INES Mapper 227

 Here are Disch's original notes:  
 ========================
 =  Mapper 227          =
 ========================
 
 
 Example Game:
 --------------------------
 1200-in-1
 
 
 Notes:
 ---------------------------
 This mapper has 8k CHR-RAM, and also has the ability to write protect it's CHR-RAM!
 
 
 Registers:
 ---------------------------
 
   $8000-FFFF:  A~[.... ..LP  OPPP PPMS]
     L = Last PRG Page Mode
     P = PRG Reg
     O = Mode
     M = Mirroring (0=Vert, 1=Horz)
     S = PRG Size
 
 
 Setup:
 ---------------------------
 
 When 'O' is set, CHR-RAM is write protected (writes have no effect).  'O' also changes the PRG mode.
 
 Note there is funky ANDs and ORs going on below depending on the modes:
 
 
                   $8000   $A000   $C000   $E000  
                 +---------------+---------------+
 O=1, S=0:       |       P       |       P       |
                 +-------------------------------+
 O=1, S=1:       |             < P >             |
                 +-------------------------------+
 O=0, S=0, L=0:  |       P       |   P AND $38   |
                 +---------------+---------------+
 O=0, S=1, L=0:  |   P AND $3E   |   P AND $38   |
                 +---------------+---------------+
 O=0, S=0, L=1:  |       P       |   P  OR $07   |
                 +---------------+---------------+
 O=0, S=1, L=1:  |   P AND $3E   |   P  OR $07   |
                 +---------------+---------------+