Re: Floating point endian issue

mcintosh@bellcore.com (Allen Mcintosh)
25 Oct 1996 22:09:46 -0400

          From comp.compilers
Related articles
Floating point endian issue tomb@microware.com (Tom Brasier) (1996-10-24)
Re: Floating point endian issue mcintosh@bellcore.com (1996-10-25)
Floating point endian issue dave@occl-cam.demon.co.uk (Dave Lloyd) (1996-10-25)
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From: mcintosh@bellcore.com (Allen Mcintosh)
Newsgroups: comp.compilers
Date: 25 Oct 1996 22:09:46 -0400
Organization: Bellcore, Morristown NJ
References: 96-10-123
Keywords: architecture, arithmetic

Tom Brasier <tomb@microware.com> wrote:


>On a big endian machine, I might declare two constants as so
>
>_value dc.l 0x01234567,0x89abcdef
>
>On a little endian machine, I can have either...
>_value dc.l 0x67452301,0xefcdab89
>-or-
>_value dc.l 0xefcdab89, 0x67452301


The answer (on a DECstation - little-endian MIPS chip) is "none of the above":


_value dc.l 0x89abcdef,0x01234567


I ran the following as a test:


#include <math.h>
main() {
                union { double d; int i[2]; }ch;
                ch.d = 4.0 * atan(1.0);
                printf("pi = %.16f = 0x%08x 0x%08x\n", ch.d, ch.i[0], ch.i[1]);
}


On a SPARC, I get this:


pi = 3.1415926535897931 = 0x400921fb 0x54442d18


and on a DECstation, I get this:


pi = 3.1415926535897931 = 0x54442d18 0x400921fb


This is consistent - bytes get stuffed into FP registers in order from
the least significant bits to the most significant bits and the exponent.
--

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