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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