
========================
=  Mapper 050          =
========================

Example Game:
--------------------------
Super Mario Bros. (JU) (Alt Levels)   (SMB2j pirate cart)


Notes:
---------------------------
No PRG-RAM.  PRG setup is bizarre, as is the scrambled PRG reg.


Registers:
---------------------------

Range,Mask:   $4020-5FFF, $4120


  $4020:  [.... HLLM]
    L,M,H = Low, middle, and high bits of PRG Reg


  $4120:  [.... ...E]  IRQ Enable (0=Disabled, 1=Enabled)



PRG Setup:
---------------------------

      $6000   $8000   $A000   $C000   $E000  
    +-------+-------+-------+-------+-------+
    | {$0F} | { 8 } | { 9 } | $4020 | {$0B} |
    +-------+-------+-------+-------+-------+


IRQs:
---------------------------

  Writing to $4120 with E=0 will disable IRQs, acknowledge the pending IRQ, and reset the IRQ counter to 0.
Writing with E=1 will just enable IRQs (but will not change anything else).

  When enabled, The IRQ counter will count up every CPU cycle.  When it makes the transition from
$0FFF->$1000, and IRQ is generated.  The counter appears to be a full 16-bits (so it will not wrap until
$FFFF)

