On 08/18/2015 10:48 AM, Holm Tiffe wrote:
Understand my quandary: I have changed not a single bit on
the hardware while cleaning and repairing it and besides
that DHU11-DHV11 Switch the board was in the published
factory setting!
DEC went to this floating address/vector scheme back in the
PDP-11 days, when they started building large systems like
the 11/70 and using PDP-11's as the console for DECsystem
20's. They'd put a huge number of I/O boards in such
systems. A choice of 3 pre-assigned CSR addresses and
interrupt vectors just would not cut it in such systems.
They had a utility you could run (at least on VAX) where you
input the number of instances of each device, and it would
print out a table of all the CSR and vectors to set them to.
So, the situation is that changing the number of one kind of
board could alter the addresses of many OTHER boards!
Most likely, some board was added or removed from the system
before you got it, and it caused the vector to now be wrong.
In some cases, you had to force a device to be at a
non-standard address, possibly because a 3rd party device
could not be configured at the address the DEC enumeration
scheme wanted to put it at. This was pretty easy to do in
later VMS systems.
Unfortunately, this type of misconfiguration is fairly hard
to detect with software. Later devices (MSCP) had an
autoconfiguration scheme where the OS would assign the CSR
and vector at boot time.
Jon
