Well the x87 does all its calculations in 80-bit, it just converts floats and doubles automatically as it reads from/writes to memory. The C library has special functions for the "long double" type which is basically the full 80-bit precision, but ever since the Microsoft compiler moved to Win32, "long double" is resolved to a 64-bit precision instead. The type still exists, but it has the same precision as a normal double. So, using a Microsoft compiler, there is no easy way to work with extended precision numbers. You pretty much have to write inline assembler code to do it.
Speaking of which, once the compiler moves to Win64, support for inline assembler will also be removed (God knows why). Microsoft intends to replace it with compiler intrinsics, i.e. "functions" that correspond to certain instructions but still lets the compiler handle most of the details. Anyone who ever tried using the MMX or SSE intrinsics know, however, that the code is wildly inefficient (often creates stack variables for things that could be done using only registers, etc).
