In the case of X86, even if using IEEE single and double precision numbers, the internal calculations are affected by a floating point control word (FCW). The internal calculations are normally 64 bit or 80 bit (long double). You can override this using inline assembly code, but there's no guarantee that some double precision library function won't set it back.
Microsoft supported 80 bit long doubles with their 16 bit compilers, but dropped support for them with their 32 bit and 64 bit compilers, and long doubles are now the same as doubles at 64 bits.
