INES Mapper 174
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Here are Bisqwit's original notes:
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= Mapper 174 =
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Example Game:
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NTDec 5-in-1 (NTSC), Famicom cartridge format
PRG ROM size: 128 kB
CHR ROM size: 64 kB
Registers:
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$8000-FFFF: A~[.... .... OPPP CCCM]
M = Mirroring (1=Horizontal, 0=Vertical)
P = when O=0 selects 16k both @ $8000 and @ $C000
when O=1, the top two bits select 32k @ $8000 and the bottom bit is ignored
O = PRG mode: selects 16 kB (when 0) or 32 kB mode (when 1).
C = Selects 8 kB page for CHR memory $0000
All of the mapper register access seems to be done in $FF00-$FFFF range, however the entire range works.
The cartridge powers on in a state where all registers are 0.
The cartridge expect the RAM contents to be kept when reset is pressed,
whereupon it boots the game. The only way to return to the main menu after
selecting a game is to power cycle the console (cold reboot).
This mapper is functionally identical to mapper 58 except the bits are moved around.
Notes:
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This mapper is probably unique to that board, constructed from
two GD74LS161As (4-bit binary counter) and one GD74LS153 (dual 4-input multiplexer).
It is a multicart that consists of games of different sizes.
- Three games with PRG size = 32 kB (The Legend of Kage, The Goonies, Arkanoid)
- One game with PRG size = 16 kB (Urban Champion)
- One game with PRG size = 8 kB (Galaxian), which shares a 16 kB page together with the main menu.
The main menu does some writes to $4028 once in a while, and one of
the games (Urban Champion) writes to $4025. The purpose of these writes
is unknown, as the cartridge does not trap those writes, and there is
(probably) no hardware in the NTDec famiclone that responds at those addresses.
It can be noted though, that $4025 is the FDS motor control register,
and the value written would safely stop a disk system motor.
It is possibly a leftover from porting the game or its library routines
from the FDS to the Famicom.
More information: [1]
