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.

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 the timer has not expired; once it has, reads should return open bus.

The game verifies that the timer expires after 107516 to 107575 M2 cycles by reading a signature from the end of the external ROM.

$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. Not all documents mention the single screen mirroring mode; its behavior should be verified against an authentic cartridge.

$F000-$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 128KB, both seen subcartridges are only 32KB, so are mirrored four times. The known UNIF dumps of these external ROMs are only 16KB in size. Naruko implies that the game only reads the first 16KB of the external ROM.

Hardware

Both the US and Japanese release of After Burner have CHR ROM split into two chips.

References

• Goroh's Sunsoft mapper doc
• Naruko's notes in the forum

 Here are Disch's original notes:
 ========================
 =  Mapper 068          =
 ========================
 
 
 Example Games:
 --------------------------
 After Burner 2
 Maharaja
 
 
 Registers:
 ---------------------------
 
 Range,Mask:   $8000-FFFF, $F000
 
   $8000:  CHR Reg 0  (2k @ $0000)
   $9000:  CHR Reg 1  (2k @ $0800)
   $A000:  CHR Reg 2  (2k @ $1000)
   $B000:  CHR Reg 3  (2k @ $1800)
 
   $C000:  [.NNN NNNN]  NT-ROM Reg 0
   $D000:  [.NNN NNNN]  NT-ROM Reg 1
   $E000:  [...R ...M]  Mirroring (see section below)
 
   $F000:  PRG Reg (16k @ $8000)
 
 
 CHR Setup:
 ---------------------------
 
       $0000   $0400   $0800   $0C00   $1000   $1400   $1800   $1C00 
     +---------------+---------------+---------------+---------------+
     |     $8000     |     $9000     |     $A000     |     $B000     |
     +---------------+---------------+---------------+---------------+
 
 
 PRG Setup:
 ---------------------------
 
       $8000   $A000   $C000   $E000  
     +---------------+---------------+
     |     $F000     |     { -1}     |
     +---------------+---------------+
 
 
 
 Mirroring:
 ---------------------------
 
 The mirroring reg has two significant bits:
 
   $E000:  [...R ...M]
 
   'M' selects H/V:
      0 = Vert
      1 = Horz
 
   'R' selects whether or not to use CHR-ROM as nametables.
      0 = normal mirroring
      1 = use CHR-ROM
 
 When 'R' is set, $C000 and $D000 are used to select 1k CHR-ROM pages to use as nametables.  They are arranged
 in either Horz or Vert mirroring fashion depending on the 'M' bit ($C000 would be used in place of NTA,
 $D000 in place of NTB).
 
  R=1, M=0:
       [ $C000 ][ $D000 ]
       [ $C000 ][ $D000 ]
 
  R=1, M=1:
       [ $C000 ][ $C000 ]
       [ $D000 ][ $D000 ]
 
 Note that CHR-ROM for nametables is taken from the last 128k of CHR.  This means you must effectively OR the
 value written to $C000/$D000 with $80.