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Date: Fri, 09 Nov 2001 09:09:09 -0800
From: Ken Rose <kenarose@earthlink.net>
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To: "Rafael 'Dido' Sevilla" <sevillar@team.ph.inter.net>
CC: Caml List <caml-list@pauillac.inria.fr>
Subject: Re: [Caml-list] how to split up a Caml float into its component bytes
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Rafael 'Dido' Sevilla wrote:
> 
> I've been writing a byte compiler for a small language using Objective
> Caml, and now am thinking about incorporating floating point support
> into the language.  I'm wondering how I would convert a floating point
> number in OCaml (which I hope I am safe in assuming is IEEE-754) into
> its equivalent bytes.  I need it to be able to output bytecode
> instructions that will load floating point constants into the virtual
> machine.  In C this is fairly trivial to do; not sure how to do it in
> OCaml.

I was having a similar problem and came up with these two functions,
intended for a C compiler.  They produce strings encoding 32 bits at a
time, to write into assembly output.  I'm not sure it handles gradual
underflow correctly, but since my target hardware doesn't, it didn't
matter to me.

 - ken

and cook_float f =
  let mant, exp = frexp f in 
  let sign = if mant < 0.0 then Int32.min_int else Int32.zero in
  let exponent = Int32.shift_left (Int32.of_int(exp + 126)) 23 in
  let mantissa = Int32.of_float((mant -. 0.5) *. 16777216.0) in
  Int32.format "0x%x" (Int32.logor sign (Int32.logor exponent mantissa)) 

and cook_double d =
  if d = 0.0 then ("0","0") else
  let mant, exp = frexp d in
  let sign = if mant < 0.0 then Int32.min_int else Int32.zero in
  let exponent = Int32.shift_left (Int32.of_int(exp + 1022)) 20 in
  let f,t = modf (((abs_float mant) -. 0.5) *. 2097152.0) in
  let highmantissa = Int32.of_float(t) in
  let highword = Int32.logor sign (Int32.logor exponent highmantissa) in
  let l, h = modf (f *. 65536.0) in
  let low1 = Int32.shift_left (Int32.of_float h) 16 in
  let low2 = Int32.of_float (l *. 65536.0) in
  let lowmantissa = Int32.of_float(f *. (ldexp 1.0 32)) in
  let lowword = Int32.logor low1 low2 in
  Int32.format "0x%x" highword, Int32.format "0x%x" lowword
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