Final Sample Questions Question 1: The design team for a simple, single-issue processor is choosing between a pipelined or non-pipelined implementation. Here are some design parameters for the two possibilities: (a) For a program with 20% ALU instructions, 10% control instructions and 75% memory instructions, which design will be faster? Give a quantitative CPI average for each case. (b) For a program with 80% ALU instructions, 10% control instructions and 10% memory instructions, which design will be faster? Give a quantitative CPI average for each case. Problem 2 The execution flow in a pipelined CPU is clearly visible by drawing the pipeline execution diagram. Draw the pipeline execution diagram of the following code, taking hazards into account What kinds of hazards are there and explain why? Show in the diagram with arrows how to fix the hazards lw $2, 16($3) sub $4, $2, $1 add $3, $4, $1 sw $3, 8($8) Problem 3. a) Calculate the performance of a processor taking into account stalls due to data cache and instruction cache misses. The data cache (for loads and stores) is the same as described in Part B and 30% of instructions are loads and stores. The instruction cache has a hit rate of 90% with a miss penalty of 50 cycles. Assume the base CPI using a perfect memory system is 1.0. Calculate the CPI of the pipeline, assuming everything else is working perfectly. Assume the load never stalls a dependent instruction and assume the processor must wait for stores to finish when they miss the cache. Finally, assume that instruction cache misses and data cache misses never occur at the same time. Show your work. • Calculate the additional CPI due to the icache stalls. • Calculate the additional CPI due to the dcache stalls. • Calculate the overall CPI for the machine. Problem 4. a. Explain in detail what the advantage of pipelining is? (5 points) b. A Single-cycle CPU is running at 500MHz. How much time does it take to execute 2 billion instructions on the CPU, assuming the perfect cache (that is, memory access takes 1 CPU clock cycle)? (3 points) c. You decided to convert the single-cycle CPU to a 5-stage pipelined CPU for performance. Assume that you completely balanced the pipeline stages. What is the operating clock frequency and why? (3 points) d. Among the same 2 billion instructions, 30% of the instructions are lw instructions and 40% of the load outcomes are used by the next instructions. How much time does it take to execute 2 billion instructions on the pipelined CPU, assuming the perfect cache? Problem5 1. For the MIPS datapath shown below, several lines are marked with “X”. For each one: • Describe in words the negative consequence of cutting this line relative to the working, unmodified processor. • Provide a snippet of code that will fail • Provide a snippet of code that will still work Problem 6 Structural, data and control hazards typically require a processor pipeline to stall. Listed below are a series of optimization techniques implemented in a compiler or a processor pipeline designed to reduce or eliminate stalls due to these hazards. For each of the following optimization techniques, state which pipeline hazards it addresses and how it addresses it. Some optimization techniques may address more than one hazard, so be sure to include explanations for all addressed hazards. (a) Branch Prediction (b) Instruction Scheduling (c) delay slots (d) increasing availability of functional units (ALUs, adders etc) (e) caches (b) Instruction Scheduling (c) delay slots (d) increasing availability of functional units (ALUs, adders etc) (e) caches Problem 7 Pipelining is used because it improves instruction throughput. Increasing the level of pipelining cuts the amount of work performed at each pipeline stage, allowing more instructions to exist in the processor at the same time and individual instructions to complete at a more rapid rate. However, throughput will not improve as pipelining is increased indefinitely. Give two reasons for this.