NES 2.0 Mapper 451

NES 2.0 Mapper 451 is used for the homebrew game Haratyler HP/MP. It is basically a homebrew TLROM-like circuit board that implements the MMC3 register's in an unusual fashion, and saves the high score to flash ROM. The game executes the AMIC A29040B flash ROM chip's "Software ID" command; if the manufacturer and model ID do not match the expected values ($37 and $86), or it detects that WRAM exists, the game replaces the first level with an unwinnable boss fight. Haratyler MP additionally provides MP3 playback from a built-in microSD card, similar to GTMP3.

Banks

• CPU $8000-$9FFF: 8 KiB fixed PRG ROM bank $00
• CPU $A000-$BFFF: 8 KiB switchable PRG ROM bank $10+x
• CPU $C000-$DFFF: 8 KiB switchable PRG ROM bank $20+x
• CPU $E000-$FFFF: 8 KiB fixed PRG ROM bank $30
• PPU $0000-$1FFF: 8 KiB switchable CHR RAM bank from 16 KiB total

Registers

Mirroring Register ($A000-$BFFF)

A~[101. .... .... ...M]
                     +- 0: Vertical, 1: Horizontal

IRQ Register ($C000-$DFFF)

A~[110. .... VVVV VVVV]

A write to this register amounts to the following writes on a normal MMC3:

• $C000=VVVVVVVV -1
• $C001=0
• If V=$FF: $E000, otherwise $E001

Bank Register ($E000-$FFFF)

A~[111. .... .... ..BA]
                    |+- PRG/CHR A13
                    +-- PRG A16

This means effectively:

BA    CPU $A000   CPU $C000   PPU $0000
 0        $10         $20        0
 1        $11         $21        1
 2        $18         $28        0
 3        $19         $29        1

Together with the fixed banks, this means that only PRG ROM banks $00/$20/$21/$28/$29/$30 are accessible. The rest serve as padding for the flash ROM chip's 64 KiB sector size.