GATE | GATE-CS-2004 | Question 64

Directions for question 63 to 64: Consider the following program segment for a hypothetical CPU having three user registers R1, R2 and R3.

 Instruction      Operation      Instruction Size(in words)
 MOV R1,5000;      R1 ¬ Memory[5000]      2
 MOV R2, (R1);      R2 ¬ Memory[(R1)]      1
 ADD R2, R3;      R2 ¬ R2 + R3              1
 MOV 6000, R2;      Memory [6000] ¬ R2      2
 HALT              Machine halts              1

Let the clock cycles required for various operations be as follows: Register to/ from memory transfer: 3 clock cycles ADD with both operands in register : 1 clock cycle Instruction fetch and decode : 2 clock cycles per word The total number of clock cycles required to execute the program is
(A) 29
(B) 24
(C) 23
(D) 20


Answer: (B)

Explanation: The clock cycles are per block; if an instruction size is 2 then it requires twice no. of clock cycles.

Instruction no.       size no. of clock cycles

1                                2                                                                       3*1+2*2

2                                1                                                                       1*3+2

3                                1(add only)                                                      2+3

4                                2                                                                       3*1+2*2

5                                1                                                                       2(fetch and decode)

Total                                                                  24

So answer is (B)

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