Xref: utzoo comp.lang.c:29483 comp.unix.ultrix:3712 comp.sys.ibm.pc:52244 comp.sources.wanted:12053
Path: utzoo!dciem!nrcaer!sce!cognos!craigs
From: craigs@cognos.UUCP (Craig Statchuk)
Newsgroups: comp.lang.c,comp.unix.ultrix,comp.sys.ibm.pc,comp.sources.wanted
Subject: Re: Routine to convert between IEEE and VAX floating point ?
Message-ID: <8452@cognos.UUCP>
Date: 7 Jun 90 16:30:34 GMT
References: <1023@rna.UUCP> <1990Jun3.084602.1662@noao.edu>
Reply-To: craigs@cognos.UUCP (Craig Statchuk)
Followup-To: comp.lang.c
Organization: Cognos Inc., Ottawa, Canada
Lines: 60

>From article <1023@rna.UUCP>, by dan@rna.UUCP (Dan Ts'o):
>> 
>> 	Does anyone have a C routine to convert from IEEE floating point
>> to VAX D format floating point ?
>

I had to do this a while back. I found it easiest to convert 
IEEE -> G_FLOAT ->D_FLOAT. As mentioned in another response all 
you have to do swap bytes and change the exponent to go from 
IEEE to G_FLOAT. The VAX RTL has the necessary routine to
change from G_FLOAT to D_FLOAT (and has another one if you wish to go
back).

Here's some code that will do the translation on the (using
bit fields yeah!):

union G_float_template {
  double asDouble;
  struct {
    unsigned long b1:16,b2:16,b3:16,b4:16;
  } asBits;
  struct {
    unsigned long mantissaL:20,exponent:11,sign:1,mantissaH:32;
  } asFields;
};

union G_float_template value;
unsigned short s_temp;
double result;
extern double MTH$CVT_G_D();
...
/* put some value into the template -- assume it is really in IEEE format */
value.asDouble = (IEEE) 1234.5

/* exchange upper 16 bits with lower 16 bits in each word */
s_temp = value.asBits.b1;
value.asBits.b1 = value.asBits.b2;
value.asBits.b2 = s_temp;
s_temp = value.asBits.b3;
value.asBits.b3 = value.asBit.b4;
value.asBits.b4 = s_temp;

/* normalize exponent bias */
if (value.asDouble != 0.0)
  net_value.asFields.exponent -= 2;

/* convert to D_FLOAT */
result = MTH$CVT_G_D(&value.asDouble);

Easy eh?

/CS



-- 
Craig Statchuk           USENET   : uunet!mitel!sce!cognos!craigs
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