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Two-digit Numeric Display - Final


Photo by Nick Hillier on Unsplash


In this post, I’ll continue the two-digit numeric display. If you miss it, click here and check it out. To finish this project, we just need to show the numbers in the matrix-pixel (the black-box in the 6502 emulator). To kickstart, our instructor gave us one example of how to display graphs, which was a lot helpful.

The first thing that I’ve noticed was the bitmap table at the bottom. So, I mimic it and made ten tables like that to represent each number (zero to nine). So far, so good! Then I grabbed the logic to display one digit, and then my nightmares just started. How to place two graphs (one for each digit)? How to switch from one number to another? How to reuse code? Where is my coffee?!

To emulate some if-elseif-else statements, I used jmp (jump). They are all over the place! However, the 6502 limits the jump range from -127 to 128. That means moving the code-blocks to satisfy all jumps limit. For example, the “main” moved down, and the “draw_lft” is above “main.”

Now the last problem is how to select the correct bitmap when the number changes. I tried arithmetic shift left, multiply by 64 and others, but no luck. Then I gave up and implemented that long compare-and-jump. It worked, but this is not elegant at all. I know that using arithmetic shift left and multiply by 64 works because the bitmap tables are stored continuously in the memory, so starting from the “num_0” I should be able to navigate to the correct digit. I just couldn’t deliver it.

Assembly has limitations. And I have it too. The simple fact that I couldn’t make a simple if-elseif-else statement discouraged me. Perhaps I have to change my mindset to work with such low-level language. See you in the next post.

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