text: +-----------+   NMI
  |           |
  |           |<----------+
  |           |
  |           |           +------------+
  |           |           |            |   IRQ0
  |           |           |            |<------------+ device0
  |    CPU    |           |            |   IRQ1
  |           |   INTR    |     PIC    |<------------+ device1
  |           |<----------+            |   IRQN
  |           |           |            |<------------+ deviceN
  |           |           |            |
  +-----------+           +------------+