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VRC2/VRC4 submapper hell
http://forums.nesdev.com/viewtopic.php?f=3&t=15869
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Author:  rainwarrior [ Sun Apr 30, 2017 7:27 pm ]
Post subject:  Re: VRC2/VRC4 submapper hell

Myask wrote:
Just saying I'd've allocated VRC2 to one and its addressings to subs, but…well, backwards compatibility hobble.

Besides, if a new dump requires submapper support, then wouldn't that pressure emulator authors to implement it?

If a submapper is required for it to function then it's already iNES 2, so... if you're allocating a new mapper at 256+ I see nothing wrong with using "submappers" to cram 16 mappers into one mapper, if you really want to do that... though again, quite moot at this point.

For the iNES 1 mapper plane of 0-255, though, I don't think it's a good idea. Preserve their existing function, and use submappers to refine it. If you're going to add new mappers to 0-255 we should try to make sure they're at least compatibly implementable as a single mapper without submappers. I.e. if the mapper can't be implemented in iNES 1, don't stick it in the iNES 1 plane.

AWJ wrote:
I thought that bootleg mappers with similar but incompatible behaviour to a licensed publisher's mapper get their own full mapper number, not a submapper (e.g. the countless multicart-MMC3 variants)

If it's incompatible: yes, it needs a new mapper.

Author:  Fisher [ Tue May 02, 2017 5:01 am ]
Post subject:  Re: VRC2/VRC4 submapper hell

Looking at the user's perspective, with exception of the unique games that works only with hacked mappers, to add these hacks don't seem a big deal, specially if the original game is already working fine.

On the developer's side, I think it's a different history.
I think making pirate games' mapper to work on your own emulator is kind of solving a riddle.
The satisfaction of the accomplishment is probably great.

What's the opinion of you guys about this?

Author:  AWJ [ Tue May 02, 2017 3:48 pm ]
Post subject:  Re: VRC2/VRC4 submapper hell

You can't implement two different incompatible IRQ systems in the same mapper, though. How do you determine which one to use for a particular ROM? You need either a ROM hash list, which completely defeats the ability to run modified, newly discovered or newly developed software, or you need some kind of runtime heuristic, which still plays havoc with new software and particularly with test ROMs (which not only have to conform to the hardware but to the emulator's heuristics)

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