VRC4
The Konami VRC4 is an ASIC mapper.
Overview
- PRG ROM size: Up to 256 KB
- 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.
See VRC4 pinout for chip pinout.
Revisions
This mapper has been used with several board revisions, each of which uses different address lines. All boards use A15-A12 the same way, along with two lines from A7-A0. 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 VRC4 #27 A0, A1 $x000, $x001, $x002, $x003 027
Some variants that use the same address lines use them in different ways. For example, VRC4d and VRC4e both use A2 and A3, however VRC4d has them reversed.
For historical reasons, some variants share a mapper number. These may be detected with a hash of the PRG ROM, or the address lines used may be expressed in NES 2.0 with submappers.
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.
