http://forums.nesdev.com/viewtopic.php?f=9&t=16559
[DDDD DDDD] $f000 (mask: $f00c)
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++++-++++- set counter to the following value (and ack any pending irq)
[.... ...E] $f008 (mask: $f00c)
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+-- 0: disable counting and hold m2 prescaler in clear, 1: enable counting and release m2 prescaler from clear
+--
counter counts up from the preset value to $ff (one tick every 256 felling edge of M2)
if it rolls to $00, irq is assrted (which will be auto-acked after next 128 falling edge of m2).
prese