25 Oct
2010
25 Oct
'10
9:07 a.m.
On 10/25/10 16:53, William Donzelli <wdonzelli at gmail.com> wrote:
The PDP-11 was in architectural ways more important than the VAX, if
nothing else than just because the VAX was basically just extending the
PDP-11.
However, I also object to the discussion about "Virtual memory" as
something new the VAX brought to the table.
Virtual memory worked just fine on a PDP-11 as well, thank you very
much, as it also worked fine on a bunch of other machines, and had been
doing for quite a while.
VAX stands for "Virtual Address eXtension", note the "extension".
Extension normally means that you modify/extend something that already
exists, in this case the virtual address. On a PDP-11, the virtual
address is 16 bits, the VAX extended it to 32 bits, which is a huge
improvement (and the biggest bottleneck of the PDP-11, as I'm sure all
people know). The physical address on a PDP-11 is 22 bits, while the
physical address on a VAX varies, but on the 11/780 I only think it was
something like 24 bits.
The VAX also introduced demand pageing, compared to the PDP-11, where
you normally didn't do that (and not all models could even possibly do
it), but demand pageing as such wasn't new either. DEC was already doing
it with the PDP-10 running TOPS-20 (and other companies had also done it).
Johnny
> Also,
the sentence starting "The VAX instruction set well revered would later on
influence" needs restructured. The paragraph after the bullet points has more of the
same error.
You could say that the designers of the 68000 were influenced by the
PDP-11, but I do not think you could say the same thing about the VAX.
When the 68000 design was started, the VAX was still well under wraps.
I can see no reason why DEC would have let the Motorola guys see the
developing architecture.
Anyway, in the mid/late 1970s, heavily microcoded very CISCy
architectures were pretty much the design route of choice. It was
everywhere. It is very difficult to pin down influences. 
26 Oct
26 Oct
11:12 a.m.
VAX stands for "Virtual Address eXtension",
note the "extension".
I always thought that VAX-11 meant it was a PDP11 that had the
'eXtension' of 'Virtual Addresses'.
Extension normally means that you modify/extend
something that already
exists, in this case the virtual address. On a PDP-11, the virtual
address is 16 bits, the VAX extended it to 32 bits, which is a huge
improvement (and the biggest bottleneck of the PDP-11, as I'm sure all
people know). The physical address on a PDP-11 is 22 bits, while the
physical address on a VAX varies, but on the 11/780 I only think it was
something like 24 bits.
Hmmm. I was under the impression there was a difference between virtual
memory/addresses and an MMU. Namely that the former impliled a larger
logical address space than the physcial address space and that if a
program tried to access a memory page that wasn't currently mapped to a
physcial area of memory, the OS would be given the chance to load the
approraite data from disk (or whatever) into RAM and map the page
appropriately.
The 11/780 logica address space is larger than the physical one, the
(memory managed) PDP11's is not.
-tony
11:26 a.m.
From: Johnny Billquist
Sent: Monday, October 25, 2010 9:08 AM
1. > The PDP-11 was in architectural ways more important than the VAX, if
nothing else than just because the VAX was basically
just extending the
PDP-11.
2. > However, I also object to the discussion about "Virtual memory" as
something new the VAX brought to the table.
3. > Virtual memory worked just fine on a PDP-11 as well, thank you very
much, as it also worked fine on a bunch of other
machines, and had been
doing for quite a while.
4. > VAX stands for "Virtual Address eXtension", note the
"extension".
Extension normally means that you modify/extend
something that already
exists, in this case the virtual address. On a PDP-11, the virtual
address is 16 bits, the VAX extended it to 32 bits, which is a huge
improvement (and the biggest bottleneck of the PDP-11, as I'm sure all
people know). The physical address on a PDP-11 is 22 bits, while the
physical address on a VAX varies, but on the 11/780 I only think it was
something like 24 bits.
5. > The VAX also introduced demand pageing, compared to the PDP-11, where
you normally didn't do that (and not all models
could even possibly do
it), but demand pageing as such wasn't new either. DEC was already doing
it with the PDP-10 running TOPS-20 (and other companies had also done it).
Addressing 1, 2, 4, and 5:
The "Extension" in 'Virtual Address Extension" does not refer to
extending
the virtual address in the PDP-11, but rather to extending the PDP-11
architecture with virtual addressing. The PDP-11's 16-bit address is real,
not virtual in the usual definition; the use of memory management to select
from within an 18- or 22-bit memory space does not make
it virtual.
The VAX-11 (note that "-11" in the names of the first models!) added the
use of demand-paged virtual memory (that is to say, disk-based storage) to
the PDP-11, then expanded the instruction set into the new 32-bit word size.
Addressing 2 and 5:
Burroughs introduced the B5000, the first computer with virtual memory
(segmented rather than paged) in 1961; the British brought out the Atlas in
1962. Multics used both segmentation and paging on the GE-645, beginning
in 1964. DEC provided a segmented memory model in the PDP-6 (1964) and
PDP-10 (1967); BB&N created a pager for the PDP-10 and brought TENEX, with
demand paging, to the world c. 1970. When DEC licensed TENEX and modified
it for the KL-10 processor (born at the Stanford AI Lab as the SuperFoonly!),
they added the working-set concept which had been discovered by (IIRC)
Denning in his research on demand-paged memory systems, and christened the
result "TOPS-20".
Addressing 3:
I don't believe that there was ever demand-paged virtual memory on the
PDP-11, but I'm willing to be shown the error of my ways. Please point me
at documentation for an operating system which did that.
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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