Pipelining and instruction cycle Consider the sequence of machine instruction given below:
\begin{array}{ll} \text{MUL} &amp; \text{R5, R0, R1} \\  \text{DIV} &amp; \text{R6, R2, R3} \\   \text{ADD} &amp; \text{R7, R5, R6} \\  \text{SUB} &amp; \text{R8, R7, R4}  \\ \end{array}
In  the above sequence, $R0$ to $R8$ are general purpose registers. In the instructions shown, the first register shows the result of the operation performed on the second and the third registers. This sequence of instructions is to be executed in a pipelined instruction processor with the following $4$ stages: $(1)$ Instruction Fetch and Decode $(IF)$, $(2)$ Operand Fetch $(OF)$, $(3)$ Perform Operation $(PO)$ and $(4)$ Write back the result $(WB)$. The $IF, OF$ and $WB$ stages take $1$ clock cycle each for any instruction. The $PO$ stage takes $1$ clock cycle for $ADD$ and $SUB$ instruction, $3$ clock cycles for $MUL$ instruction and $5$ clock cycles for $DIV$ instruction. The pipelined processor uses operand forwarding from the $PO$ stage to the $OF$ stage. The number of clock cycles taken for the execution of the above sequence of instruction is _________.
I got the answer as 15 however the correct answer is 13. Am I doing something wrong or is the answer key at fault?
The link for reference: https://gateoverflow.in/8218/gate2015-2-44
Note the sentence "The pipelined processor uses operand forwarding from the PO stage to the OF stage". That means if a result is written at cycle n, the same result can be read as an operand at the same cycle n. This happens twice in your instruction sequence. If you ignore this, your count will be too high by two.