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 A3 -> VRC4 CPU Ax (pin 03)
CPU A2 -> 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:
Mask: $F00C, see VRC4 description.
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.
Mask: $F80C
D~7654 3210
---------
.... DDDD
++++- 4 bit unsigned PCM data
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
Mask: $F00C, see VRC4 description.
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.