The HLA Standard Library reference manual contains lots of information about the HLA Standard Library's extended-precision arithmetic capabilities. You'll also want to check out the source code for several of the HLA Standard Library routines to see how to do various extended-precision operations (that properly set the flags once the computation is complete). The HLA Standard Library source code also covers the extended-precision I/O operations that do not appear in this chapter.
Donald Knuth's The Art of Computer Programming, Volume Two: Seminumerical Algorithms contains a lot of useful information about decimal arithmetic and extended-precision arithmetic, though that text is generic and doesn't describe how to do this in x86 assembly language.