From:	"Shaggs" <jud AT tminet DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: cos(90 * (PI / 180) = 6.12... (now has an idea of what happened
Date:	Mon, 31 May 1999 17:01:01 -0700
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References:	<7ii9di$sng$1 AT remarQ DOT com> <374DBEE8 DOT E73D32ED AT cartsys DOT com>
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All my values seem perfectly normal for sine and cosine, but on degrees that
are even with an axis, such as 90,18,270, etc i get some way off number,

my proccessor is a p166, no mmx, purchased when 200 was the highest, and MMX
was really new
Nate Eldredge wrote in message <374DBEE8 DOT E73D32ED AT cartsys DOT com>...
>Shaggs wrote:
>>
>> cos(90 * (PI / 180) = 6.12... could this be the result of the pentium
flaw,
>> as i have the sin and cos stored in arrays of double
>
>Hasn't this come up before?
>
>Look at the complete result, and think about what exponential notation
>means.  Then compare it with the value of DBL_EPSILON, which is the
>approximate precision of double.  Is your result within DBL_EPSILON of
>the expected 0?
>
>I don't think the Pentium FDIV bug is a likely suspect here.  Though it
>did exist, I've never heard that it was so gross as to give answers off
>by 6 in the most significant digit.  Have you even got an old enough
>Pentium that could have it?
>
>If you really want to know, this code should check for it.  It's
>borrowed from the Linux source.
>
>#include <stdio.h>
>
>static double x = 4195835.0;
>static double y = 3145727.0;
>
>int has_fdiv_bug(void)
>{
>  int result;
>  __asm__("fninit\n\t"
>   "fldl %1\n\t"
>   "fdivl %2\n\t"
>   "fmull %2\n\t"
>   "fldl %1\n\t"
>   "fsubp %%st,%%st(1)\n\t"
>   "fistpl %0\n\t"
>   "fwait\n\t"
>   "fninit"
>   : "=m" (result)
>   : "m" (x), "m" (y));
>  return result;
>}
>
>int main(void)
>{
>  printf("FDIV bug %s\n", has_fdiv_bug() ? "found" : "not found");
>  return 0;
>}
>
>--
>
>Nate Eldredge
>nate AT cartsys DOT com

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