logarithmic instructions in fpu

[jalvarado]

can anybody bring me information and examples of logarithmic  instructions in floating point unit (FPU)

[dedndave]

Ray's library and tutorial...

http://www.ray.masmcode.com/fpu.html

[jalvarado]

thanks is very useful.  :icon14: anyone can bring me more especific examples of using  logarithmic  instructions in fpu??

[dedndave]

what are you trying to do ?

many times, log values are converted to log base 2, "worked" on, then converted back
so - one of the first things you might want to learn would be converting logs between bases

log_b(a) = log_c(a) / log_c(b)

[RuiLoureiro]

thanks is very useful.  :icon14: anyone can bring me more especific examples of using  logarithmic  instructions in fpu??

              What do you mean with "more especific examples" ?
              What do you mean with "using logaritmic instructions" ?
              What do you want to do ?

[jalvarado]

so im trying to find another example like this http://www.ray.masmcode.com/tutorial/fpuchap13.htm , to understand the logic of the program using  logarithmic  instructions

[dedndave]

Rui is a good guy to help you, here
he has recently been doing a lot with logs and the FPU

[jalvarado]

Ok thanks.

Rui, Can you help me?

[qWord]

What did you not understand: the mathematics or it's implementation?

[RuiLoureiro]

so im trying to find another example like this http://www.ray.masmcode.com/tutorial/fpuchap13.htm , to understand the logic of the program using  logarithmic  instructions

Hi jalvarado,
           Sorry but the best person to explain that example is the author
           Raymond.

           Meanwhile one simple problem can be this: how do you know the
exponent y if you know z and x and the equation z=x^y ?

[jalvarado]

What did you not understand: the mathematics or it's implementation?

 it's implementation

[qWord]

log_b(a) = log_c(a) / log_c(b)

Code: [Select]

fld FP8(0.301029995663981) ; 1/log2(10)
fld FP8(0.001) ; test value = 10^-3
fyl2x
fstp realVar ; = log(0.001) = log2(0.001)/log2(10) = -3

;OR: log(x) = log10(2)*log2(x)
fldlg2
fld FP8(0.001)
fyl2x
fstp realVar


[RuiLoureiro]

jalvarado,
Based on logb(a) = logc(a) / logc(b) given by Dave,

dont forget this 2 important formulas:

            log10(x)= log2(x). (1/log2(10))

            ln(x)      = log2(x). (1/log2(e))

If we have y=(e^x-e^-x)/2, to find x=f(y) we need to use logarithms.