Front panel switches - what did they do?

Brent Hilpert hilpert at
Tue May 24 11:25:41 CDT 2016

On 2016-May-24, at 8:38 AM, Swift Griggs wrote:
> Since I'm an igmo about most machines before the mid-eighties (and still 
> fuzzy even on most of those), I'm curious about all these older machines 
> with front panel buttons and switches. What all did they do?

Primary front-panel operations were:
	1 - examine and load memory locations
	2 - examine and load registers, including the instruction counter
	3 - start and stop processor execution (run state)
		Thus with (2) you can start execution where-ever you want in memory

Also, depending on the machine:
	4 - single-step the processor, sometimes per instruction, sometimes per clock cycle or instruction phase
	5 - twiddle I/O devices

> You could 
> actually program them using the front panel right?


> Some of them 
> bootstrapped this way, too?

Yes. In the core days, the common thing to do would be to use the front panel to 'toggle in' the first bootstrap loader of some few dozen instructions/words.
Core being non-volatile, that bootstrap program would continue to reside there across power-down/up so you didn't have to re-toggle at every power-up.
Just set the execution-start address with the toggles, and run.

> What kind of "language" was used for that 
> (ie.. what were the basic mechanics)?

Raw machine code. Typically expressed in octal or hex depending on the machine instruction layout or policy, and translated to binary by the operator as you toggle in.

> Did the buttons ever change color?

> Were you considered a badass if you had switch flipping all memorized down 
> to an art?

> Were they mainly multi-position toggle switches or on/off 
> buttons?

Most common was two-position (up-down) toggles.
Some machines had rotary switches for such as register-display selection
Some used momentary-push-buttons for 1-0 bit cycling.

> They just seem to be a lot more important on older mainframes and minis. 
> Also, what was the main reason for the blinkenlights? Was it to show 
> system load or specific system states?

Primary intentions were:
	1 - initial bootstrap as mentioned above

	2 - hardware diagnosis and servicing
		Earlier and larger machines might have the state of every or near-every flip-flop in the machine brought out to an indicator,
		so you could see the entire processor state on the front panel. You could then use that to trace logic faults in the machine.
		Simple example: if the carry flip-flop lamp isn't turning on after single-stepping through an ADD instruction with registers
		loaded with operands which should result in carry . . .

	3 - (less common) program diagnosis , i.e. used as an instruction-level software debugger, if one had monopoly machine access.

The improved reliablity of LSI logic over discrete and SSI, and the creation of ROM chips of reasonable capacity (to hold the bootstrap or a monitor), would bring about the demise of the blinkenlight  front panel.
Note that only a couple of the first microcomputers had blinkenlight front panels, and they were pretty much gone from minis and mainframes by the late-70s.

> Just curious. I'm learning a ton from reading these threads on older 
> machines, but there is so much I don't know. 

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