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GATE | GATE-CS-2001 | Question 38

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Consider the following data path of a simple non-pilelined CPU. The registers A, B, A1, A2, MDR, the bus and the ALU are 8-bit wide. SP and MAR are 16-bit registers. The MUX is of size 8 × (2:1) and the DEMUX is of size 8 × (1:2). Each memory operation takes 2 CPU clock cycles and uses MAR (Memory Address Register) and MDR (Memory Date Register). SP can be decremented locally.

The CPU instruction “push r”, where = A or B, has the specification
M [SP] ← r
SP ← SP – 1
How many CPU clock cycles are needed to execute the “push r” instruction?

(A)

1

(B)

3

(C)

4

(D)

5



Answer: (D)

Explanation:

SP out, MAR in —— 2 cycles as they are 16 bit and system bus is of 8 bits

A out, MDR in ——— 1 cycle

M[MAR]<—– MDR ——- 2 cycles

So total 5 cycles,
option D is the correct answer


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Last Updated : 19 Nov, 2018
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