[rescue] Re: UNIX PC/3b1?

Dave McGuire rescue at sunhelp.org
Thu Jun 7 02:03:17 CDT 2001

On June 6, dave at cca.org wrote:
> A small PDP-11 with ethernet is easy.
> I don't understand the vast permutations of PDP-11 hardware options
> as it relates to the various versions of unix that were available
> for the 11, so I'm not sure what the minimal 11 that will run an
> acceptable unix is...

  There are two big issues...address bus width and split I/D space.
The earliest pdp11 processors have a 16-bit address bus with no memory
management whatsoever.  The next "generation" (address bus wise) is
18-bit addressing, for 256KB (actually 248KB...I/O page) of RAM with
memory management.  This was standard on soem systems, and an option
on others.  The next (and last) "generation" has 22-bit addressing
with reasonable memory management.  Some Unix implementations will run
with 22- or 18-bit addressing, but I don't think any of them will run
with 16.

  The other issue is split I/D space.  Some later Unix implementations
require it, while not all pdp11 processors have it.

  I can give you details of which ones do what if you're interested...

         -Dave McGuire

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