Path: utzoo!utgpu!water!watmath!clyde!rutgers!cmcl2!brl-adm!umd5!eneevax!noise
From: noise@eneevax.UUCP (Johnson Noise)
Newsgroups: comp.lang.c
Subject: Re: register unions
Keywords: registers unions structures
Message-ID: <1231@eneevax.UUCP>
Date: 14 Feb 88 00:37:52 GMT
References: <1229@eneevax.UUCP> <6831@ccv.bbn.COM>
Reply-To: noise@eneevax.umd.edu.UUCP (Johnson Noise)
Distribution: comp.lang.c
Organization: Elec. Eng. Dept., U of Maryland, College Park, MD 20742
Lines: 52

In article <6831@ccv.bbn.COM> kgregory@ccv.bbn.com (Keith D. Gregory) writes:
>In article <1229@eneevax.UUCP> noise@eneevax.umd.edu.UUCP 
>(Johnson Noise) writes:
>> [code examples deleted]
>>
>
>struct mystruct {
>	char	field1, field2, field3, field4;
>	};
>
>union myunion {
>	int	field1;
>	char	field2[2];
>	};
>
>
>The first example of course, would fail on the 68000 right away - you
>can't arbitrarily split a register into 4 bytes.  The second is simply
>more of the same, and the key point is that structures and unions are
>manipulated using pointer arithmetic (to access the fields therein), and
>pointers to registers can not exist.
>

	I understand now, but for slightly different reasons.  In the
code examples I gave in my previous posting, pointer arithmetic was
not necessary to access the different fields.  The problem is with
most/least significant byte/word.

move.b _blah, d0 /* and */
move.w _blah, d0

move data into the least significant part of the register.  Where

move.b _blah, _there /* and */
move.w _blah, _there

move data into the most significant part of _there (assuming _there is
aligned to a longword address).  This is obviously inconsistent.  So
what about address registers (pointers)?  Pointers and adresses are always
32 bits and occupy the same position whether in memory or registers.
Some examples:

68k assembly		C source

movea _i, a0		a0.l = &i;	i must be long
movea _j, a0		a0.w = &j;	j must be short or long
movea _k, a0		a0.b = &k;	k may be char, short, or long

move.b (a0)+, (a1)+	*a1.b++ = *a0.b++;
move.l -(a0), -(a1)	*--a1.w = *--a0.w;

etc.  Again, comments, flames?