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

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