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.

Example games:

After Burner
Maharaja (J)
Nantettatte!! Baseball (J)

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 Pattern Table ROM bank 0

Map a 2 KiB CHR ROM bank at PPU address $0000-$07FF.

$9000-$9FFF: CHR Pattern Table ROM bank 1

Map a 2 KiB CHR ROM bank at PPU address $0800-$0FFF.

$A000-$AFFF: CHR Pattern Table ROM bank 2

Map a 2 KiB CHR ROM bank at PPU address $1000-$17FF.

$B000-$BFFF: CHR Pattern Table ROM bank 3

Map a 2 KiB CHR ROM bank at PPU address $1800-$1FFF.

$C000-$CFFF: CHR Nametable ROM bank 0

Map a 1 KiB CHR ROM bank where CIRAM $000-$3FF would normally be mapped.

This applies only when $E000.4 = 1 for ROM nametable mode.
This occupies the lower nametable and pattern table.
The PPU address range of this bank follows the mirroring selection in $E000.
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: CHR Nametable ROM bank 1

Map a 1 KiB CHR ROM bank where CIRAM $400-$7FF would normally be mapped.

This applies only when $E000.4 = 1 for ROM nametable mode.
This occupies the upper nametable and pattern table.
The PPU address range of this bank follows the mirroring selection in $E000.
Only D6-D0 are used; D7 is ignored and treated as 1.

$E000-$EFFF: Nametable control

7654 3210
   |   ||
   |   ++- Nametable Mirroring Behavior (See table below.)
   +------ CIRAM / CHR-ROM Select for Nametables (PPU $2000-$2FFF):
             0 = CIRAM
             1 = CHR-ROM (Uses ROM bank selection registers $C000, $D000.)

Nametable Mirroring Behavior

Mode	$E000.(1:0)	PPU $2000-$23FF	PPU $2400-$27FF	PPU $2800-$2BFF	PPU $2C00-$2FFF
Vertical	%00	Low NT	High NT	Low NT	High NT
Horizontal	%01	Low NT	Low NT	High NT	High NT
Single-screen NT0	%10	Low NT	Low NT	Low NT	Low NT
Single-screen NT1	%11	High NT	High NT	High NT	High NT

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

$F000-$FFFF: PRG-ROM bank

Map a 16 KiB PRG ROM bank at CPU address $8000-$BFFF.

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 Disch
Comprehensive NES Mapper Document by \Firebug\, information about mapper's initial state is inaccurate.