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\subsection{Handling IEEE floating point constants}
Here we take care of converting floating point constants from
string representation to 64-bit IEEE representation.
We use the machinery developed for the Sparc by John Reppy.

Reppy's functor accepts a simple structure with a single value,
[[emitWord : int -> unit]], which emits a 16-bit word.
It produces a [[PRIMREAL]].
When [[RealConst]] is applied to the result, it produces a 
structure containing a single function, [[val realconst : string -> unit]].
This function, when applied to a string, emits the four sixteen-bit words
of the IEEE representation, most significant first.

Our job will be to convert this to something that emits the two 32-bit
words of the constant, least significant word first.
First, let's consider the state information that has to be retained
while the halfwords are being emitted, and functions that change that state.
<<state info>>=
val halfwords = ref nil : int list ref		(* halfwords already out *)
val count = ref 0				(* length of halfwords *)
fun reset_state () = (halfwords := nil; count := 0)
fun add_half h = (count := !count + 1; halfwords := h :: (!halfwords))

<<emitting a halfword>>=
fun emit_half h = 
    if !count = 3 then (emit_four (h::(!halfwords)); reset_state())
    else add_half h

@ To emit the whole list, we have to emit the words, one at a time.
We use descriptive names to remind ourselves what is signficant
(highest is most significant).
<<emitting four halfwords>>=
fun emit_four [lowest,low,high,highest] = 
			(emit_word(low,lowest);emit_word(highest,high))
  | emit_four _ = ErrorMsg.impossible "bad floating pt constant in mips"

@ Now, we bundle up the whole thing in a functor that
gets passed a structure holding [[emit_word]] and returns 
one containing [[realconst]].
<<*>>=
functor MipsReal(E: sig val emit_word : int * int -> unit end) : REALCONST = 
struct
    open E
    <<state info>>
    <<emitting four halfwords>>
    <<emitting a halfword>>
    structure IEEERealConst =
	RealConst(IEEEReal(struct val emitWord = emit_half end))
    val realconst = IEEERealConst.realconst
end

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