VRC2
The Konami VRC2 is an ASIC mapper.
Overview
- PRG ROM bank size: 8 KB at $8000, and $A000
- PRG RAM: None or 8KB
- CHR bank size: 1 KB
- Nametable mirroring: Controlled by mapper
- Subject to bus conflicts: No
The Konami VRC2 is almost identical to the VRC4, but a bit more limited.
Revisions
This mapper had two known revisions: VRC2a and VRC2b. Both revisions uses Address lines A0, A1, and A12-A15 for registers, however VRC2a has A0 and A1 "backwards" from the norm. Address $x001 on VRC2b would be $x002 on VRC2a, and $x002 on VRC2b would be $x001 on VRC2a.
Additionally, VRC2a has 7-bit wide CHR registers, whereas VRC2b has 8-bit wide CHR registers.
This page lists registers as they are in the VRC2b variant. For VRC2a registers, reverse A0 and A1 lines.
Registers
PRG Select 0 ($8000, $8001, $8002, $8003)
7 bit 0 --------- ...P PPPP | |||| + ++++- Select 8 KB PRG bank at $8000
PRG Select 1 ($A000, $A001, $A002, $A003)
7 bit 0 --------- ...P PPPP | |||| + ++++- Select 8 KB PRG bank at $A000
Mirroring Control ($9000, $9001, $9002, $9003)
7 bit 0
---------
.... ..MM
||
++- Mirroring (0: vertical; 1: horizontal;
2: one-screen, lower bank; 3: one-screen, upper bank;)
CHR Select 0 ($B000 + $B001)
$B000 $B001
7 bit 0 7 bit 0
--------- ---------
.... LLLL .... HHHH
|||| ||||
|||| ++++- High 4-bits of 1 KB CHR bank at PPU $0000
++++-------------- Low 4-bits
Note: On VRC2a, the low bit is ignored (right shift value by 1)
CHR Select 1 ($B002 + $B003)
$B002 $B003
7 bit 0 7 bit 0
--------- ---------
.... LLLL .... HHHH
|||| ||||
|||| ++++- High 4-bits of 1 KB CHR bank at PPU $0400
++++-------------- Low 4-bits
Note: On VRC2a, the low bit is ignored (right shift value by 1)
CHR Select 2 ($C000 + $C001)
$C000 $C001
7 bit 0 7 bit 0
--------- ---------
.... LLLL .... HHHH
|||| ||||
|||| ++++- High 4-bits of 1 KB CHR bank at PPU $0800
++++-------------- Low 4-bits
Note: On VRC2a, the low bit is ignored (right shift value by 1)
CHR Select 3 ($C002 + $C003)
$C002 $C003
7 bit 0 7 bit 0
--------- ---------
.... LLLL .... HHHH
|||| ||||
|||| ++++- High 4-bits of 1 KB CHR bank at PPU $0C00
++++-------------- Low 4-bits
Note: On VRC2a, the low bit is ignored (right shift value by 1)
CHR Select 4 ($D000 + $D001)
$D000 $D001
7 bit 0 7 bit 0
--------- ---------
.... LLLL .... HHHH
|||| ||||
|||| ++++- High 4-bits of 1 KB CHR bank at PPU $1000
++++-------------- Low 4-bits
Note: On VRC2a, the low bit is ignored (right shift value by 1)
CHR Select 5 ($D002 + $D003)
$D002 $D003
7 bit 0 7 bit 0
--------- ---------
.... LLLL .... HHHH
|||| ||||
|||| ++++- High 4-bits of 1 KB CHR bank at PPU $1400
++++-------------- Low 4-bits
Note: On VRC2a, the low bit is ignored (right shift value by 1)
CHR Select 6 ($E000 + $E001)
$E000 $E001
7 bit 0 7 bit 0
--------- ---------
.... LLLL .... HHHH
|||| ||||
|||| ++++- High 4-bits of 1 KB CHR bank at PPU $1800
++++-------------- Low 4-bits
Note: On VRC2a, the low bit is ignored (right shift value by 1)
CHR Select 7 ($E002 + $E003)
$E002 $E003
7 bit 0 7 bit 0
--------- ---------
.... LLLL .... HHHH
|||| ||||
|||| ++++- High 4-bits of 1 KB CHR bank at PPU $1C00
++++-------------- Low 4-bits
Note: On VRC2a, the low bit is ignored (right shift value by 1)
Unknown Latch ($6000-$6FFF)
7 bit 0
---------
.... ...L
|
+- 1-bit latch value (r/w)
On boards that lack PRG RAM, this register bit serves an unknown purpose.
Many VRC2 games such as Contra (J) and Ganbare Goemon 2 (J) write to the lowest bit of $6000, and expect to be able to read it back. Returning either open bus or 0x00 will not work, and the games will lock almost immediately after execution begins.
Emulators that use the same VRC4 core (and its 8KB of PRG RAM) for VRC2 emulation will have the effect simulated for them. However, most VRC2 boards do not contain any RAM.
Reads from $6000-6FFF return open bus OR'ed with the latch value bit. Eg LDA $6100 will always return $61, whereas LDA $6000 will return $60|latch. $7000-7FFF returns only open bus, ignoring the latch.
