INES Mapper 073

iNES Mapper 073 is used to represent the Konami VRC3 mapper.

 Here are Disch's original notes:  
 ========================
 =  Mapper 073          =
 ========================
 
 aka
 --------------------------
 VRC3
 
 
 Example Games:
 --------------------------
 Salamander
 
 
 Registers:
 --------------------------
 
 Range,Mask:   $8000-FFFF, $F000
 
   $8000:  [.... IIII]   Bits  0- 3 of IRQ reload value
   $9000:  [.... IIII]   Bits  4- 7 of IRQ reload value
   $A000:  [.... IIII]   Bits  8-11 of IRQ reload value
   $B000:  [.... IIII]   Bits 12-15 of IRQ reload value
 
   $C000:  [.... .MEA]   IRQ Control
      M = IRQ Mode (0=16-bit mode, 1=8-bit mode)
      E = IRQ Enable (0=disabled, 1=enabled)
      A = Enable-on-Acknowledge (see IRQ section)
 
   $D000:  [.... ....]   IRQ Acknowledge (see IRQ section)
 
   $F000:  [.... PPPP]   PRG Select (16k @ $8000)
 
 
 PRG Setup:
 ---------------------------
 
       $8000   $A000   $C000   $E000  
     +---------------+---------------+
     |     $F000     |     { -1}     |
     +---------------+---------------+
 
 
 IRQs:
 ---------------------------
 
 VRC3 IRQs operate differently from other VRCs.  The counter is 16 bits instead of 8 bits, and there is no
 scanline mode -- only CPU cycle mode.  Other aspects, however, are very similar.
 
 $8000-B000 set the 16-bit reload value (not the actual IRQ counter).  When $C000 is written to with the 'E'
 bit set, the reload value is copied into the actual IRQ counter.
 
 When enabled, the IRQ counter will increment by 1 every CPU cycle until it wraps, at which point the IRQ
 counter is reloaded with the reload value (relevent bits only!  see Modes below) and an IRQ is tripped.
 
 Any write to $C000 or $D000 will acknowledge the IRQ.
 
 Any write to $D000 will also copy the 'A' control bit to the 'E' control bit... enabling or disabling IRQs.
 This does not change the contents of the IRQ counter.
 
 
 Modes:
 ---------------------------
 There are 8-bit and 16-bit modes for the IRQ counter, as controlled by the 'M' bit in $C000.
 
   In 16-bit mode (M=0):
     - Counter is a full 16-bits.
     - IRQ is triggered when IRQ counter is incremented from $FFFF
 
 
   In 8-bit mode (M=1):
     - Only the low 8-bit bits of counter are used
     - IRQ is triggered when low 8 bits of IRQ counter are incremented from $FF
     - Incrementing the low bits *never* alters the high bits of the counter
     - When low 8 bits wrap, only the low 8 bits are copied from the reload value... high bits remain unchanged
     - Reloading via $C000 write will still reload all 16 bits.