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When a source file is compiled by Watcom C/C++ with one of the "fpi" or
"fpi87" options, all floating-point arguments are passed on the 80x86 stack.
 The rules for passing arguments are as follows.

 1. If the argument is not floating-point, use the procedure described
    earlier in this chapter.

 2. If the argument is floating-point, it is assigned a position on the
    80x86 stack.

+--------------------------------------------------------------------------+
|   Note:  When compiling using the "fpi" or "fpi87" options, the method   |
| used for passing floating-point arguments in the stack-based calling     |
| convention is identical to the method used in the register-based calling |
| convention.  However, when compiling using the "fpi" or "fpi87" options, |
| the method used for returning floating-point values in the stack-based   |
| calling convention is different from the method used in the              |
| register-based calling convention.  The register-based calling           |
| convention returns floating-point values in ST(0), whereas the           |
| stack-based calling convention returns floating-point values in EDX and  |
| EAX.                                                                     |
+--------------------------------------------------------------------------+