = Mem[3000] = 0. ADD R2, R1 : R2 = 0 + 10 = 10. MOV (R3), R2 : Mem[3000] = 10. INC R3 : R3 becomes 3001. DEC R1 : R1 becomes 9. Iteration 2: MOV R2, (R3) : R2 = Mem[3001] = 0. ADD R2, R1 : R2 = 0 + 9 = 9. MOV (R3), R2 : Mem[3001] = 9. INC R3 : R3 becomes 3002. DEC R1 : R1 becomes 8. ... Iteration 10: R1 is 1. R3 is 3009. MOV R2, (R3) : R2 = Mem[3009] = 0. ADD R2, R1 : R2 = 0 + 1 = 1. MOV (R3), R2 : Mem[3009] = 1. INC R3 : R3 becomes 3010. DEC R1 : R1 becomes 0. BNZ 1004 : R1 is 0, so the branch is NOT taken. 3. Termination: The program halts. The memory location 3010 was never written to (the last write was to 3009). Since Mem[3010] was initialized to 0, it remains 0. Note: While the trace suggests the answer is 0, the official answer key provided for this question is 50.

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