Path: utzoo!utgpu!attcan!uunet!husc6!cmcl2!rutgers!ucsd!nosc!humu!uhccux!mikem
From: mikem@uhccux.uhcc.hawaii.edu (Mike Morton)
Newsgroups: comp.sys.m68k
Subject: quad-aligning the 68020 stack
Keywords: 68020 alignment quad
Message-ID: <2194@uhccux.uhcc.hawaii.edu>
Date: 5 Aug 88 05:12:28 GMT
Organization: University of Hawaii
Lines: 26


Maybe this is old hat to all you '020 hackers, but I've been wondering about
aligning stack frames on four-byte boundaries.  I assume that in certain
critical situations, you should make sure your local variables are aligned
for speed.

Is there a standard way to do this?  I'll post these code fragments, in
hopes of getting someone to post something better.  These are tested on a
68000, since that's all I have.

/*  To quad-align the stack, execute this in-line.  Uses D0.w and leaves
    2 or 4 bytes on the stack. */
    move.w    sp, d0                    /* copy low bits to get alignment */
    addq.w    #2, d0                    /* toggle quad-align bit (#1) */
    and.w     #2, d0                    /* select just that bit */
    sub.w     d0, sp                    /* MIS-align the stack */
    move.w    d0, -(sp)                 /* then fix it and remember bias */

/*  To restore the stack state, execute this in-line. */
    add.w     (sp)+, sp                 /* pop bias and un-bias */

Is alignment desirable?  Is there a shorter way to do it?  Comments?

 -- Mike Morton // P.O. Box 11378, Honolulu, HI  96828, (808) 456-8455 HST
      Internet: msm@ceta.ics.hawaii.edu
    (anagrams): Mr. Machine Tool; Ethical Mormon; Chosen Immortal; etc.