Memory and I/O protection on the 940

Mark Kahrs mark.kahrs at
Wed May 4 16:00:44 CDT 2016

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

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.

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