Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hp-sdd!megatek!corona!mark
From: mark@corona.megatek.uucp (Rocket J. Squirrel)
Newsgroups: comp.arch
Subject: Re: In defense of the VAX
Message-ID: <501@megatek.UUCP>
Date: 25 Feb 89 01:42:45 GMT
References: <11037@tekecs.TEK.COM> <76700073@p.cs.uiuc.edu> <324@taniwha.UUCP> <668@m3.mfci.UUCP>
Sender: news@megatek.UUCP
Reply-To: mark@corona.UUCP (Rocket J. Squirrel)
Organization: Megatek Corp.
Lines: 57

I think this discussion of how stupid the design of the VAX is (or is not)
is interesting in its own right, but i think it is missing a little bit on
the reality side.

The following is the history of the VAX as *I* understand it, people who
were actually working at DEC back east in this time frame can feel free
to correct it.

DEC at that time (probably still) was composed of several engineering groups
that were responsible for different product lines. These groups HARDLY EVER
talked to each other. 

When the PDP-11 was obviously running out of address space, there began a
project in the PDP-11 group to fix that problem. This is why the VAX-11 
was called the "Virtual Address Extension 11". This group probably had
nobody that knew how to build a large computer. DEC had another group that
did "large" computers... the Large Computer Group (they built the DEC-10).
It is virtually certain that nobody thought about pipelined instruction 
sets, because DEC just didn't do that sort of stuff. 

One very BASIC goal of the project was to make sure that most of the 
customer's PDP-11 code would run on whatever they built, since they hoped 
to replace those PDP-11s with whatever they built. When the VAX-11/780
first shipped, most of the software was taken directly from RSX-11 and
ran under emulation.

When the 780 became a big success, it eventually happened that the small
computer people began trying to build large computers. There were several
failed attempts - partly because, as has been pointed out here at great
length, the architecture sucks for building fast machines. Eventually,
DEC gave up on having two "large" computer lines, they flushed the DEC-10 *
and put the "Large Computer Group" in charge of building a fast VAX. 
Meantime, the small computer group built the 750 and 730. The 750 is
a decent machine - but small.

The point is that the VAX is not the way it is because people were trying
to build a machine for a particular technology. It is the way it is because 
the people who were designing it wanted to be able to run PDP-11 code on it 
pretty well in emulation mode, and the compiler people wanted something that 
the PDP-11 fortran (and COBOL!) compilers had a prayer of generating code 
for (with the inevitable "just a few improvements").

-mark

* The last project of the DEC-10 group was a thing called "jupiter" which was
to be a pipelined DEC-10. This would have been quite an effort in its own
right. The DEC-10 had fixed-length instructions (which people in this group
seem to find desireable) and they were very orthogonal. Unfortunately, the
architecture encouraged the use of skips and instructions that modified a
register and immediately branched based on the result.


 
--
ucsd.edu!megatek!mark					mark thompson
	"Tiger mutters the fighter pilot's prayer: `Oh shit.'"
--