14 Jul
2015
14 Jul
'15
8:55 p.m.
Johnny Billquist wrote:
On 2015-07-14
19:52, Noel Chiappa wrote:
On Jul
13, 2015, at 8:52 PM, Johnny Billquist <bqt at
update.uu.se> wrote:
??? What segments??? The PDP-11 have a plain
simple page
table. No
segments anywhere in sight. And each page is 8K.
I know the processor handbook calls them 'pages', but I can't think
of any
other machine where pages are variable size. (I know of a couple
which offer
_two_ page sizes, but none that have a field per page which specifies
the
length of the page.) SO it's purely that the pages are not as fixed and
inflexible as on
some other machines, but each page is totally independent of the other
pages, and each works the same way, and is strictly mapped from the
virtual address.
It is probably possible for the hardware to support demand paging,
but I don't know of any DEC operating system for the PDP-11
which does. The usual situation is for the job to request the memory
which it requires, then ask to operating system to MAP the physical
memory to the correct addresses as the memory is needed. With
4 MB of physical memory in total and often only 64 KB in use at
any one time, there is usually sufficient memory to work with.
Jerome Fine