[fpc-pascal] Freepascal Floating Point Issue

[Paul Nance]

Turbo Pascal also had a BCD unit.

On Aug 22, 2017 6:56 PM, "Ralf Quint" <freedos.la at gmail.com> wrote:

> On 8/22/2017 1:39 PM, Mark Morgan Lloyd wrote:
> > On 21/08/17 22:15, Ralf Quint wrote:
> >> On 8/21/2017 3:02 PM, James Richters wrote:> I am having an issue
> >> with a simple floating point application.  I am setting a variable to
> >> a specific value and immediately after I set it,  it is not exactly
> >> what I set it to.  Here's an example>>    Draw_GX_Min:=999.999;>
> >> Writeln(Draw_GX_Min:3:30);>> The writeln results in
> >> 999.999000000000020000000000000000  >> Why is it not
> >> 999.999000000000000000000000000000  where did 0.00000000000002 come
> >> from?>Out of thin air... Well, kind of. Double floating point means
> >> 16 digitsof precision, so when you force a 30 digit precision output,
> >> anythingpast 16 digits is random garbage, at best...
> >
> > And in any event, that's probably much more precision than the GPU is
> > using to generate the final image :-)
> >
> Well, older GPUs (at least NVidia, pretty sure similar restrictions
> apply to AMD) use(d) only 32bit "single" floats, giving a 7 digit
> precision, though newer ones can also handle 64bit doubles. But there
> are quite a few differences in how certain FP operations are handled on
> those GPUs, which result in even doubles only having 14 (instead of 16)
> digits of precision at best. So while NVidia keeps mentioning IEEE754,
> their GPUs are in practice not 100% compliant.
>
> As I mentioned before, if someone needs to work a lot with floating
> point arithmetic, it really helps to get yourself acquainted to the way
> those works and all the possible pitfalls.
>
> Btw, anyone know about a BCD math implementation for Free Pascal, like
> it used to be implemented in DR CBASIC? (those were the days... ;-) )
>
> Ralf
>
>
>
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