28 Oct
2010
28 Oct
'10
2:46 p.m.
On 2010-10-28 23:02, Brent Hilpert <hilpert at cs.ubc.ca> wrote:
On 2010 Oct 28, at 10:51 AM, Chuck Guzis wrote:
I think that would be a very dubious, and hard to prove claim. :-)
> On 28 Oct 2010 at 13:20, William Donzelli
wrote:
> >> I was initially thinking about just the hardware architecture when the
> >> machines were being designed, not the software aspect, nor what the
> >> machines did (or influenced) after they were released. The
> >> aforementioned memory mapped I/O, for example.
>
> Not to be too much of a wet blanket, but how many of those DEC-unique
> innovations (even if you manage to assert that they originated with
> DEC) persist in today's hardware? Do modern PCs use memory-mapped
> I/O? The 68K, but for some Freescale relics, is history.
>
> Major innovations, such as virtual memory and orthogonal instruction
> sets and hardware-implemented stacks preceded the PDP-11.
How about the
notion that the PDP-11 was where several prior but
then-topical innovations coalesced into one machine/architecture? The VAX was riding on the coattails of the market
success of the -11
and provided existing PDP-11 installations with increasing demands with
a way forward, and it grew from there. That's a bit of a tautology, but
it is to say it did exactly what it was designed and marketed for.
Yes. The VAX did not really innovate anything. It was more of a rather
successful merge of many things, and following in the footsteps on the
PDP-11, and building further on that foundation.
On the topic of memory, I would agree with Johnny
about "virtual
addresses", but differ on "virtual memory": virtual memory to me has
always meant demand-paging where the RAM address-space seen by the user
can be larger than the physical RAM, distinct from the simple mapping
of addresses, but the use of the term is a matter of definition.
Thank you.
But what, pray tell, would you say a virtual address pointed to, then?
Magic smoke? :-)
Why are people so hung up on physical memory size vs. virtual memory
size when they need to define what virtual memory is?
I just don't get it. :-)
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
29 Oct
29 Oct
12:26 a.m.
On 2010 Oct 28, at 2:46 PM, Johnny Billquist wrote:
Physical memory via the MMU; this is not a contradiction.
On 2010-10-28 23:02, Brent Hilpert <hilpert at
cs.ubc.ca> wrote:
It may be more work than proving an individual claim of innovation, but
it is still a plausible notion to research historically.
How about the notion that the PDP-11 was where
several prior but
then-topical innovations coalesced into one machine/architecture?
I think that would be a very dubious, and hard to prove claim. :-) > ...
On the topic
of memory, I would agree with Johnny about "virtual
addresses", but differ on "virtual memory": virtual memory to me has
always meant demand-paging where the RAM address-space seen by the
user
can be larger than the physical RAM, distinct from the simple mapping
of addresses, but the use of the term is a matter of definition.
Thank you.
But what, pray tell, would you say a virtual address pointed to, then?
Magic smoke? :-) Why are people so hung up on physical memory size vs.
virtual memory
size when they need to define what virtual memory is?
I just don't get it. :-)
Because there is a fundamental difference between having your valid
address space limited to available physical RAM, even if you can map a
"virtual" address to different places in that RAM, and having a
transparently addressable (large) address space unlimited by a lesser
quantity of physical RAM. In the latter it *appears* (hence virtual)
that you have more RAM than you actually do. This has always (IME) been
the common meaning of "virtual memory".
-
I'm too many years away from programming a PDP-11, but while it may as
you suggest have been possible to do demand-paging in principle, how
complex might it have been in practice? Trapping an address fault might
be easy enough, but what about instruction restart/re-execution while
accounting for all possible side effects that may or may not have
occurred between the start of the instruction and the address fault.
IIRC, the 68000 series ran into this problem when people started trying
to do (demand paging) VM with it. When an address fault occurred, not
all of the instruction state was saved and restarting the instruction
became a problem. It was fixed in the next version but I forget where
in the series (68010->20, 20->30, etc).
10:04 a.m.
From: Johnny Billquist
Sent: Thursday, October 28, 2010 2:46 PM
Why are people so hung up on physical memory size vs.
virtual memory
size when they need to define what virtual memory is?
I just don't get it. :-)
Perhaps because that's the way the term of art "virtual memory" (whether
as segments or as pages) has been defined since it was first conceived?
Virtual addressing is necessary for virtual memory. The converse is not
true.
Rich Alderson
Vintage Computing Sr. Server Engineer
Vulcan, Inc.
505 5th Avenue S, Suite 900
Seattle, WA 98104
mailto:RichA at vulcan.com
mailto:RichA at LivingComputerMuseum.org
http://www.PDPplanet.org/
http://www.LivingComputerMuseum.org/
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