Diving into 6502 Assembly: A Beginner's Journey

Embarking on the journey of learning assembly language can be both exciting and intimidating. Recently, I had the opportunity to delve into the world of 6502 assembly language, a popular instruction set used in many classic microcomputers and gaming consoles. In this blog post, I'll share my experience of running my first piece of 6502 code, understanding its functionality, and modifying it to change the output color on the screen.

Starting with the Basics

The 6502 processor is a marvel of its time, known for its simplicity and efficiency. To get started, my professor provided me with a snippet of 6502 assembly code. The goal of this code was to fill the screen with a specific color. Here's the code I was given:



When I ran this code in a 6502 emulator, the result was a screen filled with the color yellow. This was my first tangible output from assembly code, and it was quite a thrill to see!

Disassembling and Timing Analysis

To truly understand what was happening under the hood, I needed to disassemble the code and analyze the number of bytes for each operation, the type of operation, and the number of cycles each took to execute. This analysis would allow me to calculate the total runtime of the code.


Using an Excel spreadsheet, I meticulously counted the cycles and calculated the time it took for the code to run. This process gave me a deeper appreciation for the efficiency of assembly language and the importance of optimization.

Modifying the Code

The next task was to modify the code to change the screen color to light blue. This was a simple matter of changing the color number loaded into the accumulator before the loop. Here's the modified line:

    lda #$0E    ; colour number for light blue

 

After making this change and running the code again, the emulator displayed a light blue screen, confirming the success of my modification.

Visualizing the Output

To bring this blog post to life, I would include pictures of the emulator output showing the yellow and light blue screens. 



Conclusion

My first foray into 6502 assembly language was a fascinating experience that combined hands-on coding with analytical disassembly and timing calculations. It's a testament to the power of low-level programming and its ability to provide granular control over hardware. For anyone interested in the inner workings of computers or the history of computing, I highly recommend exploring assembly language – it's a rewarding challenge that offers a unique perspective on programming.

Stay tuned for more adventures in assembly, and happy coding!



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