Do bear in mind that if you test the speed of the BIOS functions on an
emulator the result will not necessarily be indicative of what happens on
real hardware. The emulators generally just do a sqrt in native 80x86 code
and return the result.

Graham

-----Original Message-----
From: Thomas [mailto:sorcererxiii@...]
Sent: 09 December 2003 16:23
To: gbadev@yahoogroups.com
Subject: [gbadev] Re: Fixed Point Square Root


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



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