Summary: Don't assume the math coprocessor is available to the DLL.
A stand-alone DLL—that is, a DLL created for general use by many programs— can make few assumptions about the calling program. Therefore, the safest way to perform floating-point calculations is to use alternate math routines. If you link to a Microsoft high-level language, you can access these routines through a language library. These routines give the fastest results possible without a coprocessor. See Section 6.3, “Using Emulator Libraries,” for more information.
Floating-point operations in DLLs can use a coprocessor or emulator routines if you are certain that a coprocessor or emulator libraries are available.