davet@oakhill.UUCP (Dave Trissel) (01/27/85)
This answers a question Jeff Hardy at sdcsvax asked: Why does the MC68020
when switching from the supervisor stack to the interrupt stack place
context information on the supervisor as well as the interrupt stack?
This allows an interrupt routine to do a task switch without messing around
with the supervisor stack. Remember that the context data saved pertains to
the supervisor routine which was interrupted. A dispatcher changes tasks
by storing the supervisor stack pointer (with other context data) into a
Task Control Block (or equivalent.) Notice that this works regardless of
whether the dispatcher is called from a standard supervisor routine running
on the supervisor stack or from an interrupt routine running on the interrupt
stack. The dispatcher replaces the Supervisor stack pointer with a new
task's stack pointer as part of the changeover to the new task.
Notice that when an RTE is done to the end (last of) the interrupt stack
that only the generic vector word is used. The context (PC and SR)
are always obtained from the Supervisor stack. Thus whatever task
was (is now) in control is properly continued since its context was at the
top of the supervisor stack.
I hope this has been clear. We will be adding information like this to the
back of an updated MC68020 User's Guide.
Motorola Semiconductor Inc. Dave Trissel
Austin, Texas {ihnp4,seismo,ctvax,gatech}!ut-sally!oakhill!davet