VRC6

The Konami's VRC6 ASIC mapper comes in two variants. The register descriptions given here are as they exist in Akumajou Densetsu (iNES Mapper 024). The A0 and A1 lines are switched in Madara and Esper Dream 2 (iNES Mapper 026), so for those games, adjustments will need to be made ($x001 becomes $x002 and vice versa).

Overview

• PRG ROM size: Up to 256 KB
• PRG ROM bank size: 16 KB at $8000, 8 KB at $C000
• PRG RAM: Up to 8 KB
• CHR capacity: Up to 256 KB ROM
• CHR bank size: 1 KB
• Nametable mirroring: Controlled by mapper
• Subject to bus conflicts: No

See VRC6 pinout for chip pinout.

Registers

Only address lines 0, 1, and 12-15 are used for registers, therefore mirrors can be found by ANDing the address with $F003 ($DE6A mirrors $D002)

16k PRG Select ($8000-$8003)

7  bit  0
---------
.... PPPP
     ||||
     ++++- Select 16 KB PRG ROM at $8000

8k PRG Select ($C000-$C003)

7  bit  0
---------
...P PPPP
   | ||||
   +-++++- Select 8 KB PRG ROM at $C000

PPU Banking Style ($B003)

7  bit  0
---------
W.PN MMDD
| || ||||
| || ||++- PPU banking mode; see below
| || ++--- Mirroring varies by banking mode, see below
| |+------ 1: Nametables come from CHRROM, 0: Nametables come from CIRAM
| +------- CHR A10 is 1: subject to further rules 0: according to the latched value
+--------- PRG RAM enable

The VRC6 supports the use of a larger RAM to provide more nametables. However, no games used any more than the two from the Famicom's built-in CIRAM, nor ever use ROM nametables. As a result, the commercial games seen only ever write the values $20, $24, $28, $2C, $A0, $A4, $A8, and $AC to this register.

CHR Select 0…7 ($Dxxx, $Exxx)

For brevity, we refer to the registers at $D000 through $D003 and $E000 through $E003 as R0 through R7.

The lower 3 bits of the $B003 register affect where the registers are used:

[$B003] & 7 →	0	4	1 or 5	2 or 3	6 or 7
PPU bank	Register used
$0000-$03FF	R0	R0	R0	R0	R0
$0400-$07FF	R1	R1		R1	R1
$0800-$0BFF	R2	R2	R1	R2	R2
$0C00-$0FFF	R3	R3		R3	R3
$1000-$13FF	R4	R4	R2	R4	R4
$1400-$17FF	R5	R5
$1800-$1BFF	R6	R6	R3	R5	R5
$1C00-$1FFF	R7	R7
$2000-$23FF	R6	R6	R4	R6	R6
$2400-$27FF	R6	R7	R5	R7	R6
$2800-$2BFF	R7	R6	R6	R6	R7
$2C00-$2FFF	R7	R7	R7	R7	R7

For the pattern tables, when the $20s bit of $B003 is set, 2 KiB banks pass PPU A10 through (limiting the register to seven bits wide by ignoring the LSB).

For the nametables, if the $20s bit of $B003 is set and the lower 4 bits of $B003 have one of the following values, CHR A10 is replaced:

[$B003] & 15	CHR A10
0 or 7	PPU A10 ("vertical mirroring")
4 or 3	PPU A11 ("horizontal mirroring")
8 or 15	Ground
12 or 11	Vcc
all other values	not replaced

If the $20s bit is clear, in both the pattern and name tables, the full eight-bit value from the register is used, even if this causes duplication. If these modes had ever been used, we assume Konami would have connected PPU A10 to the CHR ROM manually.

IRQ control ($F00x)

$F000:  IRQ Latch
$F001:  IRQ Control
$F002:  IRQ Acknowledge

Many VRC mappers use the same IRQ system. For details on IRQ operation, see VRC IRQs.

Sound ($900x, $A00x, $B000-$B002)

For details on sound information, see VRC6 audio.

References

• Official VRC6 documentation: http://www.assemblergames.com/forums/showthread.php?48390-Several-Famicom%28NES%29-misc-dev-documents-from-Nintendo-and-Konami
• Naruko's explanation of the $B003 register according to the VRC6 documentation: http://forums.nesdev.org/viewtopic.php?f=11&t=10628