Probs w. M3119 CXY08 MUX on VAX VMS 7.3

Paul Koning paulkoning at
Tue Aug 18 14:55:23 CDT 2015

> On Aug 18, 2015, at 3:48 PM, Johnny Billquist <bqt at> wrote:
> On 2015-08-18 21:29, Paul Koning wrote:
>>> On Aug 18, 2015, at 2:00 PM, Johnny Billquist <bqt at Update.UU.SE> wrote:
>>> On 2015-08-18 19:05, Jon Elson wrote:
>>> ...
>>>> Most likely, some board was added or removed from the system before you
>>>> got it, and it caused the vector to now be wrong.
>>> The vector is usually not the first victim. The CSR address is, which cause all access to the controller to fail. But the vector often also move, causing the more obscure errors. However, most DEC OSes actually autodetected the vetor, and did not care about the actual floating assignment rules for the vectors.
>>> The thing is, all you need is to trigger an interrupt on the device, and then notice at what vector it came in, and then you go with that. This only fails when several devices happen to use the same vector.
>> Typically that would be detected as a configuration error — two devices whose autodetected vector matches.  One of the offending devices (the one seen later, presumably) would end up disabled.
> Ideally yes. However, at least RSX I think fails on that. As device vectors are detected. they are installed. So the vector detection code does not get called for the second device using the same vector, but instead you get a spurious interrupt in the autoconfiguration.

Ok.  RSTS does indeed check for duplicate vectors.  It also checks for devices interrupting at too high a priority.

It’s pretty neat code.  Back in 1977 or so when that came out, it may have been one of the first autoconfig systems, at least in DEC.  It could probe almost all devices supported by RSTS (and some not supported); the exceptions being card readers and the DT07 bus switch.  But it would do hairy things like the KMC-11 and DMC-11, for example.

Also, for disks it would discover all units and their types.


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