SoundGammon Posted February 16, 2022 Share Posted February 16, 2022 Suppose you had a 7800 and a bad Maria chip. Would you be able to turn it into a Super 2600? In theory, you could put a 2600 rom on a 7800 cart and then have full access to the 6502. You would have all the address lines and more memory, 16K of ram. One example might be Basic Programming cart that might make use. Could it work? 2 Quote Link to comment Share on other sites More sharing options...
+Karl G Posted February 16, 2022 Share Posted February 16, 2022 I believe if you lock in TIA mode, you also lock out everything from the 7800 side. 2 Quote Link to comment Share on other sites More sharing options...
RevEng Posted February 17, 2022 Share Posted February 17, 2022 Indeed. To break it down for the non-programmers, TIA access is disabled until you tell Maria otherwise. The default state of the system on power-up is pretty much 7800 mode so the bios can be executed, encryption checks done, etc. If Maria is broken, that's not going to happen. Even if you went about reengineering the system, changing the bios and bodging the internal circuitry, games using various 4k rom mirrors would mean a Super 2600 wouldn't be backward compatible with regular 2600 games. The TIA mirroring would also stomp over the useful address space, so it would be a not-so-super lobotomized 7800. 2 Quote Link to comment Share on other sites More sharing options...
CPUWIZ Posted February 17, 2022 Share Posted February 17, 2022 Decades ago, someone made Save Mary run on the 7800, using the extra RAM. I think it was V61 who released it. 2 Quote Link to comment Share on other sites More sharing options...
gambler172 Posted February 17, 2022 Share Posted February 17, 2022 25 minutes ago, CPUWIZ said: Decades ago, someone made Save Mary run on the 7800, using the extra RAM. I think it was V61 who released it. right...they also did Shooting Arcade a 2600 game for the 7800 1 Quote Link to comment Share on other sites More sharing options...
RevEng Posted February 17, 2022 Share Posted February 17, 2022 For sure, but in those cases the games were physically on 7800 carts (which avoids rom mirrors), started up 7800 mode, and enabled TIA and RAM before locking the console. A "bad Maria chip" console couldn't run them, or nor would that console allow 2600 games to do anything like that. Quote Link to comment Share on other sites More sharing options...
+batari Posted February 17, 2022 Share Posted February 17, 2022 I believe A13-A15 should be available in 2600 mode on a 7800 cart even when locked, so a 2600 ROM on a 7800 cart could give access to more space than an ordinary 2600 cart without having to use bankswitching or any fancy logic. However, you would be limited in what you could access - say, using a 48k 7800 cart, you only have access to $4000-$FFFF, and on a 2600 ROM on the cart you would then only be able to use half of that, or 24k, in segments from $5000-$5FFF, $7000-$7FFF, $9000-$9FFF, $B000-$BFFF, $D000-$DFFF, and $F000-$FFFF. So, not the full address space. AND, you would have to be careful not to attempt to access the other half of ROM space on the cart as it would cause bus contention with the TIA/RIOT. If you put the signature in the 2600 ROM so it boots in 7800 mode, I suppose you could use 7800 RAM and TIA (and the full 48k space) but wouldn't the instruction timing vary? I would think that most existing 2600 games would not work properly in this hybrid mode and it could make writing a kernel pretty challenging. Quote Link to comment Share on other sites More sharing options...
RevEng Posted February 17, 2022 Share Posted February 17, 2022 2 hours ago, batari said: If you put the signature in the 2600 ROM so it boots in 7800 mode, I suppose you could use 7800 RAM and TIA (and the full 48k space) but wouldn't the instruction timing vary? I would think that most existing 2600 games would not work properly in this hybrid mode and it could make writing a kernel pretty challenging. The PAL bios explicitly checks for 4k mirrors and goes to 2600 mode if found. The NTSC bios doesn't do the same (IIRC) but the RAM at $1800 would interfere with the 4K rom mirrors, causing bus conflicts. Quote Link to comment Share on other sites More sharing options...
+batari Posted February 17, 2022 Share Posted February 17, 2022 3 minutes ago, RevEng said: The PAL bios explicitly checks for 4k mirrors and goes to 2600 if found. The NTSC bios doesn't do the same (IIRC) but the RAM at $1800 would interfere with the 4K rom mirrors. I meant to say I was still talking about a 7800 cart, so you could get full access to the space if you booted to 7800 mode. However, I have wondered if it would be possible to boot to 7800 mode on a strict 2600 cart, even if a bit of trickery were needed to pull it off. 1 Quote Link to comment Share on other sites More sharing options...
+Karl G Posted February 18, 2022 Share Posted February 18, 2022 This approach seems like a good way to disappoint users of two different consoles at the same time. 1 8 Quote Link to comment Share on other sites More sharing options...
SoundGammon Posted February 18, 2022 Author Share Posted February 18, 2022 4 hours ago, batari said: I meant to say I was still talking about a 7800 cart, so you could get full access to the space if you booted to 7800 mode. However, I have wondered if it would be possible to boot to 7800 mode on a strict 2600 cart, even if a bit of trickery were needed to pull it off. What would happen if you remove the bios too? Wouldn't it be just a 2600? Quote Link to comment Share on other sites More sharing options...
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