Qbus split I&D?

Paul Koning paulkoning at comcast.net
Tue Mar 17 14:51:10 CDT 2015

> On Mar 17, 2015, at 3:46 PM, Brent Hilpert <hilpert at cs.ubc.ca> wrote:
> On 2015-Mar-17, at 12:27 PM, Paul Koning wrote:
>> ...
>> MRAM is non-volatile, sure.  I’m not sure its write limit is high enough to be used as a substitute for main memory.  In any case, what PDP-11 operating systems use the non-volatility of memory?  I know of one: RSTS-11.  But RSTS/E dropped that (it reboots on powerup instead).  That makes sense, given that semiconductor memory appeared fairly early in the PDP-11 product life, and none of that came with battery backup.  In other words, only some of the models in some configurations offered non-volatile memory, which made it fairly uninteresting for operating systems to support.
> Well, even if the OSs didn't support restart, non-volatility of core was useful for retention of the bootstrap program on the front-panel/pre-boot-ROM machines, just so you didn't have to re-toggle the bootstrap at every power-up.

True, though I think in practice PDP-11s came with boot ROMs.  Even fairly early 11/20s did — the M792 discrete diode array, programmed with diagonal cutters :-).


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