It’s time to write our own fmod() equivalent; lucky for us the Intel guys were kind enough as to implement quite a useful instruction called FPREM a few eons ago, which pretty much means “Partial REMinder” and what it does is rather simple: it computes the partial remainder of st(0)/st(1).

Procedure.f fmod( x.f, y.f )
  EnableASM
	  FLD DWORD[p.v_y]
	  FLD DWORD[p.v_x]
	  !NOT_READY:
		  FPREM
		  FSTSW AX
		  TEST AH, 100b
	  !JNZ NOT_READY
	  FSTP ST1
  DisableASM
  ProcedureReturn
EndProcedure

Of course with great instructions comes great limitations: in this case we cannot produce the final reminder if the difference between X and Y is bigger than 63. We can solve this by implementing a loop and checking for the C2 bit of the status word to be cleared. This is all explained in the IA32 manuals.

It’s not pretty but until fpu modulo operations are officially supported in the language, this might well be your only sane choice.