Weekly Classic Computer Trivia Question (20150112)

Johnny Billquist bqt at update.uu.se
Mon Jan 12 12:10:50 CST 2015

On 2015-01-12 17:10, Doug Ingraham wrote:
> This weeks question comes from Warren Stearns.  It is a little obscure and
> specific to the PDP-8 family of computers.
> What is the source of the greatest latency in the interrupt system on a
> PDP-8.
> I have three answers, two of the answers depends on the options fitted to
> an 8.

What do you mean by interrupt latency here?
The latency before an interrupt is actually propagated to the CPU? The 
latency before the interrupt is reacted upon by the CPU (assuming 
interrupt are ON I hope...). The latency internal to a controller before 
it even might raise the interrupt request?

> Why would this have been an important question?  Interrupt latency would be
> extremely important in the field of data collection which was one of the
> principal early uses of these machines.  My particular 8 was used for
> exactly this purpose in the Summer when it was hauled to a radar site and
> collected weather research data for the Institute of Atmospheric Sciences.
> I believe it was used for this from 1969 through 1972.  I have several
> hundred DECTapes with some of this data.  The surprising thing is we don't
> see any problems reading DECTapes that haven't been out of their box since
> the early 70's.

Why are you surprised? DECtapes have excellent reliablilty. You can 
punch holes in them and they will still work.


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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