8-bit IRQ counter

$f000: d0-d3 of IRQ latch reg are set by d0-d3 on the bus
$f002: d4-d7 of IRQ latch reg are set by d0-d3 on the bus
$f001: copy IRQ latch to IRQ countup, IRQ enable flag = d1, temp reg = d0
$f003: if temp reg is 1, set IRQ enable flag to 1, else set to 0

if (enable)
every 113 cpu cycles: inc IRQ counter, if it wraps, send an int and put contents of
latch into countup reg. do NOT set enable to 0