VRC6

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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, and bit 5 affects the behaviour of this signal (see below for details). The commercial games always left this bit set.

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 clear, CHR/CIRAM 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. This was intended for a different PCB (never used) where PPU A10 directly controls CHR A10 instead, permitting 512 KiB of CHR-ROM.

When bit 5 of $B003 is set, 2 KiB pattern table banks pass PPU A10 through (ignoring the LSB of the register). Nametables apply different rules at the same time: see below.

Mirroring

With $B003:5 set, there are four different modes corresponding to $B0003 & 3 that each have 4 nametable mirroring settings. Only mode 0 was used by Konami's commercial games.

Mode 0
[$B003] & $F $0 $4 $8 $C
Mirroring vertical horizontal 1-screen A 1-screen B
CIRAM A10 PPU A10 PPU A11 Ground (0) Vcc (1)

This mode was not intended for use with ROM nametables ($B003:4 set), because it overrides the LSB of the nametable registers with the signal intended for CIRAM A10. Because R6 and R7 are already in use to control the pattern banks, this is an undesirable configuration, and Mode 3 is recommended instead.

[$B003] & $F $0 $4 $8 $C
Mirroring horizontal pairs
2 KiB spread		 vertical pairs
2 KiB spread		 horizontal mirroring
1 KiB (even only)		 vertical mirroring
1 KiB (odd only)
$2000-$23FF R6 even R6 even R6 even R6 odd
$2400-$27FF R6 odd R7 even R6 even R7 odd
$2800-$2BFF R7 even R6 odd R7 even R6 odd
$2C00-$2FFF R7 odd R7 odd R7 even R7 odd
Mode 3 equivalent $7 $3 $F $B
Mode 1

Mode 1 uses R4-R7 to directly control 4-screen mapping. This is very effective with ROM nametables ($B003:4 set), but the LSB of each register still applies when using CIRAM.

[$B003] & $F $1 $5 $9 $D
Mirroring 4-screen
$2000-$23FF R4
$2400-$27FF R5
$2800-$2BFF R6
$2C00-$2FFF R7
Mode 2

Mode 2 uses R6-R7 to select two ROM pages for horizontal or vertical mirroring. If using CIRAM, the LSB of each register applies.

[$B003] & $F $2 $6 $A $E
Mirroring vertical horizontal vertical horizontal
$2000-$23FF R6 R6 R6 R6
$2400-$27FF R7 R6 R7 R6
$2800-$2BFF R6 R7 R6 R7
$2C00-$2FFF R7 R7 R7 R7
Mode 3

This mode is intended for ROM nametables ($B003:4 set) but the nametable banking is extended to 2 KiB pages by forcing the LSB as a 0 (even) or 1 (odd) in different configurations.

This can be used to spread 2 adjacent ROM pages each across a pair of nametables.

[$B003] & $F $3 $7 $B $F
Mirroring vertical pairs
2 KiB spread		 horizontal pairs
2 KiB spread		 vertical mirroring
1 KiB (odd only)		 horizontal mirroring
1 KiB (even only)
$2000-$23FF R6 even R6 even R6 odd R6 even
$2400-$27FF R7 even R6 odd R7 odd R6 even
$2800-$2BFF R6 odd R7 even R6 odd R7 even
$2C00-$2FFF R7 odd R7 odd R7 odd R7 even
Mode 0 equivalent $4 $0 $C $8

The nametable configurations are redundant with Mode 0, but in a different order; however, the reuse of R6 and R7 as pattern banks makes mode 0 ineffective for ROM nametables.

Other

With $B003:5 clear, CHR/CIRAM A10 always directly uses the LSB of the register used to map that range, rather than having it overridden for mirroring control. (See nametable bank table above.)

This setting was intended to be used with a different PCB that would have connected PPU A10 directly to CHR A10, enabling 512 KiB CHR-ROM (with mode 1 having 2 KiB banks). The actual behaviour here is a leftover consequence of being run on the wrong board:

[$B003] & $F $0 $1 $2 $3 $4 $5 $6 $7 $8 $9 $A $B $C $D $E $F
Mirroring H 4 V 4 H 4 V 4 H
$2000-$23FF R6 R4 R6 R6 R6 R4 R6 R6 R6 R4 R6 R6 R6 R4 R6 R6
$2400-$27FF R6 R5 R7 R7 R7 R5 R6 R6 R6 R5 R7 R7 R7 R5 R6 R6
$2800-$2BFF R7 R6 R6 R6 R6 R6 R7 R7 R7 R6 R6 R6 R6 R6 R7 R7
$2C00-$2FFF R7 R7 R7 R7 R7 R7 R7 R7 R7 R7 R7 R7 R7 R7 R7 R7

Using ROM nametables on the hypothetical board mentioned above would transform all of the "H" layouts into "horizontal pairs 2 KiB spread", and all of the "V" and "4" layouts would have disjoint 256 KiB halves for the left two and right two nametables.

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

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