Pipelining

Question

Consider the 4 stage pipeline with the following stages

S1:Instruction fetch

S2:Instruction decode

S3:Execute

S4:Write back

Program consists of 16 instructions(I1,I2,...I16).In which I6 is a unconditional branch instruction transfer the control to I12.In the pipeline branch target address will be available at the end of execution state.Each instruction spends the same amount of time in all the pipeline is 4ns.How much time is required to execute the above program without using the branch prediction?

Comments

actually 4ns is clock time given in question

Answer 1

Uncondition branch instruction is always taken.

Here, branch will be taken at the end of S₃ stage of I₆ .

i.e., at the end of 8^th clock cycle. So, I₉, I₁₀ & I₁₁ will be skipped & I₁₂ will be taken.

Since 3 instruction is skipped Pipeline will execute 13 instruction.

so, Total clock cycle = 4 + (13-1)*1 = 16

Thus, Total Execution Time = 16 * 4 ns = 64 ns.

Answer 2

..............

 

Comments

but here since nothing is said about unconditional or conditional branch therefore target address must be present at 5th stage only

therefore,

penality=5-1=4

pls correct me where I m wrong

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It clearlly mention branch is taken during ex stage .

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Yes. Here, only $I_8$ will be skipped