26 Oct
2010
26 Oct
'10
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