INES Mapper 228

iNES Mapper 228 represents the board used by Active Enterprises for Action 52 and Cheetahmen II.

Registers

Address           Data
FEDCBA98 76543210 76543210
1.MHHPPP PPS.CCCC ......CC
  |||||| ||| ||||       ||
  |||||| ||| ++++-------++- Select 8 KiB CHR ROM bank at PPU $0000
  |||||| ||+--------------- PRG bank size
  |||||| ||                 0: Put bank with bit 0 false in $8000 and
  |||||| ||                    bank with bit 0 true in $C000
  |||||| ||                 1: Put same 16 KiB bank in $8000 and $C000
  |||+++-++---------------- Select 16 KiB PRG ROM bank at CPU $8000
  |++---------------------- Select which 512 KiB PRG ROM chip to enable
  |                         (A52 uses 0, 1, and 3; bank 2 is open bus)
  +------------------------ 0: Vertical mirroring; 1: Horizontal mirroring

It is claimed that there are four 4-bit RAM locations at $4020-$4023, mirrored throughout $4020-$5FFF.

This 16-bit RAM (which would be a 74ls170 or 74ls670) is definitely not present on Action 52, per the image in NesCartDB. It is unknown whether Cheetahmen II has this IC; Nestopia does not implement it at all.

Incidentally, this is very similar to iNES Mapper 225 with a few differences:

• Two of the CHR bank bits have been shifted to the data bus.
• PRG bank size bit moved to other side of PRG bank number.
• High bit moved to other side of mirroring and applies only to PRG ROM, not CHR ROM.
• The rumored 4x4-bit RAM is at a different address.

 Here are Disch's original notes:
 ========================
 =  Mapper 228          =
 ========================
 
 
 Example Games:
 --------------------------
 Action 52
 Cheetah Men II
 
 
 Notes:
 ---------------------------
 Cheetah Men II is infamous for how freaking terrible it is.  Action 52 is none better.  These games are SO
 bad, it's hilarious.
 
 Action 52's PRG size is weird (not a power of 2 value).  This is because there are 3 seperate 512k PRG chips.
 PRG Setup section will cover details.
 
 
 Powerup and Reset:
 ---------------------------
 Apparently the games expect $00 to be written to $8000 on powerup/reset.
 
 
 Registers:
 ---------------------------
 
   $4020-4023:  [.... RRRR]  RAM  (readable/writable)
                 (16 bits of RAM -- 4 bits in each of the 4 regs)
   $4024-5FFF:    mirrors $4020-4023
 
   $8000-FFFF:    [.... ..CC]   Low 2 bits of CHR
                A~[..MH HPPP PPO. CCCC]
 
     M = Mirroring (0=Vert, 1=Horz)
     H = PRG Chip Select
     P = PRG Page Select
     O = PRG Mode
     C = High 4 bits of CHR
 
 CHR Setup:
 ---------------------------
 
       $0000   $0400   $0800   $0C00   $1000   $1400   $1800   $1C00 
     +---------------------------------------------------------------+
     |                             $8000                             |
     +---------------------------------------------------------------+
 
 
 PRG Setup:
 ---------------------------
 
 'H' bits select the PRG chip.  Each chip is 512k in size.  Chip 2 does not exist, and when selected, will
 result in open bus.  The Action 52 .nes ROM file contains chips 0, 1, and 3:
 
 chip 0:  offset 0x000010
 chip 1:  offset 0x080010
 chip 2:  -- non existant --
 chip 3:  offset 0x100010
 
 'P' selects the PRG page on the currently selected chip.
 
                $8000   $A000   $C000   $E000  
              +-------------------------------+
 PRG Mode 0:  |            <$8000>            |
              +-------------------------------+
 PRG Mode 1:  |     $8000     |     $8000     |
              +---------------+---------------+