The use alternate math option (/FPa) generates floating-point calls and selects the alternate math library for the appropriate memory model (mLIBCA.LIB). Calls to this library provide the fastest and smallest option for code intended to run on a machine without an 80x87 coprocessor. With this option, you can choose an emulator library (mLIBCE.LIB) or a math coprocessor library (mLIBC7.LIB) at link time.
You cannot link with an alternate math library if your program uses the intrinsic forms of floating-point library routines (that is, if you have compiled the program with the /Oi or /Ox option, selected the Generate Intrinsic Functions option from the Optimizations dialog box in the Programmer's WorkBench, or specified math functions in an intrinsic pragma).