Front panel switches - what did they do?

Jon Elson elson at
Tue May 24 20:49:06 CDT 2016

On 05/24/2016 12:17 PM, Paul Koning wrote:
> A couple of observations.
> Taking the PDP-11 as a fairly typical example, the switches are "data" and "address".  While running, the data switches were visible to the software, and could do something if you wanted to (typically this wasn't done).
Actually, all PDP-11s I used had one set of switches, that 
could be used for address or data, as appropriate to the 
operation.  So, the load address switch would load the 
address with 18, 22 or whatever address width that model 
had.  Otherwise, all operations only used the lower 16 
switches.  The data switches were readable as one I/O 
register.  When you read it, it reported the switches.  When 
you wrote to it, some models could display it on the panel.  
These were used in some diagnostic programs.


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