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.
Jon
