Computer Organization & Architecture: Exercises 1 [PDF]

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Computer Organization & Architecture Exercises 1 You now need to enter instructions into the CPU simulator. You do this by first creating a new program and then clicking on the ADD NEW… button in the Edit Program tab. This will display the Instructions: CPU0 window. Use this window to enter the instructions. You’ll find a list of useful CPU simulator instructions and examples at the end of the above document. You can also edit the instructions by using the edit commands available in the Edit Program tab.



Please record your work in the boxes as you go along.



1. Create an instruction that moves number 5 to register R00.



1. Execute the above instruction by simply double clicking on it in the CPU INSTRUCTIONS IN MEMORY (RAM) view. 2. Create an instruction that moves number 8 to register R01.



3. Execute it. 4. Observe the contents of R00 and R01 in the Register Set view.



5. Create an instruction that adds the contents of R00 and R01.



6. Execute it. 7. Write down which register the result is stored in.



Result is stored in R01 register.



8. Create an instruction that pushes the above result to the top of the program stack, and then execute it. Observe the result in PROGRAM STACK (RAM) view.



9. Create an instruction to push number -2 on top of the stack and execute it.



10. Create an instruction to compare the values in registers R00 and R01.



11. Execute it. 12. Record the status (i.e. set or reset) of the Z/N flags of the status register. If the box is checked then means set otherwise means not set.



13. Create an instruction to unconditionally jump to the first instruction.



14. Execute it. 15. Observe the value in the PC register. What address is it pointing to? Explain.



The program counter register is set to zero because we have jumped at the start of program. 16. Create an instruction to pop the value on top of the program stack into register R02.



17. Execute it. 18. Create an instruction to pop the value on top of the program stack into register R03.



19. Execute it. 20. Execute the last instruction again. What happened? Briefly explain.



The stack is empty because we already popped all values from stack and the value of R03 is set to zero. 21. Create a compare instruction that compares values in registers R04 and R05.



22. Manually insert two equal values in registers R04 and R05. 23. Execute the compare instruction in step 22 above. 24. Which of the status flags Z/N is set? Why?



The zero flag is set because both values are equal. 25. Manually insert a value in register R05 greater than that in register R04. 26. Execute the compare instruction in step 22 above. 27. Which of the status flags Z/N is set? Why?



No flags are set because they were neither equal nor R05 was less than R04 28. Manually insert a value in register R04 greater than that in register R05. 29. Execute the compare instruction in step 22 above. 30. Which of the status flags Z/N is set? Why?



Flag N is set because value in R04 was greater than R05.



31. Create an instruction that will conditionally jump to the first instruction if the values in registers R04 and R05 are equal (Note: You will need to execute the compare instruction first before you execute the jump instruction if you change values in R04 and R05)



32. Test this instruction by manually putting values in registers R04 and R05 and then first executing the compare instruction and then executing the jump instruction (i.e. You will need to execute the compare instruction first before you execute the jump instruction every time you change values in R04 and R05)



Exercises 2 You now need to enter instructions into the CPU simulator. You do this by first creating a new program and then clicking on the ADD NEW… button in the Edit Program tab. This will display the Instructions: CPU0 window. Use this window to enter the instructions. You can also edit the instructions by using the edit commands available in the Edit Program tab. You’ll find a list of useful CPU simulator instructions and examples in CPU Simulator Instruction Set. When a new program is created by the simulator a dedicated memory space is also created for it. The program can then store data in this memory space using the store instruction. It can also load data from this memory into a register using the load instruction. This tutorial uses these instructions. To see the contents of the program’s memory click the SHOW PROGRAM DATA MEMORY… button. You’ll also need to see the simulated console during the tutorial. The console is used to display text and input data. You can show the console window by clicking on the INPUT OUTPUT… button.



Please record your work in the boxes as you go along.



1. In the Appendix, locate the instruction, which is used to store a one byte of data in a memory location.Use it to store number 65 in address location 20 (all numbers are in decimal). This is an example of direct addressing.



2. Create an instruction to move decimal number 22 to register R01.



3. Create an instruction to store decimal number 51 in address location currently stored in register R01. This is an example of indirect addressing. Note the use of the @ prefix with R01.



