NES 2.0 Mapper 285

NES 2.0 Mapper 285 is used for the A65AS and JY-066 multicarts. The UNIF board name for both incompatible variants is BMC-A65AS. A second UNIF board name, BMC-GF-401CD refers to submapper 1. A later-discovered Russian bootleg cartridge of Highway Star (Русские Гонки) is assigned to mapper 285 submapper 1 as well, as it uses UNROM-banking while changing mirroring via latch bit 3.

Banks

• CPU $8000-$BFFF: In 16 KiB mode: 16 KiB switchable inner bank, 128 KiB switchable outer bank
• CPU $C000-$FFFF: In 16 KiB mode: 16 KiB fixed inner bank 7, 128 KiB switchable outer bank
• CPU $8000-$FFFF: In 32 KiB mode: 32 KiB switchable bank
• PPU $0000-$1FFF: 8 KiB unbanked CHR-RAM

Data Latch (CPU $8000-$FFFF)

A65AS (Submapper 0)

Bit 7654 3210
    ---------
    SPMO OBBB
    |||| |+++- Select 16 KiB inner PRG-ROM bank at CPU $8000-$BFFF in 16 KiB mode (bit 6=0)
    |||| |++-- Select 32 KiB PRG-ROM bank at CPU $8000-$FFFF in 32 KiB mode (bit 6=1)
    |||+-+---- Select 128 KiB outer PRG-ROM bank at CPU $8000-$FFFF in 16 KiB mode (bit 6=0)
    ||+------- Select nametable mirroring
    ||         0: Vertical (bit 7=0)/Single-screen, page 0 (bit 7=1)
    ||         1: Horizontal (bit 7=0)/Single-screen, page 1 (bit 7=1)
    |+-------- Select PRG banking mode
    |          0: 16 KiB
    |          1: 32 KiB
    +--------- Select nametable mirroring type
               0: Horizontal/vertical (selected via bit 3)
               1: Single-screen (selected via bit 5)

JY-066 (Submapper 1)

Bit 7654 3210
    ---------
    SPMO HBBB
    |||| |+++- Select 16 KiB inner PRG-ROM bank at CPU $8000-$BFFF in 16 KiB mode (bit 6=0)
    |||| |++-- Select 32 KiB PRG-ROM bank at CPU $8000-$FFFF in 32 KiB mode (bit 6=1)
    |||| +---- Select nametable mirroring if bit 7=0
    ||||       0: Vertical
    ||||       1: Horizontal
    ||++------ Select 128 KiB outer PRG-ROM bank at CPU $8000-$FFFF in 16 KiB mode (bit 6=0)
    ||+------- Select nametable mirroring if bit 7=1
    ||         0: Single-screen, page 0
    ||         1: Single-screen, page 1
    |+-------- Select PRG banking mode
    |          0: 16 KiB
    |          1: 32 KiB
    +--------- Select nametable mirroring type
               0: Horizontal/vertical (selected via bit 3)
               1: Single-screen (selected via bit 5)