PDP11 I/O page memory map

Charles Dickman chd at chdickman.com
Thu Mar 1 18:39:53 CST 2018

On Thu, Mar 1, 2018 at 9:04 AM, Paul Koning via cctalk
<cctalk at classiccmp.org> wrote:
> That's true when the MMU is disabled; if so it supplies 1 bits for the upper bits for page 7,
> and zeroes for the other pages.  But if the MMU is enabled, all mapping goes through its mapping
> registers, and page 7 is no longer special.  By software convention, kernel data page 7 is
> configured to point to the I/O page, but that isn't required.  If you wanted to be be perverse you
> could map the I/O page via page 6 and confuse a whole generation of programmers.

So if the I/O page is completely (all processor modes) unmapped is
there any way to recover besides a power cycle? Does the RESET
instruction disable the MMU?


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