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Re: Use of soft float?

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On 21.5.2018 г. 02:24, Clifford Heath wrote:
> On 20/05/18 21:24, Dimiter_Popoff wrote:
>> On 20.5.2018 г. 12:56, Philipp Klaus Krause wrote:
>>> While those who make heavy use of floating-point computations in
>>> embedded sytems probably go for systems with hardware support for
>>> floating-point calculations, there are plenty of systems without such
>>> support out there.
>>> And the existence of soft float implementations indicates that there is
>>> a real need for doing floating-point computations on systems without
>>> hardware floating-point support. I'd like to know a bit more about that.
>>>
>>> * What kind of computations do people use soft float for? Any real-world
>>> examples?
>>> * Which types of soft float do they use? IEEE 32- or 64-bit? Some other
>>> C standard-compliant type? 16-bit IEEE? Other 16-bit?
>>>
>>> Philipp
>>>
>>
>> I can only speak for myself of course, here it goes.
>>
>> Some 20 years ago on our first Nukeman
>> ( http://tgi-sci.com/tgi/nmtb.htm ) I needed floating point.
>> The need came late in the project, during spectrum evaluation
>> one has to calculate isotope decay during the time acquisition
>> has taken place (some isotopes have short half lives so this
>> matters a lot). This took e^x or something like that, and the
>> processor was an MC68340 - a CPU32 core (sort of a downgraded 68020)
>> which had no FPU.
>> So I wrote the FPU functions I needed - just those I needed, basic
>> arithmetic plus the exponential thing I believe, may be something
>> else I don't remember at the moment. The format was that the 68K
>> series used, called "extended precision", 96 bit. However the
>> precision was far far from it - the 68340 had 32*32->64 bit
>> multiplication so I left the mantissa precision at just 32 bits,
>> was plenty.
>
> How much precision did you even need? If you have a regular clock
> tick and not to many decays in parallel you can do exponential
> decay just by subtracting some fraction (shift and subtract)
> on each cycle.

It is not so simple, far from it really. During spectrum acquisition,
when you have a "clock tick" you don't know what is in the spectrum,
acquisition times of many hours - or days - are normal.
So all you have the acquisition time and the half lives after you
have identified the isotopes. Then the operator needs to be able
to calculate activities referenced to any particular moment, not the
moment of acquisition. Etc etc.
Steering away from FP makes sense as long as you don't have an FPU
on chip of course, but sometimes using FP - even emulated - is just
the easiest way to go. I had managed back then - prior to writing the
evaluation mentioned - to do curve fit to spectrum peaks using
fixed point, works quite well to this day - although I have migrated
some (not sure how much) of the arithmetic to FP since I have one
anyway.

Dimiter

======================================================
Dimiter Popoff, TGI             http://www.tgi-sci.com
======================================================
http://www.flickr.com/photos/didi_tgi/

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Use of soft float? Philipp Klaus Krause <pkk@spth.de> - 2018-05-20 11:56 +0200
  Re: Use of soft float? Dimiter_Popoff <dp@tgi-sci.com> - 2018-05-20 14:24 +0300
    Re: Use of soft float? Clifford Heath <no.spam@please.net> - 2018-05-21 09:24 +1000
      Re: Use of soft float? Dimiter_Popoff <dp@tgi-sci.com> - 2018-05-21 10:35 +0300
        Re: Use of soft float? Clifford Heath <no.spam@please.net> - 2018-05-22 09:33 +1000
  Re: Use of soft float? Grant Edwards <invalid@invalid.invalid> - 2018-05-20 23:39 +0000
  Re: Use of soft float? David Brown <david.brown@hesbynett.no> - 2018-05-21 12:12 +0200
    Re: Use of soft float? upsidedown@downunder.com - 2018-05-22 00:36 +0300
      Re: Use of soft float? Grant Edwards <invalid@invalid.invalid> - 2018-05-21 21:49 +0000
        Re: Use of soft float? upsidedown@downunder.com - 2018-05-22 01:41 +0300
      Re: Use of soft float? Clifford Heath <no.spam@please.net> - 2018-05-22 09:39 +1000
      Re: Use of soft float? David Brown <david.brown@hesbynett.no> - 2018-05-22 11:23 +0200
  Re: Use of soft float? Luke A. Guest <laguest@archeia.com> - 2018-06-06 04:37 +0100
    Re: Use of soft float? Reinhardt Behm <rbehm@hushmail.com> - 2018-06-06 12:54 +0800

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