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=  Mapper "18-72"      =
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Example Games:
--------------------------
Akumajou Special: Boku Dracula-kun (pirate version)


Notes:
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This mapper is another pirate VRC2b, this time with IRQs and VRC4-style
mirroring.

A mapper number hasn't been assigned to this particular board yet.


Registers:
---------------------------
There are two sets of registers: the VRC2 registers, and the Extra registers.
Writes will affect both sets of registers!


VRC2 registers range, mask: $8000-$FFFF, $F003

$8000-$8003:  [...P PPPP]   Same as VRC2b (PRG select 0)
$9000-$9003:  [.... ...M]   VRC2 mirroring control (see extra registers)
                       |
                       +- 0: vertical (A10); 1: horizontal (A11)

$A000-$A003:  [...P PPPP]   Same as VRC2b (PRG select 1)
$B000-$E003:  [.... CCCC]   Same as VRC2b (CHR select)



Extra registers range, mask: $8000-$FFFF, $C00C

$8004:        [.... ..XX]   Extended mirroring control
                      ||
                      ++- Extended mirroring control
                           0x: Let VRC2 control mirroring
                           10: one-screen, lower bank (overrides VRC2)
                           11: one-screen, upper bank (overrides VRC2)

$C004:        [.... ....]   Acknowledge IRQ
$C008:        [.... ....]   Start IRQ counter
$C00C:        [.... ....]   Acknowledge IRQ and hold IRQ counter at 0


Mirroring behavior:
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Games will write to $9004 to affect both the VRC2 mirroring register ($9000)
and the Extra mirroring register ($8004) at the same time. The result is like
VRC4 mirroring:

$9004 [.... ..MM]   Combined VRC2 and Extra mirroring control
              ||
              ++- VRC4 mirroring
                   00: Vertical
                   01: Horizontal
                   10: One-screen, lower bank
                   11: One-screen, upper bank

The VRC2 will remember the last setting written to it, even if the Extra
mirroring control is overriding it.


IRQ behavior:
---------------------------
The IRQ counter counts falling edges of PPU A13. The IRQ is asserted every time
the counter reaches 86. When the counter reaches 86, it's automatically reset
to 0. This results in one IRQ every two scanlines, as long as the counter is
running. Writing to Extra register $C004 or $C00C will acknowledge the IRQ.
Writing to Extra register $C00C will hold the counter at 0, and writing to
Extra register $C008 will allow the counter to resume.

Games will typically write to addresses like $F004/$F008/$F00C to control the
IRQ without affecting the VRC2's registers.