Midterm Exam, CSCI 312, Fall 2017

• Make sure you answer each question completely and thoughtfully.
• Please make sure your answers are clearly labeled with the question number.
• Please write neatly and make any diagrams as neat as you can. If I can't read it, it is wrong.
• Point values are listed by each question.

1. [6 points] Draw a diagram that represents the WOMBAT CPU. Label all parts.
2. Fetch-Decode-Execute
   1. [6 points] Describe what happens at each phase of the Fetch-Decode-Execute cycle.

   2. [4 points] Using RTN, describe at a component level what occurs during the Execute phase of a WOMBAT add instruction.

3. [4 points] Registers
   1. What is a special purpose register? Give an example of a special purpose register in the WOMBAT architecture.

   2. What is a general purpose register? Give an example of a general purpose register in the WOMBAT architecture.

4. [8 points] The authors of our book describe four factors that affect CPU performance. Name each and describe how it affects the performance of a program.

5. [2 points] A CPU has a clock that runs at 2.0 GHz. Assume the CPI for this machine is 1. How long does it take to execute a single instruction. Please give your answer in proper units. Please show all work.

6. A processor, running at 2GHz, has three classes of instructions with characteristics for a given program listed in the table below.

   Class	CPI	Frequency
   A	1	20%
   B	2	20%
   C	4	60%

   1. [2 points] Compute the average CPI for this machine.

   2. [2 points] If the program requires 2x10⁹ instructions, how long will it take to execute this program.

   3. [2 points] A programmer has written an optimizer for the compiler that will reduce the frequency of instructions in class C to 40% but increase the frequency of instructions in class A and B to 25% and 35% respectively. In addition, the number of instructions required for the program will change to be 2.3x10⁹. Should you use this compiler optimization and why? Show all work and an unsupported answer is wrong.

7. For a 4 input MUX
   1. [4 points] Describe the input, output and function of this MUX.

   2. [2 points] Draw a circuit to implement this MUX.

8. [6 points] Design and implement a circuit which takes two bits as input and adds a third bit such that the number of 1s in the three bits is always odd. Your answer should include a truth table, an equation and a circuit.