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Asked: 2025-04-06T08:14:51+05:30

Why isn’t my chip-8 emulator displaying the built-in font correctly when rendering hex numbers from ROM files?

I’m stuck with my Chip-8 emulator project that I’m building in Godot 4.3, and I could really use some help! I’ve gotten Pong working, and you can play it without any issues. However, when it comes to displaying numbers using the built-in font, things go sideways. Instead of seeing the correct hex representations, I’m getting these nonsense graphics, and it’s really frustrating.

To give you some context, I know that each character in the Chip-8’s font is supposed to be 4 pixels wide and 5 pixels high. For example, the character `0` is represented by the byte encoding `0xF0, 0x90, 0x90, 0x90, 0xF0`. I’m loading the font into memory correctly at offset `0x50`, and I see that the data for the font is formatted as a `PackedByteArray`.

Here’s the kicker: while I’m confident that my font-loading logic isn’t broken, the actual drawing part seems to go haywire when I attempt to render numbers. I suspect there might be an issue with how I interpret the pixel data or how I manage the drawing coordinates on the screen, but I can’t quite put my finger on it.

My draw function, based on the opcode, uses the `DXYN` instruction to draw sprites. First, it clears the VF register and calculates the display positions correctly based on the registers. Then, it loops through the sprite data to draw the pixels on the screen. However, even though the drawing instructions seem to be working fine for other shapes, the numbers are just not displaying correctly.

I’ve attached a couple of screenshots that capture my emulator’s current output. You can see what the graphical output looks like, and it’s definitely not the expected hex numbers. I’m really scratching my head here! Is there something that I might be missing in my implementation? Perhaps it’s something with how the sprite data is loaded into RAM or how we handle pixel collision and wrapping?

Any pointers or advice you could share would be immensely appreciated! I want to get this sorted so that I can focus on adding more exciting features to the emulator. Thanks in advance!

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    2 Answers

    1. Help with Displaying Numbers in Chip-8 Emulator!

      It sounds like you’re having a tough time with your Chip-8 emulator, especially with getting those numbers to display correctly. Here are a few things you might want to check out:

      • Check your Pixel Data: Make sure that your font data for each number is correctly defined in your PackedByteArray. You can try printing out the bytes before you draw to see if they match what you expect. Is the data for `0` actually `0xF0, 0x90, 0x90, 0x90, 0xF0`?
      • Draw Function Logic: Since you mentioned using the `DXYN` opcode, ensure you are interpreting the pixel data correctly in your draw loop. Remember that each row of the sprite data corresponds to a vertical slice of pixels on the display. Make sure you’re looping through the right number of bytes and getting the correct bits for each pixel!
      • Coordinate System: Sometimes issues crop up with coordinate calculations. Make sure that the coordinates for drawing are set correctly based on the current position of the sprite and that they stay within the screen bounds.
      • Collision Detection: If you’re handling pixel collision, double-check to ensure you’re not accidentally altering the pixels when drawing the font sprites. If your collision checks are not set up to handle the specific requirements of the Chip-8, it could result in unexpected output.
      • Debugging: It might help to isolate the drawing part of the code and test it with static values or hardcoded sprite data to see if the problem lies in the data fetching or the drawing part. Sometimes just visualizing the data being drawn as a grid can provide insights!

      Don’t get too discouraged! These little quirks are super common while developing emulators. Just take it step by step, and you’ll likely find the culprit. Good luck with your project!

    2. Based on your description, your font loading logic seems solid, especially considering you’ve correctly loaded the standard Chip-8 font data at offset 0x50 with appropriate byte representation. Given that sprites for things like Pong are rendering correctly, the problem likely resides specifically in how font sprite data is being processed or displayed. One common culprits involves incorrectly interpreting the sprite byte data. Remember, each byte represents 8 horizontal pixels, where a ‘1’ bit indicates a pixel should be turned on, and a ‘0’ bit means off. A typical confusion is treating the bits in byte order incorrectly, causing sprite pixels to look distorted. Be sure that your implementation correctly extracts and draws bits from left-to-right, with the most significant bit (bit 7) being the leftmost pixel and least significant (bit 0) the rightmost.

      Additionally, double-check your drawing loop in the DXYN instruction. Even a minor misalignment in x or y coordinates or improper modulo logic (for wrapping pixels around on screen edges) can scramble sprite display. Since your implementation works for Pong paddles and ball sprites, it suggests an issue specifically related to fetching font sprites from memory or in sprite indexing. Inspect your opcode parsing closely to ensure you calculate the correct font offset in memory. You might even print the memory address you’re reading from right before drawing; if that doesn’t match your expected font sprite locations, it indicates a problem with addressing. Making these checks should lead you to pinpoint the exact source of distortion you’re seeing with numeric font sprites.