PDP-11 pages/segments/etc (Was: PDP-12 at the RICM)

Johnny Billquist bqt at update.uu.se
Tue Jul 14 17:38:24 CDT 2015

On 2015-07-14 19:52, Noel Chiappa wrote:
>      > On Jul 13, 2015, at 8:52 PM, Johnny Billquist <bqt at update.uu.se> wrote:
>      > ??? What segments??? The PDP-11 have a plain simple page table. No
>      > segments anywhere in sight. And each page is 8K.
> I know the processor handbook calls them 'pages', but I can't think of any
> other machine where pages are variable size. (I know of a couple which offer
> _two_ page sizes, but none that have a field per page which specifies the
> length of the page.)

While the pages are variable in length, each page starts at an 8K 
virtual address boundary. And each page has a page table entry.
Oh, and actually, the pages are not entirely variable in size. They can 
only go up to 8K, which is where the next page starts.

SO it's purely that the pages are not a fixed and inflexible as on some 
other machines, but each page is totally independent of the other pages, 
and each works the same way, and is strictly mapped from the virtual 

> They really are more like what most machines call 'segments'. I know Unix
> doesn't use them that way (because it has such a simple-minded memory model),
> but other systems do - e.g. MERT.

I couldn't disagree more. :-)
If you only had one page, and that page covered the full virtual address 
space, then I would agree that it was a segment model.


Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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