INES Mapper 231

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 Here are Disch's original notes:  
 ========================
 =  Mapper 231          =
 ========================
 
 Example Game:
 --------------------------
 20-in-1
 
 
 
 Registers:
 ---------------------------
 
  $8000-FFFF:     A~[.... .... M.LP PPP.]
     M = Mirroring (0=Vert, 1=Horz)
     L = Low bit of PRG
     P = High bits of PRG
 
 
 
 PRG Setup:
 ---------------------------
 
 Note that 'L' and 'P' bits make up the PRG reg, and the 'L' is the low bit.
 
 
       $8000   $A000   $C000   $E000  
     +---------------+---------------+
     | $8000 AND $1E |     $8000     |
     +---------------+---------------+