Happy Birthday VAX 11/780

Johnny Billquist bqt at softjar.se
Mon Oct 25 11:07:34 CDT 2010

On 10/25/10 16:53, William Donzelli <wdonzelli at gmail.com> wrote:

>> >  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.

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 

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).


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