[TUHS] /dev/drum

Johnny Billquist bqt at update.uu.se
Thu Apr 26 07:43:42 AEST 2018

On 2018-04-25 16:39, Ronald Natalie<ron at ronnatalie.com> wrote:
>> On Apr 24, 2018, at 9:27 PM, Dan Stromberg<drsalists at gmail.com>  wrote:
>> On Sun, Apr 22, 2018 at 2:51 PM, Dave Horsfall<dave at horsfall.org>  wrote:
>>> Now, how many youngsters know the difference between paging and swapping?
>> I'm a mere 52, but I believe paging is preferred over swapping.
>> Swapping is an entire process at a time.
>> Paging is just a page of memory at a time - like 4K or something thereabout.
> Early pages were 1K.

What machines are we talking about then?
PDP-11 have 8K pages. VAX have 512 byte pages, if we talk about hardware.
(And yes, I know pages on PDP-11s are not fixed in size, but if you want 
the page to go right up to the next page, it's 8K.)

> The fun argument is what is Virtual Memory.    Typically, people align that with paging but you can stretch the definition to cover paging.
> This was a point of contention in the early VAX Unix days as the ATT (System III, even V?) didn’t support paging on the VAX where as BSD did.
> Our comment was that “It ain’t VIRTUAL memory if it isn’t all there” as opposed to virtual addressing.

Weird comment. What does that mean? On a PDP-11, all your virtual memory 
was always there when the process was on the CPU, but it might not be 
there at other times. Just as not all processes memory would be in 
physical memory all the time, since that often would require more 
physical memory than you had.
But you normally did not have demand paging, since that was not really 
gaining you much on a PDP-11. On the other hand, overlays do the same 
thing for you, but in userspace.

So you would claim that ATT Unix did not have virtual memory because it 
didn't do demand paging?


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

More information about the TUHS mailing list