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Path: utzoo!mnetor!seismo!sundc!hadron!jsdy
From: jsdy@hadron.UUCP (Joseph S. D. Yao)
Newsgroups: comp.lang.c,comp.bugs.4bsd
Subject: Re: Bug converting unsigned to double in BSD 4.[23]
Message-ID: <618@hadron.UUCP>
Date: Tue, 18-Nov-86 08:46:47 EST
Article-I.D.: hadron.618
Posted: Tue Nov 18 08:46:47 1986
Date-Received: Tue, 18-Nov-86 21:07:34 EST
References: <213@cartan.Berkeley.EDU>
Reply-To: jsdy@hadron.UUCP (Joseph S. D. Yao)
Followup-To: comp.lang.c
Organization: Hadron, Inc., Fairfax, VA
Lines: 29
Summary: On VAXen, being too tricky.
Xref: mnetor comp.lang.c:84 comp.bugs.4bsd:20

In article <213@cartan.Berkeley.EDU> ballou@brahms (Kenneth R. Ballou) writes:
>main ()
>{
>  printf ("%u\n", ~ ((unsigned) 0));
>  printf ("%lf\n", (double) (~ ((unsigned) 0)));
>  printf ("%lf\n", 4294967295.0); /* surely double is large enough for this? */
>}
>% bug
>4294967295
>-1.000000
>4294967295.000000

This didn't work (actually, worked as expected!) on our ISI 68000-
based machine.  It worked as above when I compiled and ran it on
the VAX.  What's happening on the VAX is that it's folding the
constant (~((unsigned int) 0)) to the bit pattern $-1, while the
68K is clearing and negating a register: no problem so far!  But
apparently type is lost or ignored on the VAX, because it then
does a cvtld (convert long to double).  This is clever-not-clever
use of the VAX FP instruction set, which doesn't have an unsigned
long data type.  The 68K calls a subroutine:
	jbsr	dpufloat
which comprehends and translates unsigned longs, as dpfloat does
for signed longs.  (Doing it in software has occasional advantages
over doing it in hardware.)
-- 

	Joe Yao		hadron!jsdy@seismo.{CSS.GOV,ARPA,UUCP}
			jsdy@hadron.COM (not yet domainised)