Front panel switches - what did they do?

Jon Elson elson at
Tue May 24 11:09:35 CDT 2016

On 05/24/2016 10:38 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? You could
> actually program them using the front panel right? Some of them
> bootstrapped this way, too? What kind of "language" was used for that
> (ie.. what were the basic mechanics)? 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?
The PDP-5 I did a fair bit of work on needed a bootstrap 
program loaded in from switches, it had no internal ROM for 
that.  And, whenever a program crashed, it generally wiped 
the entire contents of memory, so the boot had to be 
reloaded by hand.  We actually wore out the switches.  It 
was about 15-20 12-bit words that needed to be entered.  We 
had DECtape on that machine, so we generally entered to boot 
loader for that.

The LINC was also a 12-bit computer, but it had built-in 
boot hardware.  it was not a boot loader program in a ROM, 
but when you pressed the load button, it would execute an 
I/O command from the left switches, and the right switches 
told it where to put it in memory.  So, that was a big 
advance, a one-button boot.

You could use the switches to patch a program you were 
debugging, look at memory locations to examine temporary 
data values, etc.

A few machines had lighted switches.  These would generally 
be white buttons with lamps behind them.
The only one I know of that changed color was the power 
button ("key" in IBM-speak) on IBM 360's.  It lit red while 
the power-up sequence was in progress, then turned white 
when all power supplies were up.  IBM tape drives and disk 
drives had lighted buttons to show status, different color 
buttons and indicators gave them different colors, but they 
were generally just lit and unlit, but not multi-color.

DEC PDP machines generally had a few switches that were 
multi-position,  Such as stop/single-step and load 
address/examine, otherwise they were all on-off.

IBM 360's had a row of switches that were multi-position, 
mostly for FE diagnostic purposes.  The data and address 
switches were all on/off.


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