VRC2 and VRC4

VRC2, VRC4

Company	Konami
Complexity	ASIC
Boards	VRC2a-c VRC4a-f
PRG ROM capacity	128K 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, or 1 switchable
Bus conflicts	No
IRQ	Yes
Audio	No
iNES mappers	021, 022, 023, 025

NESCartDB

iNES 021

iNES 022

iNES 023

iNES 025

The Konami VRC2 and Konami VRC4 are two related ASIC mappers in the VRC 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:

• Wai Wai World 2 (021)
• Ganbare Goemon Gaiden 2 (021)
• Boku Dracula-kun (023)
• Tiny Toon Adventures (J) (023)
• Gradius 2 (J) (025)
• Bio Miracle Bokutte Upa (025)
• Ganbare Pennant Race (022)
• TwinBee 3 (022)
• Wai Wai World (023)
• Ganbare Goemon Gaiden (025)

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	A2	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	A2	A1	Registers	iNES mapper	submapper
VRC4a	352398	A2	A1	$x000, $x002, $x004, $x006	21	1
VRC4b	351406	A0	A1	$x000, $x002, $x001, $x003	25	1
VRC4c	352889	A7	A6	$x000, $x040, $x080, $x0C0	21	2
VRC4d	352400	A2	A3	$x000, $x008, $x004, $x00C	25	2
VRC4e	352396	A3	A2	$x000, $x004, $x008, $x00C	23	2
VRC4f	-	A1	A0	$x000, $x001, $x002, $x003	23	1

† The VRC2b PCBs include: 350603, 350636, 350926, 351179

VRC2 vs VRC4

The VRC2 has:

• PRG limit of 128k
• CHR limit of 256k
• 1 bit latch at $6000-6FFF
• Only horizontal or vertical mirroring.

The VRC4 has:

• PRG limit of 256k
• 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.

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.

VRC2 only has 4 bits of PRG select. Bit 4 is ignored.

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

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

VRC2 only has 4 bits of PRG select. Bit 4 is ignored.

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.

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)

$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 some 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.

References

• NES Mapper List by Disch
• Konami Mapper by goroh, translated by Sgt. Bowhack
• Comprehensive NES Mapper Document by \Firebug\, information about mapper's initial state is inaccurate.
• The CHR registers really are 9 bits: http://forums.nesdev.org/viewtopic.php?t=8569