12 Jan
2015
12 Jan
'15
7:43 p.m.
From: Johnny Billquist: Monday, January 12, 2015 10:55 AM
A more correct statement would be that if you enable
interrupts in your
interrupt handler you must first save all the context of the current
interrupt, so you can restore it before returning from the current
interrupt.
It's not really that hard. Essentially you must store the contents of
address 0, and probably also make sure you do not get a second interrupt
from the same device, since I doubt you want to write a reentrant
interrupt handler. (But that is normally the case anyway, since most
devices need some kind of operation to enable a new interrupt to be
generated.)
You've never tried to do it on a PDP-8, I expect. Since there's no stack,
and static storage won't be reentrant, it's a nightmare. At a minimum,
you'd need a routine to emulate "push"/"pop" and "call"
type operations,
and you'll have to remember to have interrupts off when you call them,
unless you want to use MQ to store the return address or something.
(Even then the stack routines will need to disable interrupts while they
do their thing).
Another way to view it is that interrupts effectively introduce a kind
of multi-threading, with the attendant complexity. That's really
incompatible with an "all static storage" machine like the PDP-8,
as everyone's always stomping your variables (even temporaries
and return addresses).
Vince