[TUHS] PDP-11/70 SPL

Johnny Billquist bqt at update.uu.se
Tue Mar 29 00:44:09 AEST 2016

On 2016-03-28 16:18, Noel Chiappa wrote:
>      > From: Dave Horsfall <dave at horsfall.org>
>      > SPL 7 was only used by the clock interrupt
> Err, according to the 1975 Peripherals Handbook, both are BR6. (Sorry, only
> interested in accuracy.)

I don't think anything actually used SPL 7, as far as I can remember.

>      > Slightly longer?  I think it was Lions himself who used to teach us that
>      > a lost interrupt is nasty :-(
> The interrupt isn't lost, it's just that the OS does a WAIT when it should
> perhaps return and start up some user process - but that resumption of doing
> user computations is delayed by at most 1 clock tick (some other device may
> interrupt during the WAIT, before the clock does).

Right. A lost interrupt is a potential disaster and is never acceptable.
What we're talking about here is what happens after the interrupt.

>      > Anyone here remember overlapped seeks on the RK-11 failing under Unix
> I'd be interested in the details of this. The V6 RK driver didn't use them,
> but the RK11-D does claim to support them (having spent a modest amount of
> time looking at the drawings), so I'd very much like to know what the bug was.

I think I at least played around some with this on PDP-8 systems, and 
seem to remember it working right there. But I have not done so on PDP-11s.

>      > I know that Kevin Dawson (I think) tried it on my /40 as well
> The 11/40 does not have the SPL instruction; see the '75-'76 PDP-11 Processor
> Handbook, pg. 4-5. (Again, sorry, just want to be accurate.)

This is also a pretty important point. But one which also begs the 
question how the splxxx() functions in Unix worked back then. Or did 
Unix not use this pattern and these functions back when the 11/40 was 

>      > Christ, but this is starting to sound like some religion or other.
> I am only interested in correct data.

My interest here is also very much on facts and technical points. I do 
not want this to become some religious argument.


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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