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From: jrb@petro.UUCP (Jon Boede)
Newsgroups: comp.arch,comp.lang.c
Subject: Byte .vs. Word Alignment
Message-ID: <244@petro.UUCP>
Date: Wed, 15-Apr-87 08:35:44 EST
Article-I.D.: petro.244
Posted: Wed Apr 15 08:35:44 1987
Date-Received: Sun, 19-Apr-87 05:15:08 EST
Sender: news@petro.UUCP
Organization: G.M. Andreen & Associates, Inc. San Antonio, Texas
Lines: 32
Keywords: C, 680X0, Alignment
Xref: mnetor comp.arch:968 comp.lang.c:1739
Environment: Tandy 6000, XENIX 3.01.02, MC68000


This is probably the most trivial of questions, but I've always wondered if
wordwise alignment affected efficiency on the 68000.

For example, assuming that a structure like:

	struct {
		int an_int;
		char a_char;
	} a_struct;

starts on a word boundry, would a structure placing the /char/ first and the
/int/ therefore a 1/4 word off be less, can we say, "efficient" in terms of
the manipulation needed to get to /an_int/??  Or doesn't the C compiler always
start structures on words boundries, making alignment consideration by the
programmer meaningless?

	struct {
		char a_char;
		int an_int;
	} a_struct;	/* is this less "efficient"? */

Also, whilst I'm on the subject, how much extra effort is required of the
68000 to get to specific bits than to, say, an int?  I often use structures
of bit flags to save space in files.  I can easily compute the space savings,
but an accurate guess as to the time savings is a little more elusive.

Thanks!
-- 
Jon Boede				...!{ihnp4,akgua,gatech}!petro!jrb
512/599-1847				2555 N.E. Loop 410, #1403, 78217
	"People who are incapable of making decisions are
	 the ones who hit those barrels at freeway exits."