Memory and I/O protection on the 940

Robert Ollerton rollerton at
Fri May 6 12:48:29 CDT 2016

I've always wanted to know more about the SDS 940 project since its a
relative to my SDS Sigma 9.   If you know of any of the papers on the
hardware, and OS they created I'd appreciate some pointers to where I could
read them.  Thanks!

On Wed, May 4, 2016 at 2:00 PM, Mark Kahrs <mark.kahrs at> wrote:

> Al mentioned the 940 so I thought I would fill in the details:
> Lichtenberger and Pirtle extended the hardware to include a page map: 14
> bits of VA was divided into 3 bits of page # and 11 bits of offset.  The 8
> pages were held in 2 x 24 bit words divided into a 8 x 6 bit page numbers.
> The high order bit of the mapped page number served as the Read Only bit.
> This meant that "subsystems" (a.k.a. applications) could be shared between
> users.
> Additionally, the instruction set was protected against users with specific
> prohibition against using the I/O instructions.
> This was described in FJCC 1965 with modifications done in 1964.

More information about the cctech mailing list