Saturday, January 16, 2016

Crossing Page Boundaries

For awhile, everything was working perfectly in my 3-enemy display kernel on Atari Anguna. Then this weekend, I added some new graphics (for the crocky boss monster at the end of dungeon 1).

Suddenly, things stopped working right. Enemies started jumping around horizontally instead of smoothly moving. It looked choppy and horrible. And all I did was add some new graphics data.


Well, the issue was probably pretty obvious to any veteran Atari programmers.

On the Atari, when you reuse a sprite with a new horizontal position, you often get the weird black lines that appear on the left of the screen. I wrote a post about it awhile back, how if you write to the re-positioning register on exactly the 74th clock cycle the scanline, you'd prevent the line. If you did it earlier, you'd get the line. If you did it later, it might not HMOVE at all.

I've shown pictures of this before, but here's a sample of the black lines from doing the mid-frame HMOVE

Another important piece of the puzzle is the 6502's conditional branch opcodes. In general, they take 2 clock cycles if the jump wasn't taken, and 3 if they do. BUT if the jump passes a memory page boundary, it takes 4 cycles. (the 6502 divides memory into pages of 256 bytes each).

So it turns out, by adding additional graphics data, which ended up coming BEFORE my kernel code, it relocated the kernel code slightly, pushing one of my branches across a page boundary, and changing the clock cycle count of the routine by 1. Which made the HMOVE come too late, which made my re-position fail!

Well, the solution ended up being really easy, once I figured it out. Moving all the graphical data AFTER the kernel logic fixed it, and made sure that additional changes won't end up moving the kernel code.


And with that, I have most of a re-creation of the first dungeon of Anguna finished for the Atari. Since I had decided somewhere along the line that the first dungeon would be the same, but the rest would be mapped out differently, this means it's time to start doing level design again!

Fighting the Crocky boss. Like the GBA version, the Bow and Arrows make this fight easier!

Really, at this point, the majority of the game logic is done. I still want to add a title screen, game over sequence, maybe a subscreen that shows what items/keys/etc you have, maybe a password entry system (like old NES games) so you can save your progress, and if I'm not sick of the game by then, a shop where you can buy stuff.  Wow, that sounds like a lot when I list it that way....

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