CSEP 548 - Autumn 2015 - Homework 1

Issued: Oct 1, 2015

Due: Oct 15, 11:59pm.

Last Updated: Oct 1, 2015

This homework should be able to be done after doing the readings assigned for the first lecture. This assignment is borrowed from Milo Martin's class at UPenn (source).

Instructions:

This is an individual work assignment. Sharing of answers or code is strictly prohibited. For the quantitative questions, show your work to document how you came up with your answers.

Submission:

Turn in an electronic copy in PDF format to the Catalyst dropbox

Problems:

1. Performance and CPI. Assume a typical program has the following instruction type breakdown:

   • 30% loads
   • 10% stores
   • 50% adds
   • 8% multiplies
   • 2% divides

   Assume the current-generation processor has the following instruction latencies:

   • loads: 4 cycles
   • stores: 4 cycles
   • adds: 2 cycles
   • multiplies: 16 cycles
   • divides: 50 cycles

   If for the next-generation design you could pick one type of instruction to make twice as fast (half the latency), which instruction type would you pick? Why?

2. Averaging. Assume that a program executes one branch every 4 instructions for the first 1M instructions and a branch every 8 instructions for the next 2M instructions. What is the average number instructions per branch for the entire program?

3. Amdahl's Law. A program has 10% divide instructions. All non-divide instructions take one cycle. All divide instructions take 20 cycles.

   1. What is the CPI of this program on this processor?
   2. What percent of time is spent just doing divides?
   3. What would the speedup be if we sped up divide by 2x?
   4. What would the speedup be if we sped up divide by 5x?
   5. What would the speedup be if we sped up divide by 20x?
   6. What would the speedup be if divide instructions were infinitely fast (zero cycles)?

4. Performance and ISA. Chip A executes the ARM ISA and has a 2.5Ghz clock frequency. Chip B executes the x86 and has a 3Ghz clock frequency. Assume that on average, programs execute 1.5 times as many ARM instructions than x86 instructions.

   1. For Program P1, Chip A has a CPI of 2 and Chip B has a CPI of 3. Which chip is faster for P1? What is the speedup for Program P1?
   2. For Program P2, Chip A has a CPI of 1 and Chip B has a CPI of 2. Which chip is faster for P2? What is the speedup for Program P2?
   3. Assuming that Programs P1 and P2 are equally important workloads for the target market of this chip, which chip is "faster"? Calculate the average speedup.

5. ISA Modification and Performance Metrics. You are the architect of a new line of processors, and you have the choice to add new instructions to the ISA if you wish. You consider adding a fused multiply/add instruction to the ISA (which is helpful in many signal processing and image processing applications). The advantage of this new instruction is that it can reduce the overall number of instructions in the program by converting some pairs of instructions into a single instruction. The disadvantage is that its implementation requires extending the clock period by 5% and increases the CPI by 10%.

   1. Calculate under what conditions adding this instruction would lead to an overall performance increase.
   2. Discuss qualitatively the impact this proposed change could have on energy consumption, complexity of processor design, and code generation by compilers.

Textbook problems

The following problems come from Appendix A in the Hennessy/Patterson Computer Architecture, 5th Edition.

1. (A.8: a. only) For the following we consider instruction encoding for instruction set architectures.

1. Consider the case of a processor with an instruction length of 12 bits and with 32 general-purpose register so the size of the address fields is 5 bits. Is it possible to have instruction encodings for the following?

• 3 two-address instructions
• 30 one-address instructions
• 45 zero-address instructions

2. <skip>
3. <skip>

2. (A.9) For the following assume that values A, B, C, D, E, and F reside in memory. Also assume that instruction operation codes are represented in 8 bits, memory addresses are 64 bits, and register addresses are 6 bits, and data values are 32-bit integers (4 bytes each).

   Stack	Accumulator	Register (register-memory)	Register (load-store)
   Push A Push B Add Pop C	Load A Add B Store C	Load R1,A Add R3,R1,B Store R3,C	Load R1,A Load R2,B Add R3,R1,R2 Store R3,C

   Figure A.2 The code sequence for ``C = A + B`` for four classes of instruction sets. Note that the Add instruction has implicit operands for stack and accumulator architectures and explicit operands for register architectures. It is assumed that A, B, and C all belong in memory and that the values of A and B cannot be destroyed. Figure A.1 shows the Add operation for each class of architecture.

   1. For each instruction set architecture shown in Figure A.2, how many addresses, or names, appear in each instruction for the code to compute C = A + B, and what is the total code size?
   2. Some of the instruction set architectures in Figure A.2 destroy operands in the course of computation. This loss of data values from processor internal storage has performance consequences. For each architecture in Figure A.2, write the code sequence to compute:

   C = A + B
   D = A - E
   F = C + D
   

   In your code, mark each operand that is destroyed during execution and mark each "overhead" instruction that is included just to overcome this loss of data from processor internal storage. What is the total code size, the number of bytes of instructions and data moved to or from memory, the number of overhead instructions, and the number of overhead data bytes for each of your code sequences?