Weekly Classic Computer Trivia Question (20150112)

Johnny Billquist bqt at update.uu.se
Mon Jan 12 12:21:55 CST 2015

On 2015-01-12 19:17, Johnny Billquist wrote:
> On 2015-01-12 18:19, Vincent Slyngstad wrote:
>> From: Doug Ingraham: Monday, January 12, 2015 8:10 AM
>>> What is the source of the greatest latency in the interrupt system on a
>>> PDP-8.
>> 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. The skip chain is 
just a smaller part of that whole thing.

And I assumed that the question was about when interrupts were enabled, 
which they are not at the point when the skip chain executes.

Consider the skip chain just a part of the interrupt handler. It is no 
different from an external point of view.


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

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