VRC2 and VRC4

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VRC2, VRC4
Company Konami
Complexity ASIC
Boards VRC2a-c

VRC4a-f

PRG ROM capacity 256K
PRG ROM window 8K
PRG RAM capacity 8K
PRG RAM window 8K
CHR capacity 256K

512K

CHR window 1K
Nametable mirroring H, V switchable

H, V, 1 switchable

Bus conflicts No
IRQ Yes
Audio No
iNES mappers 021, 022, 023, 025

The Konami VRC2 and Konami VRC4 are two related ASIC mappers in the VRC[1] family.

Because the VRC2 is mostly a subset of the VRC4, relevant games are often emulated as VRC4 only. iNES mappers 21, 22, 23 and 25 implement various board permutations, and NES 2.0 submappers may be used to disambiguate further.

Mapper 27 represents a related pirate mapper.

Example games:

Game Variant iNES mapper
Ganbare Pennant Race VRC2a 22
TwinBee 3 VRC2a 22
Wai Wai World VRC2b 23
Ganbare Goemon Gaiden VRC2c 25
Wai Wai World 2 VRC4a 21
Bio Miracle Bokutte Upa VRC4b 25
Gradius 2 (J) VRC4b 25
Ganbare Goemon Gaiden 2 VRC4c 21
Teenage Mutant Ninja Turtles (J) VRC4d 25
Boku Dracula-kun VRC4e 23
Tiny Toon Adventures (J) VRC4e 23

Variations

The VRC2 and VRC4 were used with several similar but incompatible boards. The primary difference between them was having the mapper address lines connected in different ways. In particular, two lines chosen from A0-A7 will be used to select registers.

VRC2
Nickname PCB A0 A1 Registers iNES mapper submapper
VRC2a 351618 A1 A0 $x000, $x002, $x001, $x003 22 0
VRC2b many A0 A1 $x000, $x001, $x002, $x003 23 3
VRC2c 351948 A1 A0 $x000, $x002, $x001, $x003 25 3
VRC4
Nickname PCB A0 A1 Registers iNES mapper submapper
VRC4a 352398 A1 A2 $x000, $x002, $x004, $x006 21 1
VRC4b 351406 A1 A0 $x000, $x002, $x001, $x003 25 1
VRC4c 352889 A6 A7 $x000, $x040, $x080, $x0C0 21 2
VRC4d 352400 A3 A2 $x000, $x008, $x004, $x00C 25 2
VRC4e 352396 A2 A3 $x000, $x004, $x008, $x00C 23 2
VRC4f - A0 A1 $x000, $x001, $x002, $x003 23 1
The VRC2b PCBs include: 350603, 350636, 350926, 351179

VRC2 vs VRC4

The VRC2 has:

  • CHR limit of 256k
  • 1 bit latch at $6000-6FFF
  • Only horizontal or vertical mirroring.

The VRC4 has:

  • CHR limit of 512k
  • Additional 8k PRG bank at $C000
  • Horizontal, vertical and 1-screen mirroring.
  • IRQ device

The VRC2a (mapper 22) additionally wires the CHR banking lines differently (see below).

The RAM decoding circuit that's part of the VRC4 itself only decodes RAM from $6000-$6FFF. For the one game with 8 KiB of RAM, an external circuit was added.

iNES mappers

iNES mappers 21, 23 and 25 each implements two address mappings at the same time:

  • 21: VRC4a, VRC4c
  • 23: VRC2b + VRC4f, VRC4e
  • 25: VRC2c + VRC4b, VRC4d

Because the address pairings do not overlap, and the games appear to use these registers in a well behaved manner, it is presumed sufficient for compatibility in most cases. Additionally, the VRC4 is always presumed for these three mappers.

With an NES 2.0 header, a submapper may be used to specify which address mapping to use, and either VRC2 or VRC4.

Banks

Common

  • PPU $0000-$03FF: 1 KiB switchable CHR bank
  • PPU $0400-$07FF: 1 KiB switchable CHR bank
  • PPU $0800-$0BFF: 1 KiB switchable CHR bank
  • PPU $0C00-$0FFF: 1 KiB switchable CHR bank
  • PPU $1000-$13FF: 1 KiB switchable CHR bank
  • PPU $1400-$17FF: 1 KiB switchable CHR bank
  • PPU $1800-$1BFF: 1 KiB switchable CHR bank
  • PPU $1C00-$1FFF: 1 KiB switchable CHR bank

VRC2

  • CPU $6000-$6FFF: 1 bit latch, or
  • CPU $6000-$7FFF: optional 8 KiB RAM
  • CPU $8000-$9FFF: 8 KiB switchable PRG ROM bank
  • CPU $A000-$BFFF: 8 KiB switchable PRG ROM bank
  • CPU $C000-$FFFF: 16 KiB PRG ROM bank, fixed to the last 16 KiB

VRC4

  • CPU $6000-$6FFF: optional 2 KiB PRG RAM bank (mirrored once), or
  • CPU $6000-$7FFF: optional 8 KiB PRG RAM bank
  • CPU $8000-$9FFF (or $C000-$DFFF): 8 KiB switchable PRG ROM bank
  • CPU $A000-$BFFF: 8 KiB switchable PRG ROM bank
  • CPU $C000-$DFFF (or $8000-$9FFF): 8 KiB PRG ROM bank, fixed to the second-last bank
  • CPU $E000-$FFFF: 8 KiB PRG ROM bank, fixed to the last bank

Registers

This page lists registers as they are in the VRC2b and VRC4f variants (iNES mapper 23).

