26 Apr
2018
26 Apr
'18
5:08 a.m.
On 2018-04-26 00:55, jnc(a)mercury.lcs.mit.edu (Noel Chiappa) wrote:
:-)
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.
Oh yes. You definitely could do demand paging based on the hardware
capabilities.
Yeah, it would actually probably have been more useful on an 11/45.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt(a)softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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...:-) 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". 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).
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Johnny Billquist