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- From: jan@pallas.neuroinformatik.ruhr-uni-bochum.de (Jan Vorbrueggen)
- Subject: Re: Goals, Cost, and Flexibility (was Re: "Training" of programmers)
- Message-ID: <JAN.92Sep12155439@pallas.neuroinformatik.ruhr-uni-bochum.de>
- In-reply-to: crowl@jade.CS.ORST.EDU's message of 11 Sep 92 18:47:44 GMT
- Organization: Inst. f. Neuroinformatik, Ruhr-Universitaet Bochum, FRG
- References: <Btx4vF.Jx6@mentor.cc.purdue.edu> <1992Sep4.151001.9886@sei.cmu.edu>
- <Bu5qD3.GAM@mentor.cc.purdue.edu> <1992Sep11.184744.19329@CS.ORST.EDU>
- Date: 12 Sep 92 15:54:39
- Lines: 62
-
- In article <1992Sep11.184744.19329@CS.ORST.EDU>
- crowl@jade.CS.ORST.EDU (Lawrence Crowl) writes:
- >And then the hardware designers, being told that something is useless, drop
- >it from the instruction set?
-
- You misunderstand what has happened in the industry. The hardware designers
- were told that something was rarely used, and therefore was not cost
- effective. As good engineers, they dropped the feature.
-
- Ah yes. And them somebody thought of an algorithm, reducing the complexity
- of some important programme. Unfortunately, because of a few cents' worth
- of cost savings in the hardware design, the implementation is ten to 1000
- times slower than necessary. Sheesh, how cost effective!
-
- Pray tell, how are you, using this philosophy of "it's only rarely used",
- going to avoid ending up with processors which can do nothing but NANDs?
- They're still Trung machines, but I'd rather not use them for my simulations.
-
- >Designing flexibility in both hardware and software is not that expensive;
- >putting in something left out may be.
-
- [...] I invite you to provide _evidence_ in support of your claim.
-
- >A simple example is that of square root. The cost of putting in square root
- >on any machine which has "classical" division hardware is essentially zero
- >at the design and construction stage,but an algorithm to do it is otherwise
- >is necessarily slow and costly.
-
- Only costly when making simplistic comparisons. If the lack of square root
- hardware increases my run time from 10 seconds to 10.0001 seconds (which is
- probably realistic for many if not most program runs), it is _not_ costly.
-
- And that _is_ a simplistic comparison. Why should "many if not most"
- programmes be good enough reason for elimiting a function? I think Herman
- is talking about incremental cost effectiveness. And to give you an example:
- When Inmos added an FPU to the transputer, they put in a barrel shifter and
- a 3bit/cycle mutiplier. Taking these as a given, they put in the microcode
- for square root and remainder because that's all it cost them: some more
- microcode. Should they just leave them out just to spite their customers?
-
- >Bit vectors do not necessarily start on a word, or even a byte, boundary,
-
- Yes, but if only one in a million don't, supporting nonaligned bit vectors
- probably isn't cost effective.
-
- And again, these decisions are limiting what gets done. Why are currently
- proposed cryptoalgorithms using key lengths that are powers of two? Surely,
- one of the reasons is that current hardware isn't able to process bit strings
- that don't fall on 64 ot 128 bit boundaries efficiently.
-
- >and again, it is easier to deal with the problem at hardware design time
- >rather than in software.
-
- This claim is strongly supported by the wide abundance of hardware
- implemented compilers and spreadsheet packages. :)
-
- Not quite that, but look at IBM's VM/HPO. A large part of the virtual machine
- assists and thus of VM is actually done in microcode, probably individually
- for every model series. Saving 25% or more of your 3090's processing power
- is certainly worth a few man year's microcoding slavery.
-
- Jan Vorbrueggen, Inst. f. Neuroinformatik, EuheUniversitaet Bochum, FRG
-