VRC6

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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).



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.

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, 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)

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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