[TUHS] Bootstrapping UNIX - how was it done

[Ron Natalie via TUHS]

Nothing magic.   Leave the halt key down and press continue (not START). 
   That single steps most othe PDP-11’s with switches.   The 11/70 even 
allowed you to choose from single step or single bus cycle.
Pressing start does a bus reset which you probably don’t want.



------ Original Message ------
>From "Noel Chiappa via TUHS" <tuhs at tuhs.org>
To tuhs at tuhs.org
Cc jnc at mercury.lcs.mit.edu
Date 3/23/2026 5:46:18 PM
Subject [TUHS] Re: Bootstrapping UNIX - how was it done

>     > From: Ron Minnich
>
>     > toggle in an address that was owned by the diode rom, 177400, whatever
>
>The 'magic number', 773030, which had to be left in the front console
>switches for the system to come up single-user (in early UNIX versions), was
>in fact an address in the BM792 ROM - clearly the address used to start the
>'start from powered off' bootstrap on their machine!
>
>     > I believe if you left HALT set, you could step it through the diode
>     > romK
>
>That was the incantation to give to the front console of PDP-11's that had
>one to cause it to single step.
>
>	Noel