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PostPosted: Sun Jun 10, 2007 10:50 am 
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Joined: Fri Nov 12, 2004 2:49 pm
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Location: Chexbres, VD, Switzerland
I've been wondering this for a while.
This is a known fact that EPROMs (or mak ROMs) pinouts are designed so that they are more or less compatible among different sizes of chips, and new adress lines are added on the smaller pinout.
Now, I wonder what a mess Nintendo did with their pinouts, because on PRG ROMs they seems to have made different boards for each single size available (with very small differences, such as NROM-128 and NROM-256, wich only have one pin wich changes from VCC to A14), but for the CHR ROMs they seems to have many sizes available for a single board, even trough the ROM pinouts are the same and the ROM placed in are all the same standards (I'm talking about sizes <128 KB).

For example, some CNROM games can have 16KB CHRROMs without wondering about having the unused A14 (which *becomes* /PGM on EPROMs) connected to the mapper instead of being connected to VCC as they theorically should. The game most likely will write 0 to the mapper, effectively connecting /PGM (or whathever) to ground.
The same applies to Dragon Warrior (SAROM), SMB/Duck Hunt (MHROM) and some other gmaes too : They have 16KB CHR ROMs instead of 32 KB (as the boards supports in theory) with their pin 27 (/PGM) connected to the mapper instead of VCC, and this don't cause any problems, so why wouldn't it be the same for PRGROMs, why wouldn't a 16KB PRG ROM work just as fine on a NROM-256 board ?


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