PDP-11/23 Debugging

Ben Sinclair bensinc at gmail.com
Mon Nov 17 10:14:40 CST 2014

Since I do have a BDV11, am I correct in that I should just leave the aux
switch alone and continue debugging elsewhere?


On Mon, Nov 17, 2014 at 9:35 AM, Noel Chiappa <jnc at mercury.lcs.mit.edu>

>     > 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.
>         Noel

Ben Sinclair
ben at bensinclair.com

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