[TUHS] /dev/drum

Johnny Billquist bqt at update.uu.se
Thu Apr 26 09:08:14 AEST 2018

On 2018-04-26 00:55, jnc at mercury.lcs.mit.edu  (Noel Chiappa) wrote:
>      > From: Johnny Billquist
>      > PDP-11 have 8K pages.
> Segments.:-)  (This is an old argument between Johnny and me, I'm not trying
> to re-open it, just yanking his chain...:-)

And if you hadn't had the ability for them to be less than 8K, you 
wouldn't even try that argument. But just because the hardware gives you 
some extra capabilities, you suddenly want to associate them with a 
technology that really gives you much less capabilities.

Either way, the next page always start at the next 8K boundary.

>      > On a PDP-11, all your virtual memory was always there when the process
>      > was on the CPU
> In theory, at least (I don't know of an OS that made use of this), didn't the
> memory management hardware allow the possibility to do demand-paging? I note
> that Access Control Field value 0 is "non-resident".

Oh yes. You definitely could do demand paging based on the hardware 

> Unix kinda-sorta used this stuff, to automatically extend the stack when the
> user ran off the end of it (causing a trap).

Ah. Good point. The same is also true for brk, even though that is an 
explicit request to grow your memory space at the other side.

DEC OSes had the brk part as well, but stack was not automatically 
extended if needed. DEC liked to have the stack at the low end of 
address space, and have hardware that trapped if the stack grew below 
400 (octal).

>      > you normally did not have demand paging, since that was not really
>      > gaining you much on a PDP-11
> Especially on the later machines, with more than 256KB of hardware main
> memory. Maybe it might have been useful on the earlier ones (e.g. the -11/45).

Yeah, it would actually probably have been more useful on an 11/45.


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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