Interrupts are serviced on the Amiga through the use of interrupt handlers
and servers.  an interrupt handler is a system routine that exclusively
handles all processing related to a particular 4703 interrupt.  An
interrupt server is one of possibly many system routines that are invoked
as the result of a single 4703 interrupt.  Interrupt servers provide a
means of interrupt sharing. This concept is useful for general-purpose
interrupts such as vertical blanking.

At system start, Exec designates certain interrupts as handlers and others
as server chains.  The ports, coper, vertb, exter, and nmi interrupts are
initialized as server chains.  Therefore, each of these may execute
multiple interrupt routines per each interrupt.  All other interrupts are
designated as handlers and are always used exclusively.

 interrupt data structure    interrupt handlers 
 environment                 interrupt servers