INES Mapper 068

iNES Mapper 068 denotes PCBs using the Sunsoft-4 mapper IC. In the US it was only used in the game After Burner. It has the unusual ability to map CHR ROM into the part of the PPU's address space used for nametables.

Overview

• PRG ROM size: Up to 256 KiB
• PRG ROM bank size: 16 KiB
• PRG RAM: Up to 8 KiB
• CHR capacity: Up to 256 KiB ROM
• CHR bank size: 2 KiB
• Nametable mirroring: Controlled by mapper
• Subject to bus conflicts: No

Banks

• CPU $6000-$7FFF: 8 KiB PRG RAM bank
• CPU $8000-$BFFF: 16 KiB switchable PRG ROM bank
• CPU $C000-$FFFF: 16 KiB PRG ROM bank, fixed to the last internal bank
• PPU $0000-$07FF: 2 KiB switchable CHR bank
• PPU $0800-$0FFF: 2 KiB switchable CHR bank
• PPU $1000-$17FF: 2 KiB switchable CHR bank
• PPU $1800-$1FFF: 2 KiB switchable CHR bank
• PPU $2000-$23FF (and $2400 or $2800): 1 KiB switchable CHR bank
• PPU $2C00-$2FFF (and $2800 or $2400): 1 KiB switchable CHR bank

Registers

$6000-$7FFF: Licensing IC

Only for the game Nantettatte!! Baseball, when WRAM is disabled ($F000.4 is 0), writes to this register reset a timer in the external option ROM cartridge. The external ROM is only readable while banked into $8000-$BFFF and the timer has not expired; once it has, reads from the external ROM should return open bus.

The game verifies that the timer expires after 107516 to 107575 M2 cycles (most likely exactly 107520=1024·105) by reading a signature from the end of the external ROM.

While WRAM is disabled, reads from $6000-$7FFF return open bus for all games. (The licensing IC isn't readable because it is connected to only M2, WRAM +CE, WRAM /CE, and R/W. None of the data pins connect.)

$8000-$8FFF: CHR bank at $0000

Map a 2 KiB CHR ROM bank into PPU $0000.

$9000-$9FFF: CHR bank at $0800

Map a 2 KiB CHR ROM bank into PPU $0800.

$A000-$AFFF: CHR bank at $1000

Map a 2 KiB CHR ROM bank into PPU $1000.

$B000-$BFFF: CHR bank at $1800

Map a 2 KiB CHR ROM bank into PPU $1800.

$C000-$CFFF: Nametable register 0

Map a 1 KiB CHR ROM bank in place of the lower nametable (CIRAM $000-$3FF). Only D6-D0 are used; D7 is ignored and treated as 1, so nametables must be in the last 128 KiB of CHR ROM.

$D000-$DFFF: Nametable register 1

Map a 1 KiB CHR ROM bank in place of the upper nametable (CIRAM $400-$7FF). Only D6-D0 are used; D7 is ignored and treated as 1.

$E000-$EFFF: Nametable control

7654 3210
   |   ||
   |   ++- Mirroring
   |       0: vertical (0101); 1: horizontal (0011);
   |       2: 1-screen (0000); 3: 1-screen (1111)
   +------ Chip select for PPU $2000-$2FFF (nametables):
           0 for CIRAM or 1 for CHR ROM

Nametable mirroring works the same way in both CIRAM and CHR ROM modes.

$F000-$FFFF: PRG bank at $8000

7  bit  0
---- ----
...E BBBB
   | ||||
   | ++++- Select 16 KiB PRG banked into $8000-$BFFF
   +------ 1:Enable PRG RAM = WRAM +CS2

Nantettatte!! Baseball repurposes some of the bits:

7  bit  0
---- ----
...E RBBB
   | ||||
   | |+++- Select 16 KiB PRG banked into $8000-$BFFF
   | +---- 1: select internal ROM
   |       0: select external ROM
   +------ 1:Enable PRG RAM = WRAM +CS2
           0:Enable licensing verification

Note that although the external ROM could be up to 128KiB, both known subcartridges contain 16KiB of data, doubled to fill a 32KiB EPROM, mirrored across the bottom 128KiB.

The fixed bank is always from the internal ROM.

Hardware

The US release of After Burner has CHR ROM split into two 28-pin chips. The Japanese releases exist both as two 32-pin 128KiB CHR ROMs with extra enables and as one 32-pin 256KiB ROM.

References

• Goroh's Sunsoft mapper doc
• Naruko's notes in the forum
• NES Mapper List by Dish
• Comprehensive NES Mapper Document by \Firebug\, information about mapper's initial state is inaccurate.