Galaxian graphical glitch

[bendm]

Hey everyone, like everyone and their dog here I'm working on writing my own NES emulator using Haxe. Mostly it's working quite well, except for a handful of games with strange graphical glitches that I haven't been able to figure out. Such as Galaxian:



After pressing "start", the game "begins" but you can't really tell right away - the controller moves the block of text at the bottom side to side (from "1 player" down to "all rights reserved") as if it were your ship, shots come from the "Y" in "2 players", and the enemies look like a garbled mess until they fly out of formation.

Not a bad ROM, because it works in every other emulator I've tried it in.

I'm at my wit's end trying to figure out what could be causing this, especially since no other games that I've tested have any similar problems. Anyone have an idea what could be going on?

[rainwarrior]

I know Galaxian was one of the few games with an 8k PRG-ROM.

An iNES file should have the PRG-ROM doubled, I think, since the minimum spec is 16k, but how big is your file?

[Dwedit]

Sounds like a mirroring/scrolling issue.

[bendm]

You're right, it was an issue with my Y increment. I re-implemented exactly as specified in the wiki: http://wiki.nesdev.com/w/index.php/PPU_ ... ing_around and the problem was solved. Thanks!

[Sik]

So the player is not a sprite? o.O Weird, since the player isn't in an area likely to cause overflow...

[lidnariq]

Nope. The first 28 sprites are used for 7 opponents out of formation, the next 4 are for enemy explosions, the next 8 are all the bullets, and the last 24 are the background starfield.