[Info-vax] Vaxes shutting off this week

[glen herrmannsfeldt]

Bill Gunshannon <billg999 at cs.uofs.edu> wrote:
 
> I see no reason why the VAX architecture could not have been continued,
> just like IBM has done with the 360 architecture.  The only thing that
> stopped it was its owners desire to kill it.  And for those who might
> argue that it was not economically feasable, how many very old VAX
> systems do we still hear about here that are still viable commercial
> operation?

Maybe IBM was lucky.  S/360 is somewhat RISCy.  Only three
different instruction lengths, and very few address modes.
You can tell the instruction length from the first byte
(actually, just the first two bits) for all instructions.
Also, only a small number of addressing modes are used,
two for a large fraction of the instructions commonly used.

The I/O instructions have completely changed, and some
other changes to privileged instructions have been
made since S/360, but the user instruction set has 
been kept backwards compatible.  

Determining the length of a VAX instruction is much
harder than just reading the first byte.  That,
and the wide range of instruction lengths  greatly
complicates the logic to issue multiple instructions
per clock cycle.  

-- glen