INES Mapper 126

From Nesdev wiki
Jump to: navigation, search

iNES Mapper 126 denotes an MMC3-based multicart mapper with an optional (C)NROM-like PRG and CHR banking modes.

  • Power Joy Classic TV Game 84-in-1 (PJ-008)
  • Gamezone 118-in-1 (AT-207)

MMC3-compatible registers ($8000-$FFFF, write)

These registers function identically to a normal MMC3.

Outer Bank/PRG Mask Register ($6000, write)

Mask: $E003

D~7654 3210
  ---------
  XYBB CPPp
  |||| |||+- PRG A17 if Y=1
  ||++-|+++- PRG A21..A18
  |||| +---- CHR A17 if X=1
  |||+------ CHR A19
  ||+------- CHR A18
  |+-------- PRG A17 mode
  |           0: PRG A17=MMC3 PRG A17 (256 KiB inner PRG bank size)
  |           1: PRG A17=p (128 KiB inner PRG bank)
  +--------- CHR A17 mode
              0: CHR A17=MMC3 CHR A17 (256 KiB inner CHR bank size)
              1: CHR A17=C (128 KiB inner CHR bank)

Note the reverse-ordered CHR A18/A19 bits.

Miscellaneous Register ($6001, write)

Mask: $E003

D~7654 3210
  ---------
  .... ..?M
          +- 0: CPU $8000-$FFFF reads PRG-ROM
             1: CPU $8000-$FFFF reads solder pad (D0/D1)

Inner CHR Bank Register ($6002, write)

Mask: $E803

D~7654 3210
  ---------
  .... CCCC
       ++++- CHR A16..A13 in CNROM mode,
             ignored otherwise

Mode Register ($6003, write)

Mask: $E003

D~7654 3210
  ---------
  L..C .?PP
  |  |   ++- PRG Banking Mode
  |  |        0: MMC3
  |  |        1: NROM-128: PRG A14=MMC3 PRG A14
  |  |        2: NROM-128: Same as 1
  |  |        3: NROM-256: PRG A14=CPU A14
  |  +------ CHR Banking Mode
  |           0: MMC3
  |           1: CNROM
  +--------- Lock registers $6000 and $6003

NROM PRG banking mode is implemented by holding the MMC3 clones' CPU A13 and A14 input low. This means that...

  • MMC3 register #6 applies across the entire CPU $8000-$FFFF address range;
  • the MMC3's PRG A13 output is substituted with CPU A13;
  • in NROM-256 mode, the MMC3's PRG A14 output substituted with CPU A14;
  • MMC3 $8000.6 bits 1-3 therefore select a 16 KiB inner bank number.

CNROM CHR banking mode is implemented by using register $6002 as a source for an 8 KiB inner bank number instead of the MMC3's CHR registers.

Note

  • Outer bank registers overlay any WRAM, and can only be written to if WRAM is enabled in the MMC3 ($A001=$80).
  • Register $6002 is not affected by the Lock bit in register $6003.
  • NES 2.0 Mapper 534 is almost identical. It connects the higher CHR address lines differently, and MMC3 register $C000 takes the inverted value.
  • The common dump of Gamezone 118-in-1 (AT-207) inverts CHR A18, which may be a protection feature or an incorrect dump.