NES 2.0 Mapper 351

NES 2.0 Mapper 351 is used for Techline XB-xxx multicarts.

Set CHR-ROM Base ($5000, write)

Mask: $F003

D~7654 3210
  ---------
  BBBB BBMM
  |||| ||++- ASIC compatibility mode for inner bank selection
  |||| ||     0/1: MMC3
  |||| ||     2: MMC1
  |||| ||     3: VRC4
  ++++-++--- Select 8 KiB CHR-ROM base

All bits are cleared on reset, so that MMC3 mode is active. VRC4 mode uses bits 2 and 3 ($004s and $008s) as VRC A0/A1 if address bit 11 ($800s) is clear, and as VRC A1/A0 if bit 11 is address set during a write.

Set PRG-ROM Base ($5001, write)

Mask: $F003

D~7654 3210
  ---------
  BBBB BB..
  ++++-++--- Select 16 KiB PRG-ROM base

All bits are cleared on reset.

Set Banking Mode ($5002, write)

Mask: $F003

D~7654 3210
  ---------
  .CCp .P.R
   |||  | +- 0: CHR-ROM mode
   |||  |    1: CHR-RAM mode
   |||  +--- Inner PRG-ROM bank size
   |||        0: 256 KiB (p=0), 32 KiB (p=1)
   |||        1: 128 KiB (p=0), 16 KiB (p=1)
   ||+------ PRG-ROM banking mode
   |||        0: ASIC (selecte by $5000.0-1)
   |||        1: NROM
   +++------ Inner CHR-ROM bank size
              1xx: 8 KiB
              01x: 128 KiB
              001: 32 KiB
              000: 256 KiB

All bits are cleared on reset. Unusually, in CHR-RAM mode (R=1), CHR-ROM becomes the second half of an enlarged PRG address space that becomes addressable via register $5001. At least one multicart containing both TLROM and UNROM games makes use of this feature and puts the UNROM game's PRG data into CHR-ROM. This seems to be possible as the mapper ASIC, PRG and CHR-ROM are under a single glob.

NROM Mirroring ($4025, write)

Mask: $FFFF

D~7654 3210
  ---------
  .... M...
       +---- 0: Vertical Mirroring
             1: Horizontal Mirroring

In NROM mode ($5002.4=1), mirroring can be changed via this FDS-inspired register as well, apparently in an attempt to become compatible with FDS conversions.