Ok, testing has revealed that the bios function is a good 10 times
faster than the 2.30 function (I didn't measure it precisely - all I
need to know is that it's faster). To get more precision, I multiply
the input by 4096 then divide the result by sqrt(4096).

(Sqrt(fixedval << 12) << 5) >> 6

Of course, this limits us to a max input of 512.0, which is
sufficient for what it's being used.

Tom


--- In gbadev@yahoogroups.com, "Thomas" <sorcererxiii@y...> wrote:
> Thanks, I'll try that out . . . for some reason my brain is not
> working on this issue (years of only having to press the square
root
> key on a calculator?). I also figured out that I can use the bios
> square root (I think) by realizing that
>
> sqrt(n * 1024) = sqrt(n)*sqrt(1024) = sqrt(n) << 5.
>
> I'll need to compare to see which method is faster/more accurate.
>
> Tom
>
>
> --- In gbadev@yahoogroups.com, "Damian Yerrick" <d_yerrick@h...>
> wrote:
> > --- In gbadev@yahoogroups.com, "Thomas" <sorcererxiii@y...> wrote:
> > > I'd like a general algorithm in C or psuedocode that could be
> > > used for different fraction sizes just be changing a #define
> > > value. I've searched and found some that worked for specific
> > > cases (like one that was 2.30)
> >
> > For reference, here's an example of a 2.30 square root function:
> > http://www.worldserver.com/turk/opensource/FractSqrt.c.txt
> >
> > > but I could figure out how to adapt it for 22.10.
> >
> > The given code gives sqrt(1<<30) == 1<<30. You want
> > sqrt(1<<10) == 1<<10 and sqrt(1<<30) == 1<<20. So use the
> > 2.30 code and just shift the result right by 10 binary places.
> > In general, if you have n fractional places (in your case 10),
> > where n is odd and 0 <= n <= 30, then shift right by (30-n)/2
> > after calling the 2.30 routine.
> >
> > Division is rather slow on ARM; don't use the division based
> > Newton's method code that somebody else just posted.
> >
> > --
> > Damian