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.