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that byte at $fff9


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According to all the literature I keep reading, if the low nybble of $fff9 in the cartridge ROM is set to any other value besides $x3 and $x7, the BIOS will reject it as an invalid cart. That's true enough.


What doesn't seem true is how $x3 and $x7 act. According to some sources (including the badly-aged BIOS code source from GCC) if the value at $fff9 in the cart ROM is set to $x7, it will display the Atari logo and fuji symbol, but if it's set to $x3 it will skip that. However, I've been studying the NTSC BIOS ROM closely, and I've noticed that before the routine at $fb84 which actually checks $fff9 is called, the BIOS ROM is switched back in. And I haven't found any occurrence of the routine at $fb84 being called elsewhere in the BIOS. (Edit: Also, there's the fact that the BIOS ROM would have to be switched in for the routine at $fb84 to be called, since it isn't copied to RAM.)


So it checks $fff9 in the BIOS ROM, NOT the cart's ROM!


I tested this by changing the byte at $fff9 from $c7 to $c3 in the Asteroids ROM and re-signing it, then running it in an emulator. Sure enough my suspicions were confirmed - the Atari logo and fuji symbol were still displayed.


So either I'm wrong or everybody else is.

Edited by doppel
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The main reason not to disable the atari logo is that the signature check still runs, so you would have a blank screen for that time. It's not like the Colecovision's startup screen that is just running a LONG delay loop. It's not so bad when only 4K of the game is signed, but a whole 16K would keep the screen blank for like a whole second. And it's a kind of cool startup logo anyhow.

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