Homework 5 Intro to Assembly Solution

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1.1 Purpose:

So far in this class, you have seen how binary or machine code manipulates our circuits to achieve a goal. However, as you have probably gured out, binary can be hard for us to read and debug, so we need an easier way of telling our computers what to do. This is where assembly comes in. Assembly language is symbolic machine code, meaning that we don’t have to write all of the ones and zeros in a program, but rather symbols that translate to ones and zeros. These symbols are translated with something called the assembler. Each assembler is dependent upon the computer architecture on which it was built, so there are many di erent assembly languages out there. Assembly was widely used before most higher-level languages and is still used today in some cases for direct hardware manipulation.

1.2 Task:

The goal of this assignment is to introduce you to programming in LC-3 assembly code. This will involve writing small programs, translating conditionals and loops into assembly, modifying memory, manipulating strings, and converting high-level programs into assembly code.

You will be required to complete the four functions listed below with more in-depth instructions on the following pages:

  1. mult.asm

  1. selectionsort.asm

  1. printvowels.asm

  1. linkedlist.asm

1.3 Criteria:

Your assignment will be graded based on your ability to correctly translate the given pseudocode into LC-3 assembly code. Check the deliverables section for deadlines and other related information. Please use the LC-3 instruction set when writing these programs. More detailed information on each instruction can be found in the Patt/Patel book Appendix A (also on Canvas under LC3 Resources). Please check the rest of this document for some advice on debugging your assembly code, as well some general tips for successfully writing assembly code.

You must obtain the correct values for each function. While we will give partial credit where we can, your code must assemble with no warnings or errors (Complx will tell you if there are any). If your code does not assemble, we will not be able to grade that le and you will not receive any points. Each function is in a separate le, so you will not lose all points if one function does not assemble. Good luck and have fun!


  • Detailed Instructions

2.1 Part 1: Implementing Multiply

To start you o with this homework, we are implementing the multiply function! Store the result of the operation in the label ANSWER. Arguments A and B are stored in memory, and you will load them from there to perform this operation. Assume the values of A and B are positive integers. Implement your assembly code in mult.asm.

Suggested Pseudocode:

a = (argument 1);

b = (argument 2);


while (b > 0) {




// note: when the while-loop ends, the value stored at ANSWER is a times b.

2.2 Part 2: Selection Sort

The second assembly function is to selection-sort all elements of an array in memory. Use the pseudocode to help plan out your assembly and make sure you are sorting it properly! Implement your assembly code in selectionsort.asm. For more information on selection sort and a good visual representation, check out this link.

Suggested Pseudocode:

x = 0;

// current swapping index in the array

len = length of array;

while(x < len – 1) {

z = x;

// index of minimum value in unsorted portion of array

y = x + 1;

// current index in array

while (y < len) {

if (arr[y] < arr[z]) {

z = y;

// update the index of the minimum value




if (z != x) {

temp = arr[x];

// perform a swap

arr[x] = arr[z];

arr[z] = temp;





2.3 Part 3: Printing Vowels

The third assembly function is to print the vowels in a null-terminated string. The label STRING will contain the address of the rst character of the string. Remember that strings are just arrays of consecutive characters. Implement your assembly code in printvowels.asm

Assume every character in the string is UPPERCASE.

To check for these vowel characters, refer to the ASCII table and remember that each of these characters are represented by a word (16-bits) in the LC-3’s memory. This is a null-terminated string, meaning that a 0 will be stored immediately after the nal character in memory!


0 is the same as ’n0’

0 is di erent from ’0’

Suggested Pseudocode:

string = “TWENTY ONE TEN”;

i = 0;

while(string[i] != ’\0’){

if(string[i] == ’A’ || string[i] == ’E’ ||

string[i] == ’I’ || string[i] == ’O’ ||

string[i] == ’U’){





Hint: Take a look at the trap vectors in Appendix A for printing characters to the console.


2.4 Part 4: Make elements of a Linked List into a string

For the nal problem, your goal is to create a string from the elements of a linked list and store that string at the label named ANSWER (don’t forget about the null terminating character mentioned in Part 3). In order to do so, look at the label we have given you:

LL: The address of a Linked List object. Similar to Java, our linked list is an object with two attributes – head and length. These two attributes are stored in memory like so:

Address of

Length of


Head Node

Linked List

lu32041u1ue tmp 93aa84de626d02ac

Every node in our linked list is another object with two attributes – next node and value. These two attributes are stored in memory like so:

Address of



next node

at node

lu32041u1ue tmp 93aa84de626d02ac

So together, our data structure would look something like this:









Address of

Length of

Head Node

Linked List







Address of

Value at

next node

head node








Value at

of next

next node


lu32041u1ue tmp 93aa84de626d02ac lu32041u1ue tmp f899bb890b25c933 lu32041u1ue tmp f899bb890b25c933

Now that you understand what data structure we are dealing with, we have provided the following pseudocode to help you begin your coding! This code will be implemented in the linkedlist.asm le.

