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 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 (most likely exactly 107520=1024·105) 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.

$F000-$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 seen subcartridges only use a 32KiB EPROM, and so are mirrored four times. Naruko implies that the game only reads the first 16iKB of the external ROM, and the known UNIF dumps of these external ROMs are also only 16KiB in size.

The fixed bank is always from the internal ROM.

Hardware

The US release of After Burner have 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

 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.

INES Mapper 068

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