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From: Russell_Williams.design_women@gateway.qm.apple.com (Russell Williams)
Newsgroups: comp.arch
Subject: Re: DEC RISC Architecture?
Message-ID: <10724@goofy.Apple.COM>
Date: 16 Oct 90 15:25:06 GMT
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References:<4462@trantor.harris-atd.com> <107038@convex.convex.com> <15007@hydra.gatech.EDU> <0093E081.85D1F940@KING.ENG.UMD.EDU> <1353@exodus.Eng.Sun.COM> <8198@scolex.sco.COM>

In article <8198@scolex.sco.COM> seanf@sco.COM (Sean Fagan) writes:
> I don't know if that's the case.  Elxsi went to a great deal of trouble 
to
> be VMS compatable, at the user-interface and programmers' level 
(high-level
> languages, not assembly, of course 8-)), and, apparantly, they were quite
> good at it.

Elxsi produced a good source-level emulation of perhaps the most common 
20% of VMS functionality;  that took care of most applications and was 
effective in convincing a very small percentage of VAX shops to buy the 
Elxsi hardware.  Elxsi faced two limitations in producing VMS emulation: 
one was an engineering team of only 6-8 engineers trying to duplicate 
DEC's thousands of engineer-years of effort.  The other was machine 
architecture.  Elxsi had the advantage of a similar flat 32-bit memory 
model, and IEEE arithmetic which was similar to a couple of DEC's alphabet 
soup of floating point formats.  These formats weren't identical though, 
and floating point applications often got slightly different results.  
These could be proven to be more accurate than the DEC results, but 
customers didn't like it anyway.  The Elxsi is a consistent big-endian 
machine and no attempt was made to emulate the VAX's inside-out/little 
endian byte ordering.

The expectations for a DEC "VMS compatible" system would be considerably 
higher.  

In most respects relevant to porting VMS application source, the Elxsi is 
similar to commercial RISC architectures (memory model, floating point 
data format, consistent endian-ness).  The last two of these architectural 
features caused gaps in  VMS emulation which Elxsi never bridged but which 
DEC could not ignore in a credibly "compatible" solution. 

Interpretation of VAX object code on a commercial RISC would probably 
produce terrible performance.  The floating point format problems remain, 
and the VAX instruction set is not suitable for emulation on most 
commercial RISCs because of the byte-aligned nature of instructions.  
Object code translation would probably fare better, but data alignment and 
floating point format problems remain.

I conclude that DEC would either design their own RISC platform or market 
a commercial RISC/VMS box with more limited compatibility and "porting 
aid" claims.