PDP-12 at the RICM

Paul Koning paulkoning at comcast.net
Tue Jul 14 09:09:24 CDT 2015

> On Jul 13, 2015, at 8:52 PM, Johnny Billquist <bqt at update.uu.se> wrote:
> On 2015-07-13 21:16, Rich Alderson wrote:
>> ...
>> [2] With memory management, 18 or 22, in 16-bit segments.  Late models could
>>     use separate instruction and data segments, for a total of 128KB in use at
>>     one time.
> ??? What segments??? The PDP-11 have a plain simple page table. No segments anywhere in sight. And each page is 8K.

To me, “segment” means a chunk of memory.  It doesn’t refer to separate page tables, or page tables broken into pieces.  And as I recall, the terms “instruction and data segments” were in common use to describe the feature.

The PDP-11 is a bit unusual in that each page is *up to* 8k, the actual size (and whether it’s the lower or the upper part that is accessible) is specified by the page description register for that page.  Also in that the physical address of the page isn’t constrained to a multiple of the (max) page size.


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