Happy Birthday VAX 11/780

Tony Duell ard at p850ug1.demon.co.uk
Tue Oct 26 13:12:45 CDT 2010

> 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 

The 11/780 logica address space is larger than the physical one, the 
(memory managed) PDP11's is not.


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