INES Mapper 178

iNES Mapper 178 is used by some games from Waixing (on a PCB named FS305), Nanjing (on a PCB named NJ0430), Jncota and Henge Dianzi as well as some educational computers from GameStar, a.k.a. Belsonic. The chipset is designed for using 8 KiB of chip-internal unbanked CHR-RAM, but can address external RAM as well, banked in 8 KiB amounts, which can be connected on the circuit board as PRG-RAM, CHR-RAM or CHR-ROM.

Registers

Mode Register ($4800)

7654 3210
---------
.... .SSM
      ||+- Nametable mirroring
      ||    0: Vertical
      ||    1: Horizontal
      ++-- PRG banking mode
            0: NROM-128/BNROM (PRG A14=CPU A14)
            1: UNROM (PRG A14..16=111b if CPU A14=1)
            2: NROM-128
            3: UNROM but with bit 0 of "fixed" bank selectable
               (PRG A15..16=11b if CPU A14=1)

Low PRG Bank Register ($4801)

7654 3210
---------
.... .LLL
      +++- PRG A16..A14

This can be considered an inner bank register for UNROM mode.

High PRG Bank Register ($4802)

7654 3210
---------
HHHH HHHH
++++-++++- PRG A24?..A17

This can be considered an outer bank register for UNROM mode.

External RAM/ROM Bank Register ($4803)

7654 3210
---------
BBBB BBBB
++++-++++- External A20..A13

If a cartridge has CHR-ROM, this register selects the 8 KiB CHR-ROM bank, otherwise it selects an 8 KiB PRG-RAM bank at CPU $6000-$7FFF.

Infrared

Some cartridges have an additional 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.
• The Waixing FS305 circuit board connects PRG A14 directly to CPU A14, making all PRG banking modes except BNROM unusable. Since it writes the correct banking mode to $4800, it does not require any special treatment.

See also

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