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What is the max size for a cartridge based app on the Coco 3?

I believe the Coco 1 & 2 cartridge maxes at 16K, while the Coco 3 is 32K. However, I know there have been cart based games for the Coco 3 that exceed 32K and I'm unclear on how this is accomplished. Let's say I want to have a 256K cart, is that possible and how might it be accomplished?

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  • I've never seen a CoCo or even heard about it, so I'm probably not worth answering this; however, banking techniques apply in general to cases like this when you have a smaller address width than storage you want to address. Would it be worthwhile to post an explanation of banking techniques without any reference to actual CoCo specifics?
    – Cactus
    May 4, 2016 at 5:28
  • This question asks a few questions, which is going to attract answers that answer only one or the other. It may not be clear you are probably looking for a discussion of banking techniques, and not just the max unbanked size -- which you already seem to know.
    – user12
    May 4, 2016 at 17:58

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I would make the assumption that, in the CoCo 3's case, a 32k, 64k or 128k ROM can be created.

According to the discussion on the CoCo Mailing list, the normal size is a little less than 16k or 32k. Any carts that require larger ROMS need to have hardware MMU's to take advantage of a bank-switching scheme. There's a seller on eBay who sells a 512k cart with a control pad, to determine which bank on the ROM the CoCo will boot to. The same could be done with software to read the data from the different banks, in effect creating a larger game or application.

Additionally, if you get to a point where you need a larger ROM, you could look at the content of the code; if there's a lot of graphics or constant data, you could compress the code, and decompress it in memory. Not much different than some disk games load the data and then decompress in memory (Sock Master's Donkey Kong for example).

Your fictitious 256k cart could be done with an add-on MMU (like Robocop used for it's 128k) to handle the bank switching. As one list member said, there doesn't appear to be any such hardware under development for this right now, but, it has been done.

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This very topic is currently being debated on the Cocolist - the answer is 'a little less than 32K' on a Coco3, unless (as Cactus states above) you resort to bank switching:

https://pairlist5.pair.net/pipermail/coco/2016-May/153680.html

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    Answers that are mostly a link aren't that great. Maybe you could expand on the contents therein in a manner that lets this Q&A be stand-alone? Links don't really work for SE because they rot surprisingly fast.
    – user12
    May 4, 2016 at 17:53
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    Fair point, I know how frustrating rot can be. Brian's reply is spot on, so I've given his a vote.
    – Tony Jewell
    May 8, 2016 at 15:11
  • Looks good. I agree.
    – user12
    May 8, 2016 at 16:28

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