4. Make a note of what you see in memory locations 20 and 22?



5. Now, let’s create a loop. First, enter the following code. The # prefix is used to denote a literal value thus distinguishing it from an address value which does not need it. R01 represents an arbitrary register; you can use any of the registers from R00 to R31. MOV #0, R01 ADD #1, R01 CMP #5, R01 JNE 0



6. The above code is not quite ready yet. The JNE instruction uses a numeric value as the address to jump to. In this case it is 0. This may not always the case so in order to make the code more flexible we will use labels to represent instruction addresses. The simulator allows you to do this. Follow the instructions below to do this: Highlight the MOV instruction Click on the INSERT BELOW… button



MOV #0, R01



Type label name L1 in the box next to the ENTER LABEL button



L1



Click the ENTER LABEL button



ADD #1, R01



The new code should look like this:



CMP #5, R01 JNE 0 HLT



Now, highlight the JNE instruction Click on the EDIT… button



MOV #0, R01



Select L1 in the drop-down list under the Source Operand section



L1



Click the EDIT button



ADD #1, R01



The new code should look like this:



CMP #5, R01 JNE $L1



7. As you can see, the label L1 represents the address of the instruction immediately



below it, i.e. the ADD instruction. So now the JNE instruction can use L1 as the address to jump to. As the label L1 can represent any address this code should work anywhere in memory making it very flexible. The above code is now ready to run. To run this program follow the instructions below: Click on the RESET PROGRAM button Highlight the MOV instruction, i.e. the first instruction of the program Adjust the speed slider to a value nearest to the value 80 Click on the RUN button After a short while the program should stop. If it appears to run too long then click on the STOP button and check your code. Correct it if necessary and repeat the above instructions once again.



When the program stops make a note of the value in R01



The value in R01 is 5 because the loop runs 5 times. 8. Now you’ll make a slight modification to the above program. Change the program



code so that the program loop is repeated as long as the value of R01 is less than or equal to 3. Make a note of the new code below:



9. Let’s create a simple subroutine. Enter the following new code. You need to create a new label L0 at the start of the subroutine. This label represents the starting address of the subroutine. You must enter the label using the ENTER LABEL button only as explained in (6). Also, make sure you select the Direct Mem radio button when entering the first operand value 24 of the OUT instruction: L0 OUT 24, 0



10.The above subroutine code simply displays the text starting at data memory location 24 and returns (see RET instruction in appendix). For it to work there needs to be some text in location 24. You can do this manually by following the steps below: Click on the SHOW PROGRAM DATA MEMORY… button. In the displayed window highlight the line 0024 under LAdd column Under Initialise Data click on the String radio button Enter some text in the text box, e.g. My name is Besim Click the UPDATE button



11.Now, a subroutine is of no use by itself. For it to be useful your program must call it using the instructions MSF followed by CAL. The CAL instruction needs to include the starting address of the subroutine. Let’s modify our code so that the above subroutine is called and it displays the text repeatedly in a loop. For example, using the code in (6) the modified program should look something like this (you need to modify the code you created in (7) above): MOV #0, R01 L1 ADD #1, R01 MSF CAL $L0 CMP #5, R01



12.The above code is now ready to run. In order to see the displayed text you need to show the console window. Click on the INPUT OUTPUT… button which will display the simulated console window. To run this program follow the instructions below: Click on the RESET PROGRAM button Highlight the MOV instruction, i.e. the first instruction of the program Adjust the speed slider to a value nearest to the value 80 Click on the RUN button



13.We need to make a small change to our subroutine. Currently the OUT instruction uses direct memory addressing, i.e. the memory address 24 is part of the instruction. We now wish to make it use indirect addressing in a way similar to that in (4). So, you’ll need to place the memory address 24 in a register. Then you need to have the OUT instruction use this register indirectly as the source of the address of the text to display. Run the code to test your modification. Make a note of the modified program code below:



Exercises - 3 Enter the instructions you create in order to answer the questions. Refer to Appendix at the end of this document to find the details on the desired instructions. You are expected to execute the instructions you created on the simulator in order to verify your answers.



A. Loops using jump and compare instructions: 1. Write a conditional statement such that if R02 is greater than (>) R01 then R03 is set to 8. (Use R01 as the first operand and R02 as the second operand)



2. Write a conditional statement such that if R02 is less than or equal to ( 0. Set the initial value of R04 to 8.



6. Write a loop that repeats until R05 is > R09. Set the initial values of R05 to 0 and R09 to 12.



B. Instructions for writing to and reading from memory (RAM) : The following instructions access the program’s data memory. You can display this memory so that you can observe the results by referring to Image 1 and the related text in section D above.



9. Locate the instruction that stores a byte in memory and use it to store number 65 in memory address location 20 (this uses memory direct addressing method).



10. Move number 51 into register R04. Use the store instruction to store the contents of R04 in memory location 21 (this uses register direct addressing method).



11. Move number 22 into register R04. Use this information to indirectly store number 58 in memory (hint: you will need to use the ‘@’ prefix for this – see the list of instructions in appendix) - (this uses register indirect addressing method).



12. Locate the instruction that loads a byte from memory into a register. Use this to load the number in memory address 22 into register R10.



C. Putting it together:



13. Challenge #2: write a loop in which 10 numbers from 48 to 57 are stored as single bytes in memory starting from memory address 24. You should use register indirect addressing method of storing the numbers in memory (see exercise 11 above).



14. Challenge #3: write a loop in which the numbers stored in memory by the program in (13) above are copied to a different part of the memory starting from address location 80