Re: intel x86 floating point power funtion in assembly?

From: Terje Mathisen (spamtrap_at_crayne.org)
Date: 12/20/04 

Date: Mon, 20 Dec 2004 21:09:31 +0000 (UTC)

Matt C. wrote:

> I've been trying to find a good power function (x^y) that is
> written in 100% intel x86 fpu assembly language. Does anybody
> have one, or do you know where I can find one?

The naive approach is to simply do exp(ln(x)*y), right?

This is relatively straightforward, except that we'd prefer to use
binary logs and exponentiation:

FYL2X calculates the product of a scale factor (i.e. Y) and the binary
logarithm of the input number (X).

F2XM1 returns the result of binary exponentiation minus 1.0, so the
following code should do it:

; Assume the caller has loaded first Y, then X onto the fp stack!
power proc
   fyl2x ; Binary log
   f2xm1 ; Result - 1
   fld1 ; Load 1.0
   faddp st(1),st(0) ; Add together and pop
   ret ; Return the result as ST(0)
power endp

Next is to detect some of the special cases:

Integer y?

Negative y?

Y close to zero?

X close to zero or one?

Terje 

-- 
- <Terje.Mathisen@hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"

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