Re: Why VAX Was the Ultimate CISC and Not RISC

John Levine <johnl@taugh.com> writes:

According to Anton Ertl <anton@mips.complang.tuwien.ac.at>:

That's not a fair comparison. VAX design started in 1975 and shipped in 1978.
The first ARM design started in 1983 with working silicon in 1985. It was a
decade later.

>
The point is that ARM outperformed VAX without using caches.  DRAM
with 800ns cycle time was available in 1971 (the Nova 800 used it).
By 1977, when the VAX 11/780 was released, certainly faster DRAM was
available.

>
How was the code density?

I have no data on that.  Interestingly, unlike the 68k, which was
outcompeted by RISCs at around the same time, the VAX did not have an
afterlife of hobbyists who produced Linux and Debian ports, so I
cannot easily make a comparison.

I know ARM was pretty good but VAX
was fantastic since they sacrified everything else to compact instructions.

I don't think they did.  They spent encoding space on instructions
that were very rare, and AFAIK instructions can be encoded that do not
work (e.g., a consant as destination).  The major idea seems to have
been orthogonality, not compactness.  They did take choices for
compactness (e.g., the call instructions that includes a bitmask for
the registers to be saved), but overall other ideas were more
important.

- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>