I have a large Win32 DLL (10 MB) that is called from
my user interface (written in C++) or from VBA in MS
Excel. In my user interface, the DLL runs in its
own space and calculates correctly. Under Excel VBA,
my DLL is having problems with double precision
accuracy. The following test passes in my user
interface but fails under my bad pentium test:

double precision chptst
double precision divtwo
double precision top
double precision bottom
data top / 4195835.0D0 /
data bottom / 3145727.0D0 /
DIVTWO = top / bottom
CHPTST = (DIVTWO * bottom) - top

In my user interface, the chptst result is zero.
Under Excel VBA, the chptst result is 0.2851266E-09.

I have tried resetting the math coprocessor in my
DLL with the following code but it is not working:

unsigned old87Status = 0;
unsigned new87ControlWord = 0;
unsigned new87ControlMask = 0;
unsigned new87result = 0;
old87Status = _status87 ();
if (old87Status != 0)
new87result = _control87 (new87ControlWord, new87ControlMask);

I have verified this behavior in both Excel 2003
and 2010. Does anyone have any ideas here ?

Sincerely,
Lynn McGuire