VRC4

The Konami VRC4 is an ASIC mapper.

Overview

• PRG ROM bank size: 8 KB at $A000, and $8000 OR $C000
• PRG RAM: Up to 8 KB
• CHR bank size: 1 KB
• Nametable mirroring: Controlled by mapper
• Subject to bus conflicts: No

The Konami VRC4 is almost identical to the VRC2, but with a bit more capabilities, such as the option to swap PRG at $C000 instead of $8000 and an IRQ counter.

Revisions

This mapper had several revisions, each of which uses different address lines for its registers. Here is a listing of known revisions, which address lines are used, and which iNES Mapper number is used to represent it:

variant   lines     registers                       iNES Mapper Number
----------------------------------------------------------------------
VRC4a:    A1, A2    $x000, $x002, $x004, $x006      021
VRC4b:    A1, A0    $x000, $x002, $x001, $x003      025
VRC4c:    A6, A7    $x000, $x040, $x080, $x0C0      021
VRC4d:    A3, A2    $x000, $x008, $x004, $x00C      025
VRC4e:    A2, A3    $x000, $x004, $x008, $x00C      023

In addition to above address lines, all revisions also use A12-A15.

Note that while some variants may use the same address lines, they use them in different ways. For example, VRC4d and VRC4e both use A2 and A3, however VRC4d has them reversed.

This page lists registers as they are in the VRC4a variant. For other variants, you can use the diagram above for determining which registers are used

Registers

PRG Swap Mode control ($9004, $9006)

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

When 'M' is clear:

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

When 'M' is set:

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

PRG Select 0 ($8000, $8002, $8004, $8006)

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

PRG Select 1 ($A000, $A002, $A004, $A006)

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

Mirroring Control ($9000, $9002)

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

CHR Select 0 ($B000 + $B002)

  $B000        $B002
7  bit  0    7  bit  0
---------    ---------
.... LLLL    .... HHHH
     ||||         ||||
     ||||         ++++- High 4-bits of 1 KB CHR bank at PPU $0000
     ++++-------------- Low 4-bits

CHR Select 1 ($B004 + $B006)

  $B004        $B006
7  bit  0    7  bit  0
---------    ---------
.... LLLL    .... HHHH
     ||||         ||||
     ||||         ++++- High 4-bits of 1 KB CHR bank at PPU $0400
     ++++-------------- Low 4-bits

CHR Select 2 ($C000 + $C002)

  $C000        $C002
7  bit  0    7  bit  0
---------    ---------
.... LLLL    .... HHHH
     ||||         ||||
     ||||         ++++- High 4-bits of 1 KB CHR bank at PPU $0800
     ++++-------------- Low 4-bits

CHR Select 3 ($C004 + $C006)

  $C004        $C006
7  bit  0    7  bit  0
---------    ---------
.... LLLL    .... HHHH
     ||||         ||||
     ||||         ++++- High 4-bits of 1 KB CHR bank at PPU $0C00
     ++++-------------- Low 4-bits

CHR Select 4 ($D000 + $D002)

  $D000        $D002
7  bit  0    7  bit  0
---------    ---------
.... LLLL    .... HHHH
     ||||         ||||
     ||||         ++++- High 4-bits of 1 KB CHR bank at PPU $1000
     ++++-------------- Low 4-bits

CHR Select 5 ($D004 + $D006)

  $D004        $D006
7  bit  0    7  bit  0
---------    ---------
.... LLLL    .... HHHH
     ||||         ||||
     ||||         ++++- High 4-bits of 1 KB CHR bank at PPU $1400
     ++++-------------- Low 4-bits

CHR Select 6 ($E000 + $E002)

  $E000        $E002
7  bit  0    7  bit  0
---------    ---------
.... LLLL    .... HHHH
     ||||         ||||
     ||||         ++++- High 4-bits of 1 KB CHR bank at PPU $1800
     ++++-------------- Low 4-bits

CHR Select 7 ($E004 + $E006)

  $E004        $E006
7  bit  0    7  bit  0
---------    ---------
.... LLLL    .... HHHH
     ||||         ||||
     ||||         ++++- High 4-bits of 1 KB CHR bank at PPU $1C00
     ++++-------------- Low 4-bits

IRQ Control ($F00x)

$F000:  IRQ Latch, low 4 bits
$F002:  IRQ Latch, high 4 bits
$F004:  IRQ Control
$F006:  IRQ Acknowledge

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