Happy Birthday VAX 11/780
RichA at vulcan.com
Tue Oct 26 13:26:29 CDT 2010
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
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
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.
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