Front panel switches - what did they do?

Swift Griggs swiftgriggs at
Tue May 24 11:44:29 CDT 2016

On Tue, 24 May 2016, Charles Anthony wrote:
> "Older machines" covers a lot of ground.....

Sorry, I should have said "machines from the 50's - 70's which used 
buttons, toggles, rockers or other switches on the front panel"

> Typically, there was a set of data switches (0/1 toggles) that could be 
> set to an address or data value, and a set of command switches 
> (momentary contact) that copied the data switches to some data register 
> or memory.

Did some of the machines have blinkenlights to show you what you were 
doing so you could see the values you were inputting? Judging from how I 
play guitar, I'd probably miskey and have to start all over etc...

> For the earlier or cheaper systems, there was no 'bootstrap ROM'; so a 
> small program that was capable of reading the first record of a paper or 
> magnetic tape into memory and running it was needed.

Was that because of cost, no availability (ie.. not invented yet), or 
what? Why didn't they have a boot ROM, BIOS, etc.. ?

> This program was documented in ''machine language' -- a list of binary 
> values that needed to be placed in specific memory locations. 
> Hypothetical example (in octal):

Ah, thanks for the example. Between this and the PPT someone posted 
earlier, I think I "get it" now.

> It reads some number of characters from the paper tape into memory, and 
> then starts executing them. Those characters will be a more complex boot 
> strap loader that will read them rest of the tape into memory and run 
> it.

Well, paper tape sure beats having to flip all those switches, it sounds 

> The data switches would be examined by the operating system during boot 
> to enable debugging (pause at certain points during boot, eg).

I wish OS's still had something like this sometimes. Using a debugger over 
serial, there are times when I'd like to step through code or stop the 
whole kit-and-kaboodle. However, there are so many timers running in OS's 
these days a lot of the time that sort of thing causes major pain, 
especially with certain problematic drivers.

> The 709 had these massively over-engineered rocker switches, reminiscent 
> of circuit breakers, and a reset switch which activated a electric motor 
> in the console which physically set the switches back to 0.

Heh, that sounds cool. Could you hear the motor running after hitting the 
switch to activate it? 

> The PDP-15 had a 'CPU speed' knob. turning it would continuously vary the
> CPU clock from 1Hz to full speed.

Ohhh, neato. I wish that was more common, too. What fun! I need a machine 
with an analog dial for the clock speed and vue meters for RAM and CPU 
capacity :-)
> The blinking lights typically would have at least the instruction 
> counter and the accumulator. Other registers might be displayed, as well 
> as the instruction being executed, operand address and value, condition 
> code bits, IO activity, interrupt status, and much more.

That makes good sense, really. It doesn't seem so mysterious when you 
describe it, now.

> Watching the instruction counter could reveal the CPU to stuck in a 
> short loop; or, if halted, what instruction it was at when it halted.

Hmm, again, that wouldn't be so bad to have even today.

> Some of the mainframes had hundreds and hundreds of lights, detailing 
> the internal state of the machine; mostly of interest to field 
> engineers.

It probably still impressed the suits when they walked the data center. 
I've done data center tours with row after row of HP or Dell x86 servers 
and it's not much to look at. 


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