out-of-mainstream minis

Johnny Billquist bqt at update.uu.se
Sun Jul 5 05:46:43 CDT 2015

On 2015-07-05 03:09, Brian L. Stuart wrote:
>> Ok. I didn't know it was some version of 7th ed. I didn't even know that
>> managed to get much beyond the PDP-11.
> At least that's the way I read the original message.  And there were
> a number of re-implementations/clones of 7th ed.

Fair enough. And yeah. if you're not doing demand paging, a 68000 should 
be enough.

>> Anyway, it's sad, because the PDP-11 hardware can easily
>> handle demand paged memory. It's just that no one really
>> found it worth the effort to  implement it when you only have
>> 8 pages, and have way more physical memory than the
>> virtual memory space can address.
> It turns out that at one time, there was an experimental version
> of UNIX at the labs that did do demand paging on an 11.  They
> didn't continue that line of development because it didn't end
> up giving any significant benefits but ran slower than a swapping
> based approach.

Not surprised.
It is a result of the same thing. Demand paging always has a cost. 
Already having all the pages in memory all the time should (obviously) 
be better, and with a PDP-11 that is easily accomplished just because 
there are only 8 pages, and you have plenty of physical memory compared 
to virtual anyway.

Funnily enough, we are almost moving back to a similar situation today, 
where people are starting to run systems without any swap space, to 
reduce paging. Paging is still there for unmodified pages, though. So 
not totally the same thing. But the reason it has become fashionable is 
partly because you in many cases now have systems with so much physical 
memory that you can do it.


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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