PDP-11/23 Debugging

Noel Chiappa jnc at mercury.lcs.mit.edu
Mon Nov 17 09:35:17 CST 2014

    > From: Ben Sinclair

    > I wasn't clear though on the discussion about the line clock... If my
    > machine is having problems with the line clock, would I need to do
    > anything to the aux switch, other than leave it in a certain position?

Well, I wouldn't call it a 'problem'. The thing is that the 11/03 and 11/23
were, unlike all other PDP-11s, designed with a line clock which _the software
could not enable/disable_. [*] The only way to turn the LTC on/off on those
machines is with that front-panel switch. When running software which _does
not_ handle line clock interrupts, turning the LTC on will blow the software
away - it will get an un-handled interrupt. (As happened to you.)

So if you're running code which does not use/handle the LTC (like XXDP,
apparently), turn it off. If you're running something that does want/need it,
turn it on. (And some software may require that it be _off_ while booting, and
turned _on_ once the system has started - Unix V6 falls into this category.)

* Two caveats. First, the 11/23-PLUS _does_ have software control over the
LTC; there's an LTC register on that board; it's only the dual-height 11/23
which does not. Second, the BDV11 card has this really elegant kludge that
basically adds an LTC register to a 11/03 or an 11/23, so on an 11/03 or 11/23
with a BDV11, you can leave the LTC switch on all the time, and the software
can enable line clock interrupts if it wants them.


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