local seq = 2

if event.type == "program" then
  interrupt(seq, 0)
  mem.a = 4
  mem.b = 3
  mem.d = 1

elseif event.type == "interrupt" then
  for direction, timing in pairs(mem) do
    if event.iid % timing == 0 then
      port[direction] = not pin[direction]
    end
  end
  interrupt(seq, event.iid + 1)
end