[TUHS] PDP-11/70 SPL

Noel Chiappa jnc at mercury.lcs.mit.edu
Tue Mar 29 00:18:42 AEST 2016

    > 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.)

    > even the published Unibus spec was known to be wrong, in order to keep
    > 3rd-party kit out of it (it was something subtle to do with buss timing,
    > so sometimes the card worked, and sometimes it didn't, doing wonders for
    > your reputation).

I don't know about that, but we built two UNIBUS DMA networking devices,
relying on the UNIBUS description in the 1975 Peripherals Handbook, and they
both worked fine (one became a product for Proteon).

    > 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).

    > 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 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.)

    > Christ, but this is starting to sound like some religion or other.

I am only interested in correct data.


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