NES 2.0 Mapper 439

NES 2.0 Mapper 439 denotes the YS2309 UxROM-like multicart PCB. Its UNIF MAPRs are BMC-DS-07 and BMC-K86B. All registers set to $00 on reset.

Latch Register ($8000-$FFFF, write)

D~[M..PP PPPP]  Address mask: $8000
   |  ++-++++- PRG A19..A14 when CPU A14=0,
   |           $3F when CPU A14=1
   +---------- Nametable mirroring, 0=Vertical, 1=Horizontal

This is basically an UxROM-like latch register for up to 1 MiB of PRG ROM, plus software-switchable mirroring.

Outer Bank Register ($6000, write)

D~[.PPP ....]  Address mask: $E001
    +++------ PRG A19..A17 if enabled via $6001

Mask Register ($6001, write)

D~[MPPP ....]  Address mask: $E001
   |||+------- 0: PRG A17 from Latch, 1: PRG A17 from $6000
   ||+-------- 0: PRG A18 from Latch, 1: PRG A18 from $6000
   |+--------- 0: PRG A19 from Latch, 1: PRG A19 from $6000
   +---------- 1: Protect Mirroring bit in Latch from being changed