INES Mapper 189

 Here are Disch's original notes:  
 ========================
 =  Mapper 189          =
 ========================
 
 
 Example Game:
 --------------------------
 Thunder Warrior
 
 
 Notes:
 ---------------------------
 This mapper is a modified MMC3.  Everything operates just as it does on the MMC3, only the normal PRG regs
 (R:6,R:7) are ignored, and a new PRG Reg is used instead.
 
 For details on MMC3, see mapper 004
 
 
 Registers:
 ---------------------------
 Regs at $6000-7FFF means no SRAM
 
 
   $4120-7FFF:  [AAAA BBBB]
      A,B:  PRG Reg
 
 
   $8000-FFFF:  Same as on MMC3
 
 
 PRG Setup:
 --------------------------
 
 'A' and 'B' bits of the $4120 reg seem to be effectively OR'd.
 That is... $30, $03, and $21  will all select page 3
 
      $8000   $A000   $C000   $E000  
     +-------------------------------+
     |             $4120             |
     +-------------------------------+