Description
Overview
2.1 Purpose
The purpose of this section of the final exam is to test your understanding of coding in LC-3 assembly,
including the use of the LC-3 calling convention to write and call subroutines.
2.2 Task
This section of the final exam is divided into two subsections. In the first, you will be implementing a short
assembly program; in the second, you will be implementing an assembly subroutine. Please see the detailed
instructions for each part on the following pages. We have provided pseudocode for both parts—you should
follow these algorithms when writing your assembly code. Your subroutine must adhere to the LC-3 calling
conventions.
2.3 Criteria
This section 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 where relevant. Please use the
LC-3 instruction set when writing these programs. Check the Deliverables section for what you must submit
to Gradescope.
You must produce the correct return values for each function. Additionally, for your subroutine, 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
conventions correctly, all of these things will happen automatically. Additionally, we will check that you
made the correct subroutine calls, so you should not try to implement a recursively subroutine iteratively.
Your code must assemble with no warnings or errors (Complx and the autograder 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. The two parts are separated into their own files, so you can still receive credit for one part if the
other does not assemble.
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3 Detailed Instructions
For this section of the final exam, you will first be implementing an equivalent to the familiar GBA function
setPixel in assembly. This program will take in as inputs a row, column, height, width, and color value
via labels, and you will write this color value to the appropriate index in a VIDEOBUFFER array.
In the second part, you will be implementing a recursive subroutine, in accordance with the LC-3 calling
convention, to calculate the modulo of two numbers a and b.
3.1 setPixel
setPixel will write a given color value to the appropriate index in a VIDEOBUFFER array, representing a
2D image with given height and width, as specified by a row and column number. setPixel will take 5
arguments via labels.
• COLOR is the color value to write to VIDEOBUFFER
• HEIGHT is the height of the VIDEOBUFFER
• WIDTH is the width of the VIDEOBUFFER
• ROW is the row number of the pixel in the VIDEOBUFFER of which you will write the color value
• COL is the column number of the pixel in the VIDEOBUFFER of which you will write the color value
The address of the VIDEOBUFFER itself is given at the memory location labelled VIDEOBUFFER.
Your program should set the value at index ROW * WIDTH + COL of the VIDEOBUFFER equal to COLOR.
You may assume that ROW and COL will be in-bounds. The following pseudocode is provided for reference:
offset = 0;
for (i = 0; i < ROW; i++) {
offset += WIDTH;
}
offset += COL;
VIDEOBUFFER[offset] = COLOR
Please refer to Grading for details on how setPixel will be graded.
3.2 mod subroutine
The mod subroutine will take one integer argument a and one positive integer argument b and compute a
% b: that is, the remainder of performing integer division between a and b.
You should follow the given pseudocode to implement this subroutine, which accounts for all cases that you
are required to handle:
mod(a, b) {
if (a < 0) {
return mod(a + b, b);
}
if (a < b) {
return a;
}
return mod(a – b, b);
}
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Examples:
• mod(5, 2) returns 1
• mod(3, 4) returns 3
• mod(-7, 6) returns 5
Note: The precise mathematical definition of modulo here is as follows: mod(a,b) = a − (ba ÷ bc × b).
As such, this version of modulo will always return a remainder with the same sign as the divisor b (always
positive in this case), regardless of the sign of the dividend a. Additionally, note that while b is guaranteed
to be positive, a may be negative and your subroutine must handle this case (as a reminder, the provided
pseudocode does this already).
Please refer to Grading for details on how mod will be graded.
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4 Grading
Point distribution for this portion of the final exam is broken down as follows:
• setPixel (5 points): Writing the correct color value to the correct index of VIDEOBUFFER.
• mod return value (10 points): Returning the correct value of a % b from the mod subroutine.
• Other requirements (5 points): The mod subroutine must follow the LC-3 calling convention. Specifically, it must fulfill the following conditions:
– Your mod subroutine must be recursive and call itself according to the pseudocode’s description.
If the autograder claims that you are making an unknown subroutine call to some label in your
code, it may be that your code has two labels without an instruction between them. Removing
one of the labels should appease the autograder.
– 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.
5 Deliverables
Turn in the following files on Gradescope during your assigned final exam period:
1. setPixel.asm
2. mod.asm
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6 Local Autograder
To run the autograder locally, follow the steps below depending upon your operating system:
• Mac/Linux Users:
1. Navigate to the directory your files are in (in your terminal on your host machine, not in
the Docker container via your browser)
2. Run the command sudo chmod +x grade.sh
3. Now run ./grade.sh
• Windows Users:
1. In Git Bash (or Docker Quickstart Terminal for legacy Docker installations), navigate to the
directory your files are in
2. Run chmod +x grade.sh
3. Run ./grade.sh
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7 LC-3 Assembly Programming Requirements
7.1 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. 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.
3. Following from 4., you can randomize the memory and load your program by going to File > Advanced
Load and selecting RANDOMIZE for registers and memory.
4. 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.
5. The stack will start 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.
6. Do NOT execute any data as if it were an instruction (meaning you should put HALT or RET instructions
before any .fills).
7. Do not add any comments beginning with @plugin or change any comments of this kind.
8. You should not use a compiler that outputs LC3 to do this assignment.
9. Test your assembly. Don’t just assume it works and turn it in.
10. Comment your code! (not a hard requirement, but will make your life much easier) 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.
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
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8 Appendix
8.1 Appendix A: LC-3 Instruction Set Architecture
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