GATE 2015 CS (CS2) – Question 53
A four-stage pipeline has IF (fetch/decode), OF (operand fetch), PO (perform operation), WB (writeback). IF,OF,WB take one cycle; PO takes one for ADD/SUB, three for MUL, five for DIV. PO-to-OF forwarding is available. Find the total cycles for:
MUL R5,R0,R1
DIV R6,R2,R3
ADD R7,R5,R6
SUB R8,R7,R4Practise this question in The GATE Grind →
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Correct answer: 13
Explanation
The first instruction enters PO in cycle three and occupies cycles three through five, then writes in six. DIV occupies PO in cycles six through ten. ADD obtains its dependent operands via forwarding and executes in eleven; SUB executes in twelve. Final writeback is in thirteen. Thus thirteen cycles suffice.