INES Mapper 234

Mapper 234 describes the Maxi 15 multicart, which allows the combination of slightly-modified 32k/32k CNROM and 64k/64k NINA-03 games. The only instance of this board was released with 512k/512k, but seems to be laid out in a way that would support a total of 1M/1M in 4 512k ROMs.

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 =  Mapper 234          =
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 Example Game:
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 Maxi 15
 
 
 Notes:
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 Typical for the Atari 2600, but strange for the NES: Registers lie at
 $FF80-$FFFF but bankswitching happens on reads, as well as writes. Bus
 conflicts are thus avoided by storing the library of desired bankswitch
 values in ROM.
 
 Example:
 
   LDA $FF80  ; where $FF80 contains $62
 
 would (ignoring bus conflicts) have the same effect on the mapper as:
 
   LDA #$62
   STA $FF80
 
 
 Registers:
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 Range,Mask:   $FF80-FFFF, $FFF8
 
   $FF80, $FF88, $FF90, $FF98:  [MOQq BBBb]  Reg 0
     M = Mirroring (0=Vert, 1=Horz)
     O = Mode (0=CNROM, 1=NINA-03)
     B,b = Block selection
     q = ROMs 3+4 /Enable (0=normal, 1=disable ROM further from cartridge edge)

         Since the cartridge was distributed with only ROMs 1+2
          populated, this is irrelevant.

         This bit seems to have been intended to have been an extra
          address line for ROMs 3+4, enabling a total of 1.5M/1.5M in the
          cartridge, but a mistake prevents it from working.

     Q = ROM switch (0=enable ROMs 1+2, 1=enable ROMs 3+4)
         i.e. 0 for normal operation

   $FFC0, $FFC8, $FFD0, $FFD8:  [.... ..LL]  Reg 1
     L = Lockout defeat (charge pump drive)
   
   $FFE8, $FFF0:   [.cCC ...P]               Reg 2
    C,c = CHR page
    P = PRG page
 
 
 Once the bottom 6 bits of Reg 0 contain a non-zero value, Reg 0 and Reg 1
  are locked and cannot be changed until the system is reset.
 
 Reg 2 is never locked.
 
 
 
 CHR Setup:
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 8k page @ $0000 selected by the following:
 
     'O'     CHR page
   ---------------------
      0      %BB BbCC
      1      %BB BcCC
 
 
 PRG Setup:
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 32k page @ $8000 selected by the following:
 
     'O'     PRG page
   ---------------------
      0      %BBBb
      1      %BBBP
 
 
 
 On Powerup/Reset:
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 Regs all filled with 0 and unlocked.