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From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.c
Subject: Re: Beginning C question.
Message-ID: <3472@goanna.cs.rmit.oz.au>
Date: 26 Jul 90 01:39:44 GMT
References: <10997@chaph.usc.edu> <7703@uudell.dell.com> <25440@nigel.udel.EDU> <4561@cvl.umd.edu>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 27

In article <4561@cvl.umd.edu>, arensb@cvl.umd.edu (Andrew Arensburger) writes:
> In article <25440@nigel.udel.EDU> gdtltr@freezer.it.udel.edu (Gary Duzan) writes:
> >In article <7703@uudell.dell.com> jrh@mustang.dell.com (James Howard) writes:
> >=>        short a=0x20df;
> >=>        short b=0x3244;
> >=>        int   c;
> >=>
> >=>	c = (a<<16) + b;
> >=>
> >=>I used "short" because they're 16 bits on this machine, and ints are 32.

> 	James's example works correctly because the following assumptions
> are true:

Bad news, friends, the example DOESN'T work correctly, even if the
half-word sex of the machine is right.  Suppose the high bit of b is 1.
That's the _sign_ bit, remember.  The effect in that case is as if 1
were subtracted from a.  (I tried it, and that _does_ happen.)

Instead, use
	c = (a << 16) | (b & 0xFFFF);
or make a and b "unsigned short" and do
	c = (int)((a << 16) + b);

-- 
Science is all about asking the right questions.  | ok@goanna.cs.rmit.oz.au
I'm afraid you just asked one of the wrong ones.  | (quote from Playfair)