Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!amdcad!ames!sdcsvax!sdcc6!calmasd!gjo
From: gjo@calmasd.GE.COM (Glenn Olander)
Newsgroups: comp.arch
Subject: RISC data alignment
Message-ID: <2635@calmasd.GE.COM>
Date: 23 Jan 88 02:11:23 GMT
Reply-To: gjo@calmasd.UUCP (Glenn Olander)
Organization: GE Calma, San Diego R&D
Lines: 21

Please forgive a possibly neophyte-type question, but is it true that
there may be an inherent incompatibility between RISC and conventional
machines?  In particular, I believe that many RISC machines require data
to be aligned on a natural boundary, e.g. longwords must be referenced
on a 4-byte boundary.  This requires compilers to make accomodations to
ensure that such alignment always occurs, even if it means padding a
data structure which contains mixed types of data, for example a C
structure with a mixture of shorts, longs, and doubles.

If this is true, then it would seem to also be true that a C structure
could have different lengths, depending on whether it was compiled 
on a RISC or non-RISC machine.  Further, it would seem that
if that C structure were written out to a file, it could only be read
properly by a machine of the same type as that which wrote it.

Does such incompatibilty truly exist?  If I create a file on a Sun/4
will I be able to read it on a Sun/3?

Glenn Olander
GE Calma
gjo%calmasd.ge.com@sdcsvax.ucsd.edu