PDP-11 architecture Was: internet blocking problem ?

Johnny Billquist bqt at update.uu.se
Fri Sep 11 10:19:49 CDT 2015

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.


More information about the cctalk mailing list