Weekly Classic Computer Trivia Question (20150112)
vrs at msn.com
Mon Jan 12 12:43:29 CST 2015
From: Johnny Billquist: Monday, January 12, 2015 10:21 AM
> On 2015-01-12 19:17, Johnny Billquist wrote:
>> On 2015-01-12 18:19, Vincent Slyngstad wrote:
>>> One answer might be "the skip chain". Since interrupts weren't
>>> vectored, it was necessary to query each device in priority order to see
>>> if it was currently requesting interrupt service.
>> Ah. Good point. I forget to include this in the possible interpretations
>> of interrupt latency.
> Actually. This needs some correction. The interrupt skip chain itself is
> not the latency. The skip chain is just some extra time required to
> actually jump to the correct interrupt handler. The interrupts are
> (normally) off until the interrupt handler finishes.
They'd better be off until the last device is in service, or you'll get
re-entry problems in the interrupt service routine. In practice, I've
never seen interrupts on during a PDP-8 service routine.
> The skip chain is just a smaller part of that whole thing.
OK, but it is a significant overhead compared to later (vectored)
interrupt system designs, and means it takes longer to get to
where you're going.
The un-vectored design also means more problems with
re-entry, since all the interrupts call the same routine. And
the JMS instruction design makes it a bear to write re-entrant
code on a PDP-8.
I think it comes back to the "what do we mean by latency"
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