PRG Swap Mode control ($9002, $9003) VRC4

7  bit  0
---------
.... ..M.
       |
       +-- Swap Mode

This register is VRC4 only.

When 'M' is clear:

  • the 8 KiB page at $8000 is controlled by the $800x register
  • the 8 KiB page at $C000 is fixed to the second last 8 KiB in the ROM

When 'M' is set:

  • the 8 KiB page at $8000 is fixed to the second last 8 KiB in the ROM
  • the 8 KiB page at $C000 is controlled by the $800x register

PRG Select 0 ($8000, $8001, $8002, $8003)

7  bit  0
---------
...P PPPP
   | ||||
   +-++++- Select 8 KiB PRG bank at $8000 or $C000 depending on Swap Mode

VRC2 does not have a Swap Mode. The bank is always at $8000.

PRG Select 1 ($A000, $A001, $A002, $A003)

7  bit  0
---------
...P PPPP
   | ||||
   +-++++- Select 8 KiB PRG bank at $A000

Mirroring Control ($9000, $9001, $9002, $9003)

7  bit  0
---------
.... ..MM
       ||
       ++- Mirroring (0: vertical; 1: horizontal; 2: one-screen, lower bank; 3: one-screen, upper bank)

VRC2 supports only vertical or horizontal mirroring. Bit 1 is ignored.

VRC4 only has mirroring control at $9000 and $9001. $9002 and $9003 are used to select PRG swap mode instead (see above).

VRC2-using games are usually well-behaved and only write 0 or 1 to this register, but Wai Wai World in one instance writes $FF instead[2].

CHR Select 0 low($B000), high($B001)

  $B000        $B001
7  bit  0    7  bit  0
---------    ---------
.... LLLL    ...H HHHH
     ||||       | ||||
     ||||       +-++++- High 5 bits of 1 KiB CHR bank at PPU $0000
     ++++-------------- Low 4 bits

VRC2 only has 4 high bits of CHR select. $B001 bit 4 is ignored.

On VRC2a (mapper 22), the low bit is ignored (right shift value by 1).

CHR Select 1 low($B002), high($B003)

  $B002        $B003
7  bit  0    7  bit  0
---------    ---------
.... LLLL    ...H HHHH
     ||||       | ||||
     ||||       +-++++- High 5 bits of 1 KiB CHR bank at PPU $0400
     ++++-------------- Low 4 bits

VRC2 only has 4 high bits of CHR select. $B003 bit 4 is ignored.

On VRC2a (mapper 22), the low bit is ignored (right shift value by 1).

CHR Select 2…7 ($C000-$EFFF)

The other six CHR bank selects continue the pattern:

Write to CPU address 1KB CHR bank affected
(low 4 bits) (high 5 bits)
$C000 $C001 $0800-$0BFF
$C002 $C003 $0C00-$0FFF
$D000 $D001 $1000-$13FF
$D002 $D003 $1400-$17FF
$E000 $E001 $1800-$1BFF
$E002 $E003 $1C00-$1FFF

IRQ Control ($F00x) VRC4

$F000:  IRQ Latch, low 4 bits
$F001:  IRQ Latch, high 4 bits
$F002:  IRQ Control
$F003:  IRQ Acknowledge

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

Microwire interface ($6000-$6FFF) VRC2

This only exists on VRC2.

How it was supposed to work:

7  bit  0
---------
.... .SCD
      |||
      ||+- Data to EEPROM (write) or from EEPROM (read)
      |+-- Clock to EEPROM (write only)
      +--- Chip Select (write only)

How it works in practice:

7  bit  0
---------
.... ...L
        |
        +- 1-bit latch value (r/w)

Reads from $6000-6FFF return open bus for the top 7 bits. Reads from $7000-7FFF only ever return open bus.

The VRC2 was supposed to have shipped with a Microwire interface for save games. However, Konami never used it, seemingly due to a defect in the VRC2.

Across all the VRC2-using boards, the Data from EEPROM pin has been connected to many different things. On 351618 (22) it's connected to ground. On 350603, 350636, and 351179 it floats. On 350926 it's connected to Data to EEPROM. On 351948 (25) it's connected to ground but extra hardware keeps the VRC2 from seeing reads from $6000 so as to not conflict with PRG RAM.

Several games, including Contra (J) and Ganbare Goemon 2 (J), rely on the two Data pins being connected to each other, and so expect to be able to read the written value back. In these cases, the LSB agrees with the last value written and the upper seven bits are open bus, e.g. both LDA $6100 and LDA $6000 will return $60|latch. Returning neither open bus nor 0x00 will work, and these games will lock almost immediately after execution begins. Fortunately, no games ever rely on any other behavior.

Emulators that use the same VRC4 core (and its PRG RAM) for VRC2 emulation will have the effect simulated for them. However, only 351948 contains any RAM.

Hardware

The VRC2 and VRC4 exist in a 40-pin PDIP package.

References

  1. Forum thread: Several Famicom Nes Documents in Japanese indicate that VRC stands for "Virtual ROM Controller"
  2. Forum thread: incorrect mirroring in Wai Wai World, and VRC2 testing.