RE: [f-cpu] Re: Floating-Point?

[Hans Summers <Hans.Summers@tudor.com>]

> Hans Summers a écrit :
> > 
> > > On Thu, Aug 16, 2001 at 01:49:27PM +0800, Glenn Alexander wrote:
> > > > Hi. This is my first post to this list. My background is as
> > > > a hardware technician, not a chip designer so bear with me.
> > > >
> > > > On Thursday 16 August 2001 03:41, you wrote:
> > > > > Michael Riepe a écrit :
> > > > >
> > > > > In sparc, there is no 80 bits float but 128 bits (2
> > > > > registers used in the same time). We don't need 1 cycle
> > > > > multiplication so it could be done for the fcpu.
> > > >
> > > > I am thinking that taking the two-register approach might be
> > > > over-complicating matters. Since F-CPU is intended to later
> > > > be scaled above 64 bits, if someone wanted 128-bit floats
> > > > in the future they would impliment a 128-bit F-CPU.
> > > > Especially for the FC-0 and probably for the FC-1, KISS
> > > > (Keep It Simple for us Stupid people).
> > >
> > > 128-bit `quadruple precision' (like SPARC) is IMHO the 
> > > way to go, but not in the FC0.  For now, let's stick to 
> > > 32-bit and 64-bit (with 80-bit `double extended' used 
> > > inside the FP unit, to maintain IEEE compliance).
> > >
> > Could someone explain to me why 128-bit FP is desireable? I 
> > am struggling
> 
> It's very easy : almost all scientific calculation ! This include
> electric simulation (spice), aerodynamic, and so on... For a lot of
> mathematician 32 bits are a none sense ! For laughing, they said that
> they didn't want to take a plane any more ! Don't forget that every
> compiler defined float as double (64b) by default. With 32 
> bit there is too much rounding problem. The only killing application 
> for it is the image processing. That's wy some of them want 256 
> bits fp number.

Sorry I still can't see it. Which electric or aerodynamic simulations
require 30 decimal places of accuracy? 

In my scientific days I never came across a calculation needing anything
like 128 bits of precision. "Almost all scientific calculation" seems to me
like a huge exagerration. The limits of measurement are often to be measured
in percent, so why would you want to calcuate to 30 decimal places when you
can only measure to one or two? As for electrical simulation, when resistors
usually have a 1% error, and capacitors 5 or 10%, where are the 30 decimal
places needed? 

Fine, for the sake of comfort, use doubles (64-bit). And I am sure that some
areas genuinely need more precision. But I still believe the number of
applications requiring this much precision to be very small. For a general
purpose processor implementing 128-bit FP is a waste of resources. 

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