Lab 3 - Follow up

Introduction

In this project, I modified a 6502 Assembly program originally designed for addition and turned it into a subtraction calculator. I used and adapted code from Krins Kumar’s blog post. This post satisfies that requirement by clearly labeling borrowed, modified, and new code segments.

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Source and Permission

• Original Code: “SPO Week 3.1 ‐ Full Lab” by Krins Kumar.

• Permission: Granted by the author, with freedom to skip attribution—but I’m including it as per my lab requirements.

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(The full code includes the DRAW_IMAGE, PRINT_STR, GET_NUMBER, UPDATE_GFX subroutines, along with the pixel data for digits. I have placed [BORROWED] or [MODIFIED] markers on relevant lines, and [NEW] markers where I added lines that did not exist previously in Krins’s code.)

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Key Changes and Additions

1. Arithmetic: Changed from ADC (addition) to SBC (subtraction) in BCD mode, with logic to handle negative results by swapping numbers.

2. Variable Renaming: e.g., value → SUM_BCD, XPOS → CUR_X, etc.

3. New Variables: SUB_NUM for the subtrahend, TEMP_BCD for swapping.

4. Updated Prompts: New strings asking the user for first and second numbers.

5. Minus Sign Output: If first < second, print “–” and swap.

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Conclusion

This project involved modifying a 6502 assembly program to implement subtraction instead of addition while maintaining proper BCD arithmetic. The code structure was improved for readability, and key functions were optimized.

🔗 GitHub Repositoryhttps://github.com/samartho4/SPO-600/Lab3