[TUHS] porting to different systems, Bootstrapping UNIX - how was it done

[Paul Winalski via TUHS]

On Mon, Mar 23, 2026 at 9:15 PM George Michaelson via TUHS <tuhs at tuhs.org>
wrote:

> In like sense TOPS-10 was ported from the 10 to the 20 but that was
> at worst a marginal conversion. Obviously the entire suite of DEC RSTS type
> operating systems were cross ported to the variants of the PDP11 on an
> as-emerged basis, but I suspect like tops10/20 that was hardly a "port" in
> any real sense
>
> The original PDP-10 did not have virtual memory capability.  BBN designed
their own paging hardware and wrote their own operating system called
TENEX.  DEC did implement paging hardware for the KL10 processor, which was
used in the DECSYSTEM-20.  But the paging system differed from the BBN
design and TENEX would not run on the DECSYSTEM-20.  By then TENEX was
extremely popular in DEC's PDP-10 customer base.  DEC bought the rights to
TENEX and ported it to the DECSYSTEM-20, the main changes being to cope
with the different paging system.  This port was TOPS-20.

TOPS-10 would run on some DECSYSTEM-20 models, in particular those using
the KS10 CPU.

-Paul W.