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by klodolph
1774 days ago
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That’s what the “hardware collision” bit is about. You place sprite 0 at a certain location on screen, and you can find out when that sprite gets drawn (with some certain other restrictions / assumptions). IIRC, the collision bit gets set when a pixel from the sprite is drawn overlapping a background pixel. This is how early games like Super Mario Bros. drew the status bar, but Super Mario Bros. could just spin the CPU waiting for this. There are two other mechanism I know off the top of my head. One is to put some circuitry on the cartridge, and then arrange the usage of tiles such that all the background tiles are in one bank and all the foreground tiles are in another. If you do this right, you get an address line which cycles high and low once each scanline–and you can put a circuit on the cartridge which counts the scanlines, triggering an interrupt. This was only available if you could put that circuitry (usually, a “mapper”) on the cartridge. The last method is to count cycles. I would note that among other differences, this is much, much easier on a Game Boy. The Game Boy has a register you can read which tells you which row you are on. Not the only thing that’s easier on a Game Boy—there’s also a hblank interrupt, and the tilemap is larger than the screen. Anyone interested in NES programming but unsure about how much they like dealing with obscure technical problems may want to try Game Boy programming first to get a taste for it. |
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