NES 2.0 Mapper 551

NES 2.0 Mapper 551 is a variant of INES Mapper 178 used for several Jncota re-releases of RPG games that were originally published by C&E and Supertone:

• 聖火列傳 (Shènghuǒ Lièzhuàn) and its title screen hack 侠客传奇 (Xiákè Chuánqí), originally released for INES Mapper 240
• 荊軻新傳 (Jīngkē Xīnzhuàn) and its title screen hack 战国风云 (Zhànguó Fēngyún), originally released for INES Mapper 240

Compared to INES Mapper 178, mirroring is hard-wired, and the chipset's internal CHR-RAM is not used in favor of CHR-ROM.

Banks

• CPU $6000-$7FFF: 8 KiB unbanked PRG-RAM
• CPU $8000-$FFFF: 16/32 KiB PRG-ROM bank, switchable
• PPU $0000-$1FFF: 8 KiB switchable CHR-ROM bank
• Nametable mirroring: Hard-wired

Registers

Mode Register ($4800)

7654 3210
---------
.... .SS.
      ||
      ||
      ||
      ++-- PRG banking mode
            0: NROM-256/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.

CHR-ROM Bank register ($4803)

7654 3210
---------
BBBB BBBB
++++-++++- CHR-ROM A20?..A13