Happy Birthday VAX 11/780

Rich Alderson 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 
   > 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


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