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| == Overview ==
| | #REDIRECT [[VRC2 and VRC4]] |
| * Manufacturer: Konami
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| * PRG ROM bank size: 8 KB at $A000, and $8000 OR $C000
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| * PRG RAM: Up to 8 KB
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| * CHR bank size: 1 KB
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| * Nametable [[mirroring]]: Controlled by mapper
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| * Subject to [[bus conflict]]s: No
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| The Konami VRC4 is almost identical to the [[VRC2|VRC2]], but with a bit more capabilities, such as the option to swap PRG at $C000 instead of $8000, an IRQ counter, and a 5-bit data bus for PRG pages instead of 4-bit.
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| == Revisions ==
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| 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:
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| <pre>
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| variant lines registers iNES Mapper Number
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| ----------------------------------------------------------------------
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| VRC4a: A1, A2 $x000, $x002, $x004, $x006 021
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| VRC4b: A1, A0 $x000, $x002, $x001, $x003 025
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| VRC4c: A6, A7 $x000, $x040, $x080, $x0C0 021
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| VRC4d: A3, A2 $x000, $x008, $x004, $x00C 025
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| VRC4e: A2, A3 $x000, $x004, $x008, $x00C 023
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| </pre>
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| In addition to above address lines, all revisions also use A12-A15.
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| 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.
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| 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
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| == Registers ==
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| === PRG Swap Mode control ($9004, $9006) ===
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| 7 bit 0
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| ---------
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| .... ..M.
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| +-- Swap Mode
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| When 'M' is clear:
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| * the 8 KB page at $8000 is controlled by the $800x register
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| * the 8 KB page at $C000 is fixed to the second last 8 KB in the ROM
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| When 'M' is set:
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| * the 8 KB page at $8000 is fixed to the second last 8 KB in the ROM
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| * the 8 KB page at $C000 is controlled by the $800x register
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| === PRG Select 0 ($8000, $8002, $8004, $8006) ===
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| 7 bit 0
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| ---------
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| ...P PPPP
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| +-++++- Select 8 KB PRG bank at $8000 or $C000 depending on Swap Mode
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| === PRG Select 1 ($A000, $A002, $A004, $A006) ===
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| 7 bit 0
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| ---------
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| ...P PPPP
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| +-++++- Select 8 KB PRG bank at $A000
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| === Mirroring Control ($9000, $9002) ===
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| 7 bit 0
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| ---------
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| .... ..MM
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| ||
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| ++- [[Mirroring]] (0: vertical; 1: horizontal;
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| 2: one-screen, lower bank; 3: one-screen, upper bank;)
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| === CHR Select 0 ($B000 + $B002) ===
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| $B000 $B002
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| 7 bit 0 7 bit 0
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| --------- ---------
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| .... LLLL .... HHHH
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| |||| ||||
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| |||| ++++- High 4-bits of 1 KB CHR bank at PPU $0000
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| ++++-------------- Low 4-bits
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| === CHR Select 1 ($B004 + $B006) ===
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| $B004 $B006
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| 7 bit 0 7 bit 0
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| --------- ---------
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| .... LLLL .... HHHH
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| |||| ||||
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| |||| ++++- High 4-bits of 1 KB CHR bank at PPU $0400
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| ++++-------------- Low 4-bits
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| === CHR Select 2 ($C000 + $C002) ===
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| $C000 $C002
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| 7 bit 0 7 bit 0
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| --------- ---------
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| .... LLLL .... HHHH
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| |||| ||||
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| |||| ++++- High 4-bits of 1 KB CHR bank at PPU $0800
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| ++++-------------- Low 4-bits
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| === CHR Select 3 ($C004 + $C006) ===
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| $C004 $C006
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| 7 bit 0 7 bit 0
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| --------- ---------
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| .... LLLL .... HHHH
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| |||| ||||
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| |||| ++++- High 4-bits of 1 KB CHR bank at PPU $0C00
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| ++++-------------- Low 4-bits
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| === CHR Select 4 ($D000 + $D002) ===
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| $D000 $D002
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| 7 bit 0 7 bit 0
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| --------- ---------
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| .... LLLL .... HHHH
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| |||| ||||
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| |||| ++++- High 4-bits of 1 KB CHR bank at PPU $1000
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| ++++-------------- Low 4-bits
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| === CHR Select 5 ($D004 + $D006) ===
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| $D004 $D006
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| 7 bit 0 7 bit 0
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| --------- ---------
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| .... LLLL .... HHHH
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| |||| ||||
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| |||| ++++- High 4-bits of 1 KB CHR bank at PPU $1400
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| ++++-------------- Low 4-bits
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| === CHR Select 6 ($E000 + $E002) ===
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| $E000 $E002
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| 7 bit 0 7 bit 0
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| --------- ---------
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| .... LLLL .... HHHH
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| |||| ||||
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| |||| ++++- High 4-bits of 1 KB CHR bank at PPU $1800
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| ++++-------------- Low 4-bits
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| === CHR Select 7 ($E004 + $E006) ===
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| $E004 $E006
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| 7 bit 0 7 bit 0
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| --------- ---------
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| .... LLLL .... HHHH
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| |||| ||||
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| |||| ++++- High 4-bits of 1 KB CHR bank at PPU $1C00
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| ++++-------------- Low 4-bits
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| === IRQ Control ($F00x) ===
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| <pre>
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| $F000: IRQ Latch, low 4 bits
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| $F002: IRQ Latch, high 4 bits
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| $F004: IRQ Control
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| $F006: IRQ Acknowledge
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| </pre>
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| Many VRC mappers use the same IRQ system. For details on IRQ operation, see [[VRC IRQs]].
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