INES Mapper 234

 Here are Disch's original notes:  
 ========================
 =  Mapper 234          =
 ========================
 
 
 Example Game:
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 Maxi 15 (PAL)
 
 
 Notes:
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 Verrrry strange.  Registers seem to lie at $FF80-$FFFF... the kicker is that *reading* those addresses also
 trigger a register write (the value written is the value being read).
 
 Example:
 
   LDA $FF80  ; where $FF80 contains $62
 
 would 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:  [MO.. BBBb]  Reg 0
     M = Mirroring (0=Vert, 1=Horz)
     O = Mode
     B,b = Block selection
 
   $FFC0, $FFC8, $FFD0, $FFD8:  [???? ????]  Reg 1
 
 
   $FFE8, $FFF0:   [.cCC ...P]               Reg 2
    C,c = CHR page
    P = PRG page
 
 
 Once Reg 0 contains a non-zero value, it is locked and cannot be changed until the system is reset.
 
 ?Same is true for Reg 1?
 
 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:
 ---------------------------
 
 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.