From: cross@spitfire.i.gajendra.net
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.
If you're just talking about RISC design techniques generically,
then I dunno, maybe, sure, why not, but that's a LOT of
speculation with hindsight-colored glasses. 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.
>There can also be no doubt that a RISC-type machine would have
>exhibited the same performance advantages (at least in integer
>performance) as a RISC vs CISC 10 years later. The 801 did so
>vs. the /370, as did the RISC processors vs, for example, the
>680x0 family of processors (just compare ARM vs. 68000).
>
>Or look at the performance of the TTL implementation of HP-PA,
>which used PALs which were not available to the VAX 11/780
>designers, so it could be clocked a bit higher, but at
>a multiple of the performance than the VAX.
>
>So, Anton visiting DEC or me visiting Data General could have
>brought them a technology which would significantly outperformed
>the VAX (especially if we brought along the algorithm for graph
>coloring. Some people at IBM would have been peeved at having
>somebody else "develop" this at the same time, but OK.
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. But as with all alternate history, this is
completely unknowable.
- Dan C.
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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