PDP-11 architecture Was: internet blocking problem ?

Johnny Billquist bqt at update.uu.se
Fri Sep 11 10:41:06 CDT 2015

On 2015-09-11 17:33, Warner Losh wrote:
> On Fri, Sep 11, 2015 at 9:19 AM, Johnny Billquist <bqt at update.uu.se> wrote:
>> On 2015-09-11 16:49, Warner Losh wrote:
>>> On Fri, Sep 11, 2015 at 8:39 AM, Johnny Billquist <bqt at update.uu.se>
>>> wrote:
>>> On 2015-09-11 16:36, Noel Chiappa wrote:
>>>>        > From: Jon Elson elson
>>>>>        > I actually LIKED the PDP-11 architecture quite a LOT, but the
>>>>> limited
>>>>>        > memory was a big killer.
>>>>> The good thing about the PDP-11 was the 16-bit word size. (It resulted
>>>>> in
>>>>> what's probably the most elegant architecture, in bang/buck terms, of
>>>>> all
>>>>> time.) The bad thing about the PDP-11 was the 16-bit word size. (For the
>>>>> reason you point out.)
>>>> WHile I agree that the PDP-11 is a wonderful architecture, it really is a
>>>> few bits short of perfect, both for addressing, and for opcode
>>>> allocation.
>>>> The is obvious when you look at the EIS and FPP extensions, which could
>>>> not retain the general instruction layout format because of a lack of
>>>> bits.
>>> I loved the PDP-11 architecture, until I wanted to run programs on it that
>>> relied on the overlay manager and the overlays got to be 8 or 9 deep. Then
>>> it was... painful.
>> Uh? What do that have to do with the PDP-11 architecture? Overlays are a
>> thing specific to the operating system and linker, and looks and works
>> differently in different OSes on the PDP-11, if they have them at all.
>> Mind you, even having said that, overlays are just a userland
>> implementation of demand paging. Conceptually they are dead easy.
>> Now, the overlay description language, as well as the capabilities in RSX,
>> can make people seasick. But once you've worked with them for a while, you
>> realized that most of the time it is not that tricky.
> Well, the 16-bit address space forced a maximum limit on the text size of
> the program that could be in memory at any time. This made the overlay
> stuff trickier, since you had to use some kind of overlays (either managed
> by the linker or the OS or some experimental home-grown stuff we tried).
> Sure, the details were OS specific, but the tight memory of the
> architecture forced some kind of mechanism. If your text space was <64k,
> then OS demand paging was good. But if you needed more, there were no real
> good choices.

Ok. Well, yes, I believe we all agree that a larger address space would 
have been nice. Even DEC noted that one pretty soon.
Just like Jon Elson said initially. :-)

Overlays were a way to try getting around the address space limitations. 
I still would not consider overlays as any part of the PDP-11 
architecture. But maybe that is just me.


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