INES Mapper 246

 Here are Disch's original notes:  
 ========================
 =  Mapper 246          =
 ========================
 
 
 Example Game:
 --------------------------
 Fong Shen Bang - Zhu Lu Zhi Zhan
 
 
 Notes:
 --------------------------
 
 Regs lie at $6000-67FF, but SRAM exists at $6800-7FFF.
 
 Don't know if there's only 6k of SRAM, or if there's 8k, but the first 2k is inaccessable.  I find the latter
 more likely.
 
 
 Registers:
 ---------------------------
 
 Range,Mask:   $6000-67FF, $6007
 
 
   $6000-6003:  PRG Regs
   $6004-6007:  CHR Regs
 
 
 CHR Setup:
 ---------------------------
 
       $0000   $0400   $0800   $0C00   $1000   $1400   $1800   $1C00 
     +---------------+---------------+---------------+---------------+
     |     $6004     |     $6005     |     $6006     |     $6007     |
     +---------------+---------------+---------------+---------------+
 
 
 PRG Setup:
 ---------------------------
 
       $8000   $A000   $C000   $E000  
     +-------+-------+-------+-------+
     | $6000 | $6001 | $6002 | $6003 |
     +-------+-------+-------+-------+
 
 
 Powerup/Reset:
 ---------------------------
 $6003 set to $FF on powerup (and probably reset, but not sure).

The present hardware seems to be:

• Four 74LS670 4 x 4bit dual-ported memory
• One GAL16V8
• PRG ROM (up to 2 MiB), CHR ROM (up to 512 KiB), PRG RAM (8 KiB)
• Miscellaneous resistors, diodes, and a battery to power the PRG RAM

Correspondingly, it seems more likely that:

• all four PRG banks are $FF on powerup, but all go to whatever the value in the 74LS670s is after some unknown event (the first write?)
• More PRG RAM may be accessible (up to 8176 bytes) and there are probably fewer mirrors of the registers

See also:

• http://forums.nesdev.org/viewtopic.php?t=13969 Why does Fong Shen Bang only work in Famiclones?