CS 2110 Timed Lab 3: Assembly with Subroutines Solution

Please take the time to read the entire document before starting the assignment. It is your responsibility to follow the instructions and rules.
1 Timed Lab Rules - Please Read
1.1 General Rules
4. Resources you are allowed to use during the timed lab:
• Assignment files
• Previous homework and lab submissions
• Your mind
• Blank paper for scratch work (please ask for permission from your TAs if you want to take paper from your bag during the Timed Lab)
5. Resources you are NOT allowed to use:
• The Internet (except for submissions)
• Any resources that are not given in the assignment
• Textbook or notes on paper or saved on your computer
• Email/messaging
• Contact in any form with any other person besides TAs
6. Before you start, make sure to close every application on your computer. Banned resources, if found to be open during the Timed Lab period, will be considered a violation of the Timed Lab rules.
7. We reserve the right to monitor the classroom during the Timed Lab period using cameras, packet capture software, and other means.
1.2 Submission Rules
1. Follow the guidelines under the Deliverables section.
2. 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 files, what you turn in is what we grade. In addition, your assignment must be turned in via 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 Gradescope.
3. Do not submit links to files. We will not grade assignments submitted this way as it is easy to change the files after the submission period ends.
1.3 Is collaboration allowed?
Absolutely NOT. No collaboration is allowed for timed labs.
2 Overview
2.1 Purpose
The purpose of this timed lab is to test your understanding of the LC-3 calling convention by testing your ability to call subroutines by using the stack.
2.2 Task
You will implement one subroutine listed below in LC-3 assembly language. Please see the detailed instructions for the subroutine on the following pages. We have provided pseudocode for the subroutine, and suggest that you follow the algorithm when writing your assembly code. Your subroutine must adhere to the LC-3 calling convention.
1. tl3.asm
2.3 Criteria
Your assignment will be graded based on your ability to correctly translate the given pseudocode for a subroutine (function) into LC-3 assembly code, following the LC-3 calling convention. Please use the LC-3 instruction set when writing these programs. Check the deliverables section for what you must submit to gradescope and the deadlines.
You must obtain the correct values for each function. In addition, registers R0-R5 and R7 must be restored from the perspective of the caller, so they contain the same values after the caller’s JSR subroutine call. Your subroutine must return to the correct point in the caller’s code, and the caller must find the return value on the stack where it is expected to be. If you follow the LC-3 calling convention correctly, each of these things will happen automatically.
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 file and you will not receive any points.
3 Detailed Instructions
For this Timed Lab, you will be implementing a function named sumPowers, which takes in three arguments:
• head: The head of a linked list
• exp: A natural number
• func: The address of a function named func
To complete the timed lab, sumPowers must do the following: For each node in the linked list starting at head, you will calculate (node.data)exp, and add this value to a running sum named sum. Once this has been done for each node in the linked list, you will return the value func(sum). Pseudocode for what is expected can be found below. However, first a quick refresher on what a linked list looks like in LC-3 Assembly.
3.1 Linked List Data Structure
The below figure depicts how each node in our linked list is laid out. Each node will have two attributes: the next node, and a value for that node.
Memory Memory Memory Memory Head Node x4000 x4001 x4008 x4009
x4000 x4008 x0000
Address of Address of Data Address Data next Node next node of next node(null)
4 Pseudocode and Checkpoints
4.1 Pseudocode
Note: the function pow(int val, int exp) is provided for you at label POW in tl3.asm.
int sumPowers(Node head, int exp, function func) { int sum = 0; while(head != Null) { sum += pow(head.data, exp); head = head.next;
} return func(sum); }
4.2 Checkpoints (70 points)
In order to get all of the points for this timed lab, your code must meet these checkpoints:
• Checkpoint 1 (10 points): Store the address of the first node in the linked List to the label FIRSTNODE. This would be the address x4000 in the linked list example found above the pseudocode.
• Checkpoint 2 (25 points): Iterate through the linked list and for each node, calculate the value pow(node.data, exp) and add it to the sum.
• Checkpoint 3 (25 points): Once head == Null, store the value of sum into the label FINISHEDSUM.
• Checkpoint 4 (10 points): Match the pseudocode by returning the value func(sum) at the end of the subroutine. Hint: if you can’t figure out how to do this step, just return sum instead so that the auto-grader doesn’t complain and you will receive 60 points for the checkpoints.
All of the labels (FIRSTNODE, FINISHEDSUM, POW) mentioned above are provided in tl3.asm.
4.3 Other Requirements (30 points)
Your subroutine must follow the LC-3 calling convention. Specifically, it must fulfill the following conditions:
• When your subroutine returns, every register must have its original value preserved (except R6).
• When your subroutine returns, the stack pointer (R6) must be decreased by 1 from its original value so that it now points to the return value.
• During the execution of your subroutine, you must make n calls to pow, where n = length of the linked list, and one call to func (as described in the pseudocode).
5 Deliverables
Turn in the files
6 LC-3 Assembly Programming Requirements
6.1 Overview
1. Your code must assemble with NO WARNINGS OR ERRORS. To assemble your program, open the file with Complx. It will complain if there are any issues. If your code does not assemble you WILL get a zero for that file.
2. 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.
3. 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
Bad Comment ; if counter == 0 don’t loop again
ADD R3, R3, -1 ; Decrement R3
BRp LOOP ; Branch to LOOP if positive
4. 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 file.
5. Following from 3. You can randomize the memory and load your program by doing File - Randomize and Load.
6. 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.
7. Start the stack at xF000. The stack pointer always points to the last used stack location. This means you will allocate space first, then store onto the stack pointer.
8. Do NOT execute any data as if it were an instruction (meaning you should put .fills after HALT or RET).
9. Do not add any comments beginning with @plugin or change any comments of this kind.
10. You should not use a compiler that outputs LC3 to do this assignment.
11. Test your assembly. Don’t just assume it works and turn it in.
7 Appendix
7.1 Appendix A: LC-3 Instruction Set Architecture