14 Jul
2015
14 Jul
'15
10:52 a.m.
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.)
They really are more like what most machines call 'segments'. I know Unix
doesn't use them that way (because it has such a simple-minded memory model),
but other systems do - e.g. MERT.
Noel
14 Jul
14 Jul
3:38 p.m.
On 2015-07-14 19:52, Noel Chiappa wrote:
While the pages are variable in length, each page starts at an 8K
virtual address boundary. And each page has a page table entry.
Oh, and actually, the pages are not entirely variable in size. They can
only go up to 8K, which is where the next page starts.
SO it's purely that the pages are not a 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.
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.) They really are more like what most machines call
'segments'. I know Unix
doesn't use them that way (because it has such a simple-minded memory model),
but other systems do - e.g. MERT.
I couldn't disagree more. :-)
If you only had one page, and that page covered the full virtual address
space, then I would agree that it was a segment model.
Johnny
--
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
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
3448
days inactive
3449
days old
2 comments
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