Re: [Mingw-users] MinGW and Floating Point

[Richard C. W. <drsavage@q.com>]

Kein-Hong and Ben:

Looking at the user's manual for the programming language of the second 
test, I see that the calculations were made using the PC's "80-bit, extended 
real" format.  I believe that means that the mantissa is kept as an 80-bit 
number.  It appears that the other formats available on the PC's FPU are 
64-bit for "double precision" and 32-bit for "single precision".  Using 
doubles instead of floats in the MinGW version of the test program, I get 
the following results:

Pixel_X = 100  x = 1.00000140625000000000   1.000001406250000
Pixel_X = 101  x = 1.00000142031250010000   1.000001420312500
Pixel_X = 102  x = 1.00000143437500010000   1.000001434375000
Pixel_X = 103  x = 1.00000144843749990000   1.000001448437500
Pixel_X = 104  x = 1.00000146250000000000   1.000001462500000
Pixel_X = 105  x = 1.00000147656250000000   1.000001476562500
Pixel_X = 106  x = 1.00000149062500010000   1.000001490625000
Pixel_X = 107  x = 1.00000150468749990000   1.000001504687500
Pixel_X = 108  x = 1.00000151875000000000   1.000001518750000
Pixel_X = 109  x = 1.00000153281250000000   1.000001532812500
Pixel_X = 110  x = 1.00000154687500010000   1.000001546875000
Pixel_X = 111  x = 1.00000156093749990000   1.000001560937500
Pixel_X = 112  x = 1.00000157500000000000   1.000001575000000
Pixel_X = 113  x = 1.00000158906250000000   1.000001589062500
Pixel_X = 114  x = 1.00000160312500010000   1.000001603125000
Pixel_X = 115  x = 1.00000161718749990000   1.000001617187500
Pixel_X = 116  x = 1.00000163125000000000   1.000001631250000
Pixel_X = 117  x = 1.00000164531250000000   1.000001645312500
Pixel_X = 118  x = 1.00000165937500010000   1.000001659375000
Pixel_X = 119  x = 1.00000167343749990000   1.000001673437500

which are pretty much identical to the third column.  So in this test case, 
it was just a difference in the precision of the calculations.  In my 
Mandelbrot program, I actually rewrote it to use doubles, rather than 
floats, and I didn't see much difference.  Perhaps I did something wrong, or 
perhaps it only makes a small difference.  Regardless, mystery solved. 
Thanks for your help.


Regards,

Rich Wagner