gpiosim —
General Purpose Input/Output Simulator
The gpiosim pseudo-device simulates a 64-bit wide
  gpio(4) device. The state of the
  simulated device can be manipulated through the
  sysctl(8) interface. For this
  purpose, access the "hw.gpiosim<N>.value"
  sysctl(8) variable, where
  "<N>" denotes the number of the
  gpiosim instance.
Both edge and level interrupts are simulated. The
    "hw.gpiosim<N>.ms"
    sysctl(8) variable will change
    the amount of time between
    callout(9) ticks for
    simulated level interrupts.