Suggested Pseudocode:

length = LL.length; curr = LL.head; ANSWER = char[length]; i = 0;

while (curr != null) { ANSWER[i] = curr.value; curr = curr.next; i++;


//HINT: What can an LDI instruction be used for? //This character array will already be setup for you


  • Deliverables

Turn in the les

  1. mult.asm

  1. selectionsort.asm

  1. printvowels.asm

  1. linkedlist.asm

on Gradescope by February 25th at 11:55pm.

There will be a 6 hour late period until February 26th at 5:55am. You will receive a 25% deduction during this late period. Anything after that will result in an automatic 0.

Note: Please do not wait until the last minute to run/test your homework, history has proved that last minute turn-ins will result in long queue times for grading on Gradescope. You have been warned.


  • Running Autograder and Debugging LC-3 Assembly

When you turn in your les on gradescope for the rst time, you might not receive a perfect score. Does this mean you change one line and spam gradescope until you get a 100? No! You can use a handy tool known as tester strings.

  1. First o , we can get these tester strings in two places: the local grader or o of gradescope. To run the local grader:

Mac/Linux Users:

      1. Navigate to the directory your homework is in. In your terminal, not in your browser

      1. Run the command sudo chmod +x grade.sh

      1. Now run ./grade.sh

Windows Users:

      1. On docker quickstart, navigate to the directory your homework is in

      1. Run ./grade.sh

When you run the script, you should see an output like this:

lu32041u1ue tmp e45b9da4425ec1a3

Copy the string, starting with the leading ’B’ and ending with the nal backslash. Do not include the quotation marks.

Side Note: If you do not have docker installed, you can still use the tester strings to debug your assembly code. In your gradescope error output, you will see a tester string. When copying, make sure you copy from the rst letter to the nal backslace and again, don’t copy the quotations.

lu32041u1ue tmp b426df2f5807f41a

2. Secondly, navigate to the clipboard in your docker image and paste in the string.


lu32041u1ue tmp 59c83fe32a099627

3. Next, go to the Test Tab and click Setup Replay String

lu32041u1ue tmp 50679a09ea4b2f42

4. Now, paste your tester string in the box!

lu32041u1ue tmp 65f259b4673a5bcb

  1. Now, complx is set up with the test that you failed! The nicest part of complx is the ability to step through each instruction and see how they change register values. To do so, click the step button. To change the number representation of the registers, double click inside the register box.

lu32041u1ue tmp 771b85563ead4aa0

  1. If you are interested in looking how your code changes di erent portions of memory, click the view tab and indicate ’New View’


lu32041u1ue tmp 33d64da67805f37a

  1. Now in your new view, go to the area of memory where your data is stored by CTRL+G and insert the address

lu32041u1ue tmp 5f1d4504255446b2

  1. One nal tip: to automatically shrink your view down to only those parts of memory that you care about (instructions and data), you can use View Tab ! Hide Addresses ! Show Only Code/Data. Just be careful: if you misclick and select Show Non Zero, it may make the window freeze (it’s a known Complx bug).

lu32041u1ue tmp 1b16c37187ae855c


  • Appendix

5.1 Appendix A: ASCII Table

lu32041u1ue tmp cae225c45f77d840

Figure 1: ASCII Table | Very Cool and Useful!


5.2 Appendix B: LC-3 Instruction Set Architecture

lu32041u1ue tmp 96c5150524f1471b


5.3 Appendix C: LC-3 Assembly Programming Requirements and Tips

  1. Your code must assemble with NO WARNINGS OR ERRORS. To assemble your program, open the le with Complx. It will complain if there are any issues. If your code does not assemble you WILL get a zero for that le.

  1. Comment your code! This is especially important in assembly, because it’s much harder to interpret what is happening later, and you’ll be glad you left yourself notes on what certain instructions are contributing to the code. Comment things like what registers are being used for and what less intuitive lines of code are actually doing. To comment code in LC-3 assembly just type a semicolon (;), and the rest of that line will be a comment.

  1. Avoid stating the obvious in your comments, it doesn’t help in understanding what the code is doing.

Good Comment

ADD R3, R3, -1 ; counter–

BRp LOOP ; if counter == 0 don’t loop again

Bad Comment

ADD R3, R3, -1 ; Decrement R3

BRp LOOP ; Branch to LOOP if positive

  1. DO NOT assume that ANYTHING in the LC-3 is already zero. Treat the machine as if your program was loaded into a machine with random values stored in the memory and register le.

  1. Following from 3. You can randomize the memory and load your program by doing File – Randomize and Load.

  1. Use the LC-3 calling convention. This means that all local variables, frame pointer, etc. . . must be pushed onto the stack. Our autograder will be checking for correct stack setup.

