INES Mapper 040

iNES Mapper 040 describes yet another pirate port of SMB2j from the FDS to a cartridge. It is very similar to mapper 50.

This description was produced from the source code of FCEUX and Nestopia.

 Registers:
 ---------------------------
 Range,Mask:   $8000-FFFF, $E000

   $8000:  Disable and acknowledge IRQ
   $A000:  Enable IRQ
   $E000:  8 KiB bank mapped at $C000
 
 PRG Setup:
 ---------------------------
 
       $6000   $8000   $A000   $C000   $E000  
     +-------+-------+-------+-------+-------+
     | { 6 } | { 4 } | { 5 } | $E000 | { 7 } |
     +-------+-------+-------+-------+-------+

Like mapper 50, this hardware produces an interrupt 4096 M2 cycles after the IRQ is enabled. Both FCEUX and Nestopia's source disable the interrupt when it fires.

It seems most likely the corresponding hardware is a CD4020, functioning as a 13-bit counter, and that if the software didn't acknowledge the interrupt for another 4096 cycles it would self-acknowledge.