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.
