Instructions for First CPU Simulator Exercise

 

Bring up the Simple CPU Simulator referred to in these instructions in a separate, side-by-side browser window, if you haven't already.

Write your answers to the questions below in the provided Markdown template and submit it to Kit. Hint: you can reset the CPU Simulator back to its initial condition at any time by reloading the page.

The set of assembly language instructions that this CPU Simulator can execute appears at the bottom of this page.

Getting Started

  1. Translate the assembly language program that is provided when you load the CPU Simulator page to machine code by clicking on the "==>" button between the program and the main memory columns. [This translation process is called "assembling", and the software that does the translation is called an "assembler".]

    What changed in the simulator as a result? If you\'re not sure, reload the page and assemble the program again. Why did the assembler translate the number of lines that it did — why not more or less?

  2. Enter address 0 (or 00) into the Instruction Address or Program Counter (PC) field. Click on the "Fetch" button. What happened in the simulator?
  3. What do you think will happen when you click on the "Execute" button? Write your answer before executing. Also write down the values in any registers or memory addresses that you think might change as a result of executing this instruction. Now execute the instruction by clicking on the "Execute" button. What happened? Write down your actual results and see if they match your expected results.
    Expected Results Actual Results
       
       
       
  4. Continue to Fetch and Execute until the simulator halts. How many cycles are required?
  5. How many Fetch/Execute cycles do you think would be needed to add the contents of R0, R1, and R2 and put the result in R3? Document your expected results. Modify the assembly language and test your hypothesis. (Don\'t forget the HALT instruction.) Document the program you used and the actual number of cycles required, including the cycles needed to halt the program.
    Expected Results Actual Results
       
       
       
    Assembly Language Program:

     

Playing with the Program Counter

  1. Using your program from question 5, experiment with different start instructions. (Assuming you execute the HALT instruction each time you test your program, you will have to reload the page and re-type in your program for each question below.)
    1. What happens if you start at address 1?
    2. What happens if you start at address 5?
    3. What happens if you start at address 9?
    4. What happens if you start at address 14?
  2. What happens if you change the Instruction Address (PC) mid-program? For example, what happens if you change the PC to 01 just before fetching the HALT instruction? (In other words, if your HALT instruction is in memory location 02, change the PC from 02 to 01 before fetching instruction 02.)
  3. What happens if you Fetch multiple times before you click the Execute button?