VRC1

The Konami VRC1 is an ASIC mapper.

Overview

• PRG ROM size: Up to 128 KB
• PRG ROM bank size: 8 KB at $8000, $A000, and $C000
• PRG RAM: None
• CHR bank size: 4 KB
• Nametable mirroring: Controlled by mapper
• Subject to bus conflicts: No

Registers

PRG Select 0 ($8000-$8FFF)

7  bit  0
---------
.... PPPP
     ||||
     ++++- Select 8 KB PRG ROM at $8000

PRG Select 1 ($A000-$AFFF)

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

PRG Select 2 ($C000-$CFFF)

7  bit  0
---------
.... PPPP
     ||||
     ++++- Select 8 KB PRG ROM at $C000

Mirroring Control, CHR bits ($9000-$9FFF)

7  bit  0
---------
.... .BAM
      |||
      ||+- Mirroring  (0: Vertical; 1: Horizontal)
      |+-- High Bit of 4 KB CHR bank at PPU $0000
      +--- High Bit of 4 KB CHR bank at PPU $1000

CHR Select 0 ($E000-$EFFF)

7  bit  0
---------
.... CCCC
     ||||
     ++++- Low 4 bits of 4 KB CHR bank at PPU $0000

These bits combined with bit 1 of $9000 make a 5-bit CHR selection.

CHR Select 1 ($F000-$FFFF)

7  bit  0
---------
.... CCCC
     ||||
     ++++- Low 4 bits of 4 KB CHR bank at PPU $1000

These bits combined with bit 2 of $9000 make a 5-bit CHR selection.