NES 2.0 Mapper 409

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Revision as of 20:48, 17 January 2023 by Rainwarrior (talk | contribs) (→‎Address Latch ($C000-$CFFF, write): if we can't agree on what "mask" means it's not useful, describe why this disagrees with $C000-CFFF in the heading instead, and explain what we know and don't know)
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NES 2.0 Mapper 409 denotes the retroUSB DPCMcart circuit board, used for the homebrew music cartridge "A Winner is You" released by retroUSB.. It is basically an extremely oversized version of UNROM that latches twelve bits of the address, rather than the data, bus for selecting banks from a 64 MiB PRG address space.

Despite being called "DPCMcart", it cannot actually bank DPCM samples because of its fixed bank. Instead, A Winner is You uses the large ROM for PCM sample storage, played back with timed CPU code.

Banks

  • CPU $8000-$BFFF: 16 KiB PRG bank, switchable
  • CPU $C000-$FFFF: 16 KiB PRG bank, fixed to last bank
  • PPU $0000-$1FFF. 8 KiB CHR RAM, unbanked
  • Mirroring: Hard-wired

Registers

Address Latch ($C000-$CFFF, write)

A~FEDC BA98 7654 3210
  -------------------
  1... BBBB BBBB BBBB
       ++++-++++-++++- Selects bank via PRG A25..A14 at CPU $8000-BFFF (CPU A14=0)

This register may respond to all of $8000-FFFF, but A Winner is You only uses the $C000-CFFF range.

See also

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