INES Mapper 246

iNES Mapper 246 seems to be a singleton for the Taiwanese game 封神榜 (Fēngshénbǎng: Fúmó Sān Tàizǐ).

It provides 2 KiB of RAM from $6800-$6FFF.

 Mask:   $FFE7
 
 $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 cold boot. (All other registers probably also, but not required)

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

There is additionally some weird behavior on reads from $FFE4-$FFFF:

• PRG ROM access when A15..A5 AND A2 are all HIGH should instead occur from a bank where PRG A17 is high, i.e. ( [$6003] | $10 )

However, the existing dumps in GoodNES have been pre-patched to not require this.

See also:

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