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Two-digit Numeric Display

Photo by Nick Hillier on Unsplash


Hi! I'm continuing my blog about my SPO classes. After a brief introduction in Assembly, we are good to hit Lab3. Our instructor kindly let us choose one project out of five. And of course, we decided to go with the easiest!

We had to do a two-digit numeric display where the numbers are incremented or decremented by pressing plus and minus key in the keyboard. Soon the challenges were reviled as we dive into how to code it. Should we treat every digit separated or together? How to print them into the display?

After a moment of reflection, we decided to handle the digits independently to facilitate the printing display. Also, we had to add a bit-map representation of the numbers because the 6502 chip doesn’t know any font.

In this post, I’ll show you the code with the logic to increment and decrement without displaying anything. You can monitor the address $13 and $14 to make sure that it is working.

Let me explain how it works. When the program starts, it will set $13 and $14 to zero, and then it will fall into the infinite loop called “main.” This loop will know if one of the keys was pressed and then jump to the proper function. Let’s say that you hit the plus key. It will call the “incr_l” subroutine,  incrementing the low digit and checking if it is more than 9 (#$0A). If so, it will call the “incr_h” that will increment the high digit and set the low digit to zero. If the high digit gets more than 9, the counter resets to zero. The same logic is applied to decrementing, but instead, it limits at zero and reset it to 99.

In the next post, I’ll add the numeric display. See you.


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