12 Jan
2015
12 Jan
'15
12:38 p.m.
On 2015-01-12 20:43, Vincent Slyngstad wrote:
From: Johnny Billquist: Monday, January 12, 2015 10:55
AM
Of course I've done it. I suspect there is not much I have not done on a
PDP-8. I've written programs on PDP-8 systems for over 30 years by now...
No, you do not need a push/pop to have interrupts enabled while in an
interrupt handler on a PDP-8. As long as you know the same interrupt
will not trigger, all you need to do is store the return address from 0
somewhere else before ION, and then write it back to 0 after IOFF.
Like I said, with the provisions that you do not reuse the same
functions from different interrupt handlers, and you know that the same
device don't interrupt again.
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).
Yes. And some of the same issues are true no matter what architecture.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt at softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol