INES Mapper 048

 Here are Disch's original notes:  
 ========================
 =  Mapper 048          =
 ========================
 
 
 Example Games:
 --------------------------
 Bubble Bobble 2 (J)
 Don Doko Don 2
 Captain Saver
 
 
 Notes:
 --------------------------
 Most dumps of mapper 048 games floating around are erroneously labelled as mapper 033.  Mapper 033 does not
 have IRQs, mapper 048 does, and mirroring on each is handled a bit differently.  Apart from that, the two are
 very similar.
 
 This mapper is very similar to MMC3 in a lot of ways, including how the IRQ counter operates.
 
 
 Registers:
 --------------------------
 
 Range,Mask:   $8000-FFFF, $E003
 
   $8000:    PRG Reg 0 (8k @ $8000)
   $8001:    PRG Reg 1 (8k @ $A000)
 
   $8002:    CHR Reg 0 (2k @ $0000)
   $8003:    CHR Reg 1 (2k @ $0800)
   $A000:    CHR Reg 2 (1k @ $1000)
   $A001:    CHR Reg 3 (1k @ $1400)
   $A002:    CHR Reg 4 (1k @ $1800)
   $A003:    CHR Reg 5 (1k @ $1C00)
 
   $C000:    IRQ Reload
   $C001:    IRQ Clear
   $C002:    IRQ Enable
   $C003:    IRQ Acknowledge
 
   $E000: [.M.. ....]   Mirroring
     0 = Vert
     1 = Horz
 
 
 PRG Setup:
 ---------------------------
 
       $8000   $A000   $C000   $E000  
     +-------+-------+-------+-------+
     | $8000 | $8001 | { -2} | { -1} |
     +-------+-------+-------+-------+
 
 
 CHR Setup:
 ---------------------------
 
       $0000   $0400   $0800   $0C00   $1000   $1400   $1800   $1C00 
     +---------------+---------------+-------+-------+-------+-------+
     |     $8002     |     $8003     | $A000 | $A001 | $A002 | $A003 |
     +---------------+---------------+-------+-------+-------+-------+
 
 
 
 IRQs:
 ---------------------------
 
 IRQs on this mapper seem to behave exactly like MMC3's IRQs, with 2 exceptions:
 
  1)  The written reload value is inverted.  EG:  Writing $06 to the reload reg on this mapper would be like
 writing $F9 on MMC3.  You can simply XOR the writes with $FF and it will work just like MMC3.
 
  2)  The IRQ seems to trip a little later than it does on MMC3.  It looks like about a 4 CPU cycle delay from
 the normal MMC3 IRQ time.  Failure to put in this delay results in shaking and other graphical quirks in some
 games.
 
 
 The registers on this mapper corespond directly to regs on the MMC3:
 
   048  -  MMC3
 ---------------
  $C000   $C000   (XOR written value with $FF)
  $C001   $C001
  $C002   $E001
  $C003   $E000
 
 
 For details on MMC3 IRQ operation, see mapper 004.