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### GATE CSE 2014 Set 3

Numerical
An instruction pipeline has five stages, namely, instruction fetch $$(IF),$$ instruction decode and register fetch $$(ID/RF)$$ instruction execution $$(EX),$$ memory access $$(MEM),$$ and register writeback $$(WB)$$ with stage latencies $$1$$ ns, $$2.2$$ $$ns,$$ $$2$$ $$ns,$$ $$1$$ $$ns,$$ and $$0.75$$ $$ns,$$ respectively ($$ns$$ stands for nanoseconds). To gain in terms of frequency, the designers have decided to split the $$ID/RF$$ stage into three stages $$(ID, RF1, RF2)$$ each of latency $$2.2/3$$ $$ns,$$ Also, the $$EX$$ stage is split into two stages $$(EX1, EX2)$$ each of latency $$1$$ ns. The new design has a total of eight pipeline stages. A program has $$20$$% branch instructions which execute in the $$EX$$ stage and produce the next instruction pointer at the end of the $$EX$$ stage in the old design and at the end of the $$EX2$$ stage in the new design. The IF stage stalls after fetching a branch instruction until the next instruction pointer is computed. All instructions other than the branch instruction have an average $$CPI$$ of one in both the designs. The execution times of this program on the old and the new design are $$P$$ and $$Q$$ nanoseconds, respectively. The value of $$P/Q$$ is _____________.

2

### GATE CSE 2014 Set 1

Numerical
Consider a $$6$$-stage instruction pipeline, where all stages are perfectly balanced. Assume that there is no cycle-time overhead of pipelining. When an application is executing on this $$6$$-stage pipeline, the speedup achieved with respect to non-pipelined execution if $$25$$% of the instructions incur $$2$$ pipeline stall cycles is________________.

3

### GATE CSE 2014 Set 3

Consider the following processors ($$ns$$ stands for nanoseconds). Assume that the pipeline registers have zero latency.
$$P1:$$ Four-stage pipeline with stage latencies $$1$$ $$ns,$$ $$2$$ $$ns,$$ $$2$$ $$ns,$$ $$1$$ $$ns.$$
$$P2:$$ Four-stage pipeline with stage latencies $$1$$ $$ns,$$ 1$$.5$$ $$ns,$$ $$1.5$$ $$ns,$$ $$1.5$$ $$ns.$$
$$P3:$$ Five-stage pipeline with stage latencies $$0.5$$ $$ns,$$ $$1$$ $$ns,$$ $$1$$ $$ns,$$ $$0.6$$ $$ns,$$ $$1$$ $$ns.$$
$$P4:$$ Five-stage pipeline with stage latencies $$0.5$$ $$ns,$$ $$0.5$$ $$ns,$$ $$1$$ $$ns,$$ $$1$$ $$ns,$$ $$1.1$$ $$ns.$$

Which processor has the highest peak clock frequency?

A
$$P1$$
B
$$P2$$
C
$$P3$$
D
$$P4$$
4

### GATE CSE 2013

Consider an instruction pipeline with five stages without any branch prediction: Fetch Instruction $$(FI),$$ Decode Instruction $$(DI),$$ Fetch Operand $$(FO),$$ Execute Instruction $$(EI)$$ and Write Operand $$(WO).$$ The stage delays for $$FI, DI, FO, EI$$ and $$WO$$ are $$5$$ $$ns,$$ $$7$$ $$ns,$$ $$10$$ $$ns,$$ $$8$$ $$ns$$ and $$6$$ $$ns$$, respectively. There are intermediate storage buffers after each stage and the delay of each buffer is $$1$$ $$ns.$$ A program consisting of $$12$$ instructions $${{\rm I}_1},{{\rm I}_2},{{\rm I}_3},......,\,\,{{\rm I}_{12}}$$ is executed in this pipelined processor. Instruction $${{\rm I}_4}$$ is the only branch instruction and its branch target is $${{\rm I}_9}$$. If the branch is taken during the execution of this program, the time (in $$ns$$) needed to complete the program is
A
$$132$$
B
$$165$$
C
$$176$$
D
$$328$$

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