Determining correct mapper number with bad iNES headers

[jwdonal]

How do emulators determine the correct mapper number when the iNES ROM file header is bad. For example, I've got a dump of Dick Tracy with the following values:

$Address: $06 = $21
$Address: $07 = $44

The above (according to the iNES file format) would result in a mapper number of $42. But really it should be $02 (UNROM).

My VeriNES emulator does not properly play the game because it doesn't recognize the mapper number. However, if I load the same ROM into Nestopia it plays the game fine. How does Nestopia know that it's really not mapper $42 ????????????

THANKS!

Jonathon

[Dwedit]

The low nibble of byte 7 is a dead giveaway of a bad header.
Since it's 4, it's a bad header, throw away the high nibble.
You look at bits #2 and #3 of Byte #7 of the header.

....xx.. <- 00 = okay, 10 = Nes 2.0
Anything else: throw out byte #7 of the header because it's a "DiskDude!" rom.
"DiskDude!" roms have 01 for those two bits.

44 in hex is an uppercase D.

[blargg]

I think the usual algorithm for iNES is if the last byte of the header is not zero, treat the last 8 bytes of the header as zero, i.e.

Code: Select all

if ( header [15] != 0 )
{
    for ( int i = 8; i <= 15; i++ )
        header [i] = 0;
}

[jwdonal]

Nice!! Thanks guys!!

[BootGod]

Nestopia (and some other emulators as well) check the CRC against an internal database, if a match is found, it uses its own info instead of the header. This is most likely what's happening in this case.

[jwdonal]

That's really cool. I'd certainly like to do that with my emu (eventually). Is there a database available of Game-to-CRC mappings somewhere that I could use in the future? Or does anyone know where Nestopia stores this lookup-table so I could just steal it (lol, giving credit where credit is due of course)?

Thanks!

Jonathon

[tepples]

That's really cool. I'd certainly like to do that with my emu (eventually). Is there a database available of Game-to-CRC mappings somewhere that I could use in the future?

You might want to talk to bootgod about that. His NesCartDB (sample entry) has a CRC for every game. It'd be a lot cheaper than buying all the games and dumping them in CopyNES.

...pyNES.

...pyNES.

...pyNES.

Oh wait, am I channeling Dr. Robotnik?

[jwdonal]

Haha, oh man, tepples tepples tepples. LOL

Anyway, that is an awesome site! Never been there before. That must have taken him a heck of a long time to create! I was able to download the zipped XML file from his homepage. I'm not sure if he has it in any other formats but I can easily parse the XML. I'll wait to see if he replies to this post before bugging him.

Thanks for the tip!

Jonathon

[etabeta]

notice that the checksums in that xml file are calculated *without* the header. so you have to ignore the first 0x10 bytes of the file if you want to recognize those crc.

[BootGod]

XML is the only external format I have right now. It doesn't get as in depth as the website profiles do, but should have everything you need as far as emulation is concerned.