INES Mapper 178

iNES Mapper 178 is used by some games from Waixing and Henge Dianzi as well as some educational computers. It always uses CHR-RAM and can use up to 32 KiB of PRG-RAM, banked in 8 KiB amounts.

Registers

Mode Register ($4800)

7654 3210
---------
.... .PPM
      ||+- Select nametable mirroring type
      ||    0: Vertical
      ||    1: Horizontal
      ++-- Select PRG banking mode
            0: 32 KiB PRG-ROM bank at CPU $8000-$FFFF, low bank bit 0 ignored
            1: 16 KiB PRG-ROM bank at CPU $8000-$BFFF comprised of selectable high and low banks
               16 KiB PRG-ROM bank at CPU $C000-$FFFF comprised of selectable high and fixed low bank 7
            2: 16 KiB PRG-ROM bank at CPU $8000-$BFFF comprised of selectable high and low banks
               16 KiB PRG-ROM bank at CPU $C000-$FFFF being a mirror of $8000-$BFFF
            3: 16 KiB PRG-ROM bank at CPU $8000-$BFFF comprised of selectable high and low banks
               16 KiB PRG-ROM bank at CPU $C000-$FFFF comprised of selectable high, fixed low bank bits 1-2
               and selectable low bank bit 0 (i.e. low bank register content OR 6)

Banking modes 2 and 3 are rarely used. One multicart that uses them is 4-in-1 (SO1).

Low PRG Bank Register ($4801)

7654 3210
---------
.... .LLL
      +++- 16 KiB bank number bits 0-2.

Bit 0 is ignored in 32 KiB PRG banking mode.

High PRG Bank Register ($4802)

7654 3210
---------
HHHH HHHH
++++-++++ 16 KiB bank number bits 3-10.

The final 16 KiB PRG-ROM bank number thus is $4801 OR ($4802 SHL 3).

WRAM Bank Register ($4803)

7654 3210
---------
.... ..BB
       ++ Select 8 KiB WRAM bank at CPU $6000-$7FFF

Infrared

Apparently some cartridge have an infrared sensor. Infrared receiving is working by having two registers:

 $5000-$5FFF:  [.... ...V]  (read only)
    V = read current bit from IR sensor
 $6000-$7FFF:  [I... ....]  (write only)
    I = set to activate interrupts by infrared signal

Errata

• The commonly-available ROM images of
  • 宠物: 小精灵 IV (Chǒngwù: Xiǎo Jīnglíng IV), headerless CRC32 0xB0B13DBD
  • Education Computer 32-in-1 (Game Star) [!], headerless CRC32 0xF834F634

will not work when emulated using the above description. The meaning of registers $4801 and $4802 is swapped, and an emulator must shift the value of $4802 one bit to the left before ORing with $4801. While it cannot be ruled out that these games use an alternative version of the hardware, the more likely explanation is that these are simply bad dumps (notwithstanding GoodNES 3.23b's [!] tag) where the dumper confused the two bank registers.

• Supposedly, the new PRG bank is only committed when $4801 is written. Emulating this supposed behavior however will break Ping Pong, while commiting the PRG bank after any $480x write does not seem to break anything else.
• Most of the educational computers using this mapper require Dendy video timing and will freeze at various points when run with NTSC timing.

See also

• https://forums.nesdev.org/viewtopic.php?t=16657