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.
