VRC6
| Company | Konami |
| Games | 3 in NesCartDB |
| Complexity | ASIC |
| Boards | 351951, 351949A |
| PRG ROM capacity | 256K |
| PRG ROM window | 16K + 8K |
| PRG RAM capacity | 8K |
| PRG RAM window | 8K |
| CHR capacity | 256K |
| CHR window | 1K |
| Nametable mirroring | H, V, or 1, switchable |
| Bus conflicts | No |
| IRQ | Yes |
| Audio | Yes |
| iNES mappers | 024, 026 |
The Konami's VRC6 ASIC mapper comes in two variants:
- VRC6a - iNES Mapper 024 used for Akumajou Densetsu (Konami PCB 351951).
- VRC6b - iNES Mapper 026 used for Madara and Esper Dream 2 (Konami PCB 351949A).
The difference between the two variants switches the A0 and A1 lines. The registers described on this page are for mapper 24, but for mapper 26 the register addresses must be adjusted ($x001 becomes $x002 and vice versa).
See VRC6 pinout for chip pinout.
Banks
- CPU $8000-$BFFF: 16 KB switchable PRG ROM bank
- CPU $C000-$DFFF: 8 KB switchable PRG ROM bank
- CPU $E000-$FFFF: 8 KB PRG ROM bank, fixed to the last bank
- CHR $0000-$03FF: 1 KB switchable CHR ROM bank
- CHR $0400-$07FF: 1 KB switchable CHR ROM bank
- CHR $0800-$0BFF: 1 KB switchable CHR ROM bank
- CHR $0C00-$0FFF: 1 KB switchable CHR ROM bank
- CHR $1000-$13FF: 1 KB switchable CHR ROM bank
- CHR $1400-$17FF: 1 KB switchable CHR ROM bank
- CHR $1800-$1BFF: 1 KB switchable CHR ROM bank
- CHR $1C00-$1FFF: 1 KB switchable CHR ROM bank
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)
The addresses described here are for mapper 24. The registers for mapper 26 can be found by swapping bits 0 and 1 of the address.
variant lines registers Mapper Number
=================================================================
VRC6a: A0, A1 $x000, $x001, $x002, $x003 024
VRC6b: A1, A0 $x000, $x002, $x001, $x003 026
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, the three commercial VRC6 games neither provided extra nametable RAM, nor used ROM nametables. As a result these games only ever write the values $20, $24, $28, $2C, $A0, $A4, $A8, and $AC to this register.
CIRAM A10 is always connected to CHR A10.
CHR Select 0…7 ($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] & $03 → | 0 | 1 | 2 or 3 |
|---|---|---|---|
| CHR bank | Register used | ||
| $0000-$03FF | R0 | R0 | R0 |
| $0400-$07FF | R1 | R1 | |
| $0800-$0BFF | R2 | R1 | R2 |
| $0C00-$0FFF | R3 | R3 | |
| $1000-$13FF | R4 | R2 | R4 |
| $1400-$17FF | R5 | ||
| $1800-$1BFF | R6 | R3 | R5 |
| $1C00-$1FFF | R7 | ||
| [$B003] & $07 → | 0, 6, or 7 | 1 or 5 | 2, 3, or 4 |
|---|---|---|---|
| Nametable bank | Register used | ||
| $2000-$23FF | R6 | R4 | R6 |
| $2400-$27FF | R6 | R5 | R7 |
| $2800-$2BFF | R7 | R6 | R6 |
| $2C00-$2FFF | R7 | R7 | R7 |
When bit 5 of $B003 is set, 2 KiB banks pass PPU A10 through (ignoring the LSB of the register).
When bit 5 of $B003 is clear, CHR A10 will be controlled directly by the register LSB, causing 2 KiB banks to have duplicate 1 KiB halves. Existing Konami games did not use this configuration.
Mirroring
For the nametables, if bit 5 of $B003 is set and the lower 4 bits of $B003 have one of the following values, CHR/CIRAM A10 is replaced:
| [$B003] & $0F | CHR A10 |
|---|---|
| 0 or 7 | PPU A10 ("vertical mirroring") |
| 4 or 3 | PPU A11 ("horizontal mirroring") |
| 8 or F | Ground ("one screen A") |
| C or B | Vcc ("one screen B") |
| all other values | not replaced |
When CIRAM is used (bit 4 clear), bit 5 should normally be set (override CIRAM A10) so that the replaced signal can be used to apply mirroring. Otherwise vertical mirroring will be forced.
IRQ control ($F00x)
For details on IRQ operation, see VRC IRQs. Many VRC mappers use the same IRQ system.
The VRC6 IRQ can be used to count either CPU cycles, or scanlines as a multiple of CPU cycles.
7 bit 0
---------
$F000: LLLL LLLL - IRQ Latch
$F001: .... .MEA - IRQ Control
$F002: .... .... - IRQ Acknowledge
- L - reload value for latch
- M - mode (1=cycle, 0=scanline)
- E - enable IRQ
- A - acknowledge bit
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
- VRCVI Chip Info by Kevin Horton
- VRCVI by goroh (sound info is inaccurate)
- Comprehensive NES Mapper Document by \Firebug\, information about mapper's initial state is inaccurate.
- VRC6 mirroring test ROM
