INES Mapper 052

These notes were updated on 2013-11-19 in response to this thread on the forum.

iNES Mapper 052, like several other multicart mappers, supplements the MMC3's RAM with a single register for switching between unmodified MMC3-compatible games.

 Registers:
 ---------------------------
 
   $6000-7FFF:  [WMCC SBPP]  Multicart reg
     P = PRG Block (bits 1,0)
         = PRG A18 and A17
     B = CHR+PRG Block Select bit (PRG and CHR bit 2)
         = CHR and PRG A19
     S = PRG Block size (0=256k   1=128k)
         = 0=PRG A17 comes from MMC3  1=A17 comes from this register
     C = CHR Block (bits 1,0)
         = CHR A18 and A17
     M = CHR Block size (0=256k   1=128k)
         = 0=CHR A17 comes from MMC3  1=A17 comes from this register
     W = 1=Disable multicart register and enable RAM
         0=allow further writes to multicart register
 
   $8000-FFFF:  Same as MMC3 for selected block
 
 This register can only be written to if PRG-RAM is enabled and writable (see $A001).
 
 PRG Setup:
 ---------------------------
 
    'S'    PRG-AND    PRG-OR
    ------------------------
     0       $1F    %BP0 0000
     1       $0F    %BPP 0000
 
  'B' and 'P' bits make a 3-bit value used as PRG-OR (left shift 4).  When 'S' is clear, the low bit of that
 value is forced to 0.
 
 PRG swapping behaves just like a normal MMC3 within this selected block
 
 CHR Setup:
 ---------------------------
 
    'M'    CHR-AND    CHR-OR
    ------------------------
     0       $FF    %BC 0000 0000
     1       $7F    %BC C000 0000
 
  'B' and 'C' bits make a 3-bit value used as CHR-OR (left shift 7).  When 'M' is clear, the low bit of
 that value is forced to 0.
 
 CHR swapping behaves just like a normal MMC3 within this selected block
 
 Powerup and Reset:
 ---------------------------
 
 $6000 set to 0 on reset and powerup.

The multicart register here can be implemented using two ICs: a 74273 (8 bit latch with asynchronous clear) and a 74153 (used as two independent one-of-two multiplexers). The 74273's /CLEAR input should be connected to something like

 CPU M2 or CPU A0 --|>|--+-+-- /CLEAR
                         | |
                         R C
                         | |
                       gnd gnd

• Diode: 1N914 or any other small signal diode
• Resistor: approximately 10kΩ
• Capacitor: approximately 1nF