NES 2.0 Mapper 266

NES 2.0 Mapper 266 is used by City Fighter IV, a hack of Master Fighter II that adds (very grungy) PCM speech output. Given that its resolution (four bits) is worse than the NES/Famicom's own DAC at $4011 (seven bits), it's not immediately obvious what the purpose of including its own DAC is, other than perhaps the possibility of low-pass filtering the 3.5 kHz-sampled PCM data on the circuit board. Its UNIF board name is UNL-CITYFIGHT.

The circuit board mounts a VRC4 clone (A2/A3) with two of its higher CPU address lines mixed up:

CPU A2  -> VRC4 CPU Ax (pin 03)
CPU A3  -> VRC4 CPU Ay (pin 04)
CPU A13 -> VRC4 CPU A14 (pin 02)
CPU A14 -> VRC4 CPU A13 (pin 01)

... and two additional registers that are selected via the VRC4 /WR9003 pin, e.g. by writing to $900C, and distinguished via CPU A11 ($0800). This results in the following effective registers:

Mirroring Control ($9000)

Mask: $F00C, see VRC4 description.

PRG Select ($900C)

Mask: $F80C

D~7654 3210
  ---------
  .... PP..
       ++--- PRG A16..A15

Similar to INES Mapper 189, VRC4's fine-grained PRG banking is replaced with a single 32 KiB bank switch.

DAC Output ($980C)

Mask: $F80C

D~7654 3210
  ---------
  .... DDDD
       ++++- 4 bit unsigned PCM data

CHR Select ($A00x/$B00x/$D00x/$E00x)

Mask: $F00C

$D000/$D004 (LSB/MSB): Select 1 KiB CHR-ROM bank at PPU $0000-$03FF
$D008/$D00C (LSB/MSB): Select 1 KiB CHR-ROM bank at PPU $0400-$07FF
$A000/$A004 (LSB/MSB): Select 1 KiB CHR-ROM bank at PPU $0800-$0BFF
$A008/$A00C (LSB/MSB): Select 1 KiB CHR-ROM bank at PPU $0C00-$0FFF
$B000/$B004 (LSB/MSB): Select 1 KiB CHR-ROM bank at PPU $1000-$13FF
$B008/$B00C (LSB/MSB): Select 1 KiB CHR-ROM bank at PPU $1400-$17FF
$E000/$E004 (LSB/MSB): Select 1 KiB CHR-ROM bank at PPU $1800-$1BFF
$E008/$E00C (LSB/MSB): Select 1 KiB CHR-ROM bank at PPU $1C00-$1FFF

IRQ Control ($F00x)

Mask: $F00C, see VRC4 description.

Notes

Captured waveform of the audio speech when using R-2R as DAC

The game uses the VRC4's pseudo-scanline mode during speech output. FCEUX only emulates a downward-counting CPU cycle counter, counting on every other cycle, causing speech to be played at too low a pitch compared to hardware.