The system software has been designed to correctly handle all system
hardware interrupts at levels 1 through 6. A separate set of input
lines, designated INT2* and INT6* have been routed to the expansion
connector for use by external hardware for interrupts. These are known as
the external low- and external high-level interrupts.

These interrupt lines are connected to the peripherals chip and create
 interrupt levels 2 and 6 , respectively. it is recommended that you take
advantage of the interrupt handlers built into the operating system by
using these external interrupt lines rather than generating interrupts
directly on the processor interrupt lines.