Front panel switches - what did they do?

Noel Chiappa jnc at
Thu May 26 05:02:52 CDT 2016

    > From: Swift Griggs

    > I'm curious about all these older machines with front panel buttons and
    > switches. What all did they do? 

In addition to reading/writing memory locations, and basic machine control
(boot, start, stop, continue, single-step, etc), some machines had additional
functionality, but what it was (if any) varied widely from machine to machine.

E.g. the KA10, the first model of the PDP-10, had a front panel which also
allowed you to (among other things):

  - execute the contents of the data switches as an instruction
  - either stop the CPU, or execute an interrupt (switch selected),
	when the address in the address switches was used for (switch
	-- instruction fetch
	-- data fetch
	-- data write
  - repeat the previous key-press indefinitely (at a selectable speed)

The latter one could be used for all sorts of things. I once watched someone
halt the machine, put it in single-step mode, hit 'continue', and then
'repeat': by turning the 'repeat speed' knob up and down it was possible to
cause the CPU to run at varying speeds, down to 1 instruction/second! I
imagine that key could have also been used to clear memory by putting 0 in
the address and data switches, hitting 'deposit' and then 'deposit next', and
then 'repeat' (with the repetition rate turned to the max).

You'd have to read the processor manual for each machine to know exactly what
it could do from the front panel. E.g. some of the PDP-11's (/04, /34, /45 and
/70, IIRC) had a mode where you could single-step the microcode. I recall
using this on our /45 to debug it when the RETURN instruction broke... :-)


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