Mapper 185 questions

Discuss emulation of the Nintendo Entertainment System and Famicom.

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BootGod
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Mapper 185 questions

Post by BootGod »

According to the docs i'm looking at this is CNROM with VROM disable. I'm wondering, is this a "made-up" mapper? I'll use Mighty Bomb Jack as an example. It uses a CNROM board which appears to be no different than any other CNROM board. It has a 16KB CHR ROM, but the first 8KB is dummy data. Setting it's mapper to 185 instead of 3, allows you to erase the unused block. Is that why this mapper was created, to save space? or does it actually serve a purpose besides that?

Here is the doc reference:

Code: Select all

Mapper 185: VROM-disable 

Appears to be used for CNROM games with only one (used) 8K VROM bank, however, VROM appears to be bigger than 8K, the games verify the the presence of the unused (empty) VROM banks on startup, and get caught in an endless loop if they do not exist.
  8000h-FFFFh  (De-)select VROM bank

 
Values used to switch VROM on/off are:
  Off  On   Title
  F0h  0Fh  Bird Week
  00h  33h  B-Wings
  00h  11h  Mighty Bomb Jack
  20h  22h  Sansuu 1 Nen - Keisan Game
  20h  22h  Sansuu 2 Nen - Keisan Game
  00h  FFh  Sansuu 3 Nen - Keisan Game
  13h  21h  Spy vs Spy

 
Above games are working when mapping an empty VROM bank (FFh-filled) either when (X)=13h, or when (X AND 0Fh)=0.
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Dwedit
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Post by Dwedit »

It was probably created because the game doesn't actually contain the extra vrom, so including the garbage data in the rom dump would be incorrect.
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BootGod
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Post by BootGod »

Why would that dummy data show up then if the ROM was actually only 8KB. Trying to read it as 16KB would result in a mirror of the first 8KB I thought. It seems to me that they had to pad out the ROM to make it work with the board. In my eyes, wouldn't the "correct" way be to leave the padding in and keep it assigned as the same board?
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Quietust
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Post by Quietust »

Those games are special - if you attempt to select a particular bank, it causes the the CHR ROM to be unmapped and results in the PPU reading open bus from $0000-$1FFF.
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BootGod
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Post by BootGod »

Is the Mighty Bomb Jack this list is refering too the Japanese version? Are they using a special board? I've got 2 US carts of it, both use completely normal CNROM boards, and dump with the empty bank in front of the actual 8K.

For the hell of it, I tried formating a couple of games in the list, Mighty Bomb Jack (J) and Bird Week (J). I reassigned them to mapper 3, doubled the # of VROM banks, and inserted a dummy 8KB VROM bank. They seemingly worked just fine.
CaH4e3
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Post by CaH4e3 »

PCB image, please?
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kyuusaku
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Post by kyuusaku »

I'm not sure about the US versions but all of the Japanese versions disable the ROM, instead of thinking of it as a switch, think of the latch driving /OE. I think some of them also do it from $6000-FFFF, not just $8000.
BootGod
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Post by BootGod »

Hello CaH4e3, glad to see you pop in as it's your US dump of MBJ that brought this question up :)

Sorry for the horrible pic, my camera is an old piece of crap. If there is something you want me to take a closer look at, let me know.

Image
Image

EDIT: Somewhat better pics
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