Pipelining FAQ1. What is pipelining?Slide 32. How does it work?Slide 53. What are the performance gains?Slide 7Slide 8Slide 9Slide 104. What are the disadvantages?Slide 12Slide 135. Solutions to hazardsPipelining FAQAlex Jay CS 147 F’071. What is pipelining?•Ans: Computer design technique to increase instruction throughput. •Individual instructions are not sped up. Instead batches of instructions are more efficiently executed.•Execution time of pipelined and non-pipelined computer for a single instruction is the same.•When executing multiple instructions, pipelining decreases the speed of the entire job. 1. What is pipelining?2. How does it work?•As each instruction moves through the pipeline stages, the next instruction is moved into the vacated pipeline stage•Ideally, each stage in a pipelined system takes an equal amount of time to complete.•5 stage pipeline •IF = instruction fetch, ID = instruction decode, EX = execute, MEM = memory acces, WB = register write back2. How does it work?3. What are the performance gains? •Speedup: the ratio of the average non-pipelined instruction execution time per average pipelined execution time.pipelineddunpipelineTTSpeedup 3. What are the performance gains? stagesdunpipelinetToverheadtTstagespipelined )max(3. What are the performance gains? Instruction latency = 50+50+60+60+50+50= 320 ns Time to execute 100 instructions = 100*320 = 32000 ns3. What are the performance gains? The length of pipelined stage = MAX(lengths of unpipelined stages) + overhead = 60 + 5 = 65 ns Instruction latency = 6x65 ns =390nsTime to execute 100 instructions = 65*6*1 + 65*1*99 = 390 + 6435 = 6825 ns3. What are the performance gains? What is the speedup obtained from pipelining? Solution: Average instruction time not pipelined = 320 ns Average instruction time pipelined = 65 ns Speedup = 320 / 65 = 4.92•Non-pipelined design prevents branch delays. This makes them easier and cheaper to make.•Instruction latency is higher in pipelined designs because of added flip flops into the data path.4. What are the disadvantages?4. What are the disadvantages?Pipeline hazards.–Structural: The simultaneous use of the same resources.–Control: Program branches and overall flow.–Data: Data dependencies between instructions.5. Solutions to hazards•Software inserted No-Ops into the instructions.•Hardware inserted Stalls. Similar to No-ops.•Branch prediction schemes. Assume a correct path and prefetch that instruction branch. ( can have up to 90%
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