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?
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.
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?
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.
Quietust, QMT Productions
P.S. If you don't get this note, let me know and I'll write you another.
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.
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.