PDP-12 at the RICM

Johnny Billquist bqt at update.uu.se
Tue Jul 14 17:29:58 CDT 2015

On 2015-07-14 16:09, Paul Koning wrote:
>> 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.

Yeah. Segment is something I usually associate with the solution done in 
the 8086 family, where you essentially have a segment register which 
gives the base, and then you work from there. Essentially all memory is 
one chunk.

The 8086 have both an instruction and a data segment, which just means 
that some instructions refer to data, and that uses the data segment 
register, while instructions (obviously) are addressed through the 
instruction segment register. I never worked with the 8086, but don't it 
actually also have a third segment register? Not that I can remember 
what it was used for... Oh, and all references are relative to the 
segment register (should be obvious, but I figured I should point it out.)

> 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.

Yes, the fact that pages on a PDP-11 do not have to be the full 8K is a 
bit special. Or, was. Machines nowadays are actually more reminding me 
of the PDP-11 than other contemporary systems. Today we are once more at 
really large page sizes, and actually variable length pages. I don't 
remember if page physical address needs to be on a multiple of the page 
size on todays machines, but since page sizes can vary I don't think so. 
So very much like the PDP-11...


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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