  1. Start the stack at xF000. The stack pointer always points to the last used stack location. This means you will allocate space rst, then store onto the stack pointer.

  1. Do NOT execute any data as if it were an instruction (meaning you should put . lls after HALT or RET).

  1. Do not add any comments beginning with @plugin or change any comments of this kind.

  1. Test your assembly. Don’t just assume it works and turn it in.


5.4 Appendix D: Rules and Regulations

5.4.1 General Rules

  1. Starting with the assembly homeworks, any code you write should be meaningfully commented for your bene t. You should comment your code in terms of the algorithm you are implementing; we all know what each line of code does.

  1. Although you may ask TAs for clari cation, you are ultimately responsible for what you submit. This means that (in the case of demos) you should come prepared to explain to the TA how any piece of code you submitted works, even if you copied it from the book or read about it on the internet.

  1. Please read the assignment in its entirety before asking questions.

  1. Please start assignments early, and ask for help early. Do not email us the night the assignment is due with questions.

  1. If you nd any problems with the assignment it would be greatly appreciated if you reported them to the author (which can be found at the top of the assignment). Announcements will be posted if the assignment changes.

5.4.2 Submission Conventions

  1. All les you submit for assignments in this course should have your name at the top of the le as a comment for any source code le, and somewhere in the le, near the top, for other les unless otherwise noted.

  1. When preparing your submission you may either submit the les individually to Canvas/Gradescope or you may submit an archive (zip or tar.gz only please) of the les. You can create an archive by right clicking on les and selecting the appropriate compress option on your system. Both ways (uploading raw les or an archive) are exactly equivalent, so choose whichever is most convenient for you.

  1. Do not submit compiled les, that is .class les for Java code and .o les for C code. Only submit the les we ask for in the assignment.

  1. Do not submit links to les. The autograder does not understand it, and we will not manually grade assignments submitted this way as it is easy to change the les after the submission period ends.

5.4.3 Submission Guidelines

  1. You are responsible for turning in assignments on time. This includes allowing for unforeseen circum-stances. If you have an emergency let us know IN ADVANCE of the due time supplying documenta-tion (i.e. note from the dean, doctor’s note, etc). Extensions will only be granted to those who contact us in advance of the deadline and no extensions will be made after the due date.

  1. You are also responsible for ensuring that what you turned in is what you meant to turn in. After submitting you should be sure to download your submission into a brand new folder and test if it works. No excuses if you submit the wrong les, what you turn in is what we grade. In addition, your assignment must be turned in via Canvas/Gradescope. Under no circumstances whatsoever we will accept any email submission of an assignment. Note: if you were granted an extension you will still turn in the assignment over Canvas/Gradescope.

  1. There is a 6-hour grace period added to all assignments. You may submit your assignment without penalty up until 11:55PM, or with 25% penalty up until 5:55AM. So what you should take from this is not to start assignments on the last day and plan to submit right at 11:54AM. You alone are responsible for submitting your homework before the grace period begins or ends; neither Canvas/Gradescope, nor


your aky internet are to blame if you are unable to submit because you banked on your computer working up until 11:54PM. The penalty for submitting during the grace period (25%) or after (no credit) is non-negotiable.

5.4.4 Syllabus Excerpt on Academic Misconduct

Academic misconduct is taken very seriously in this class. Quizzes, timed labs and the nal examination are individual work.

Homework assignments are collaborative, In addition many if not all homework assignments will be evaluated via demo or code review. During this evaluation, you will be expected to be able to explain every aspect of your submission. Homework assignments will also be examined using computer programs to nd evidence of unauthorized collaboration.

What is unauthorized collaboration? Each individual programming assignment should be coded by you. You may work with others, but each student should be turning in their own version of the assignment. Submissions that are essentially identical will receive a zero and will be sent to the Dean of Students’ O ce of Academic Integrity. Submissions that are copies that have been super cially modi ed to conceal that they are copies are also considered unauthorized collaboration.

You are expressly forbidden to supply a copy of your homework to another student via elec-tronic means. This includes simply e-mailing it to them so they can look at it. If you supply an electronic copy of your homework to another student and they are charged with copying, you will also be charged. This includes storing your code on any site which would allow other parties to obtain your code such as but not limited to public repositories (Github), pastebin, etc. If you would like to use version control, use github.gatech.edu

5.4.5 Is collaboration allowed?

Collaboration is allowed on a high level, meaning that you may discuss design points and concepts relevant to the homework with your peers, share algorithms and pseudo-code, as well as help each other debug code. What you shouldn’t be doing, however, is pair programming where you collaborate with each other on a single instance of the code. Furthermore, sending an electronic copy of your homework to another student for them to look at and gure out what is wrong with their code is not an acceptable way to help them, because it is frequently the case that the recipient will simply modify the code and submit it as their own.


lu32041u1ue tmp 1e1a114d19b38143

Figure 2: Collaboration rules, explained colorfully