GATE | GATE-CS-2001 | Question 38

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: (B)

Explanation:

Push ‘r’
Consist of following operations
M [SP] !r
SP! SP – 1
‘r’ is stored at memory at address stack pointer currently is, this take 2 clock cycles.
SP is then decremented to point to next top of stack. So total cycles = 3
Hence (b) is correct option.

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