From: tkoenig@netcologne.de   
      
   Dan Cross  schrieb:   
   > In article <1070cj8$3jivq$1@dont-email.me>,   
   > Thomas Koenig   wrote:   
   >>Dan Cross  schrieb:   
   >>> In article <106uqki$36gll$4@dont-email.me>,   
   >>> Thomas Koenig   wrote:   
   >>>>Dan Cross  schrieb:   
   >>>>> In article <44okQ.831008$QtA1.573001@fx16.iad>,   
   >>>>> Scott Lurndal  wrote:   
   >>>>>>[snip]   
   >>>>>>We tend to be spoiled by modern process densities.   The   
   >>>>>>VAX 11/780 was built using SSI logic chips, thus board   
   >>>>>>space and backplane wiring were significant constraints   
   >>>>>>on the logic designs of the era.   
   >>>>>   
   >>>>> Indeed.  I find this speculation about the VAX, kind of odd: the   
   >>>>> existence of the 801 as a research project being used as an   
   >>>>> existence proof to justify assertions that a pipelined RISC   
   >>>>> design would have been "better" don't really hold up, when we   
   >>>>> consider that the comparison is to a processor designed for   
   >>>>> commercial applications on a much shorter timeframe.   
   >>>>   
   >>>>I disagree.  The 801 was a research project without much time   
   >>>>pressure, and they simulated the machine (IIRC at the gate level)   
   >>>>before they ever bulit one.  Plus, they developed an excellent   
   >>>>compiler which implemented graph coloring.   
   >>>>   
   >>>>But IBM had zero interest in competition to their own /370 line,   
   >>>>although the 801 would have brought performance improvements   
   >>>>over that line.   
   >>>   
   >>> I'm not sure what, precisely, you're disagreeing with.   
   >>>   
   >>> I'm saying that the line of though that goes, "the 801 existed,   
   >>> therefore a RISC VAX would have been better than the   
   >>> architecture DEC ultimately produced" is specious, and the   
   >>> conclusion does not follow.   
   >>   
   >>There are a few intermediate steps.   
   >>   
   >>The 801 demonstrated that a RISC, including caches and pipelining,   
   >>would have been feasible at the time.  It also demonstrated that   
   >>somebody had thought of graph coloring algorithms.   
   >   
   > This is the part where the argument breaks down.  VAX and 801   
   > were roughly contemporaneous, with VAX being commercially   
   > available around the time the first 801 prototypes were being   
   > developed.  There's simply no way in which the 801,   
   > specifically, could have had significant impact on VAX   
   > development.   
      
   Sure. IBM was in less than no hurry to make a product out of   
   the 801.   
      
   >   
   > If you're just talking about RISC design techniques generically,   
   > then I dunno, maybe, sure, why not,   
      
   Absolutely.  The 801 demonstrated that it was a feasible   
   development _at the time_.   
      
   >but that's a LOT of   
   > speculation with hindsight-colored glasses.   
      
   Graph-colored glasses, for the register allocation, please :-)   
      
   >Furthermore, that   
   > speculation focuses solely on technology, and ignores the   
   > business realities that VAX was born into.  Maybe you're right,   
   > maybe you're wrong, we can never _really_ say, but there was a   
   > lot more that went into the decisions around the VAX design than   
   > just technology.   
      
   I'm not sure what you mean here.  Do you include the ISA design   
   in "technology" or not?   
      
   [...]   
      
   > While it's always fun to speculate about alternate timelines, if   
   > all you are talking about is a hypothetical that someone at DEC   
   > could have independently used the same techniques, producing a   
   > more performance RISC-y VAX with better compilers, then sure, I   
   > guess, why not.   
      
   Yep, that would have been possible, either as an alternate   
   VAX or a competitor.   
      
   > But as with all alternate history, this is   
   > completely unknowable.   
      
   We know it was feasible, we know that there were a large   
   number of minicomputer companies at the time.  We cannot   
   predict what a succesfull minicomputer implementation with   
   two or three times the performance of the VAX could have   
   done.  We do know that this was the performance advantage   
   that Fountainhead from DG aimed for via programmable microcode   
   (which failed to deliver on time due to complexity), and   
   we can safely assume that DG would have given DEC a run   
   for its money if they had system which significantly   
   outperformed the VAX.   
      
   So, "completely unknownable" isn't true, "quite plausible"   
   would be a more accurate description.   
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