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Date: Sun, 3 Nov 1996 03:31:12 +1100
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199611021631.DAA10756@godzilla.zeta.org.au>
To: freebsd-hackers@FreeBSD.org, j@uriah.heep.sax.de
Subject: Re: VM answer requested
Sender: owner-hackers@FreeBSD.org
X-Loop: FreeBSD.org
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>> I'd like to be able to turn on the bit so that I get a fault (and it
>> kills the offending process) when an unaligned access occurs.
>
>Adding support for this should be fairly simple.  I wonder why you

Especially since it was added on 1995/01/14 in rev.1.46 of locore.s :-).
Applications have to set PSL_AC when they want alignment faults.
Alignment faults generate SIGBUS.

gcc still generates misaligned accesses, e.g.,

---
struct foo {
    short x[2];
};

struct bar {
    short pad;
    struct foo r;
} bar1, bar2;

main()
{
    asm("pushfl; popl %eax; orl $0x40000,%eax; pushl %eax; popfl");
    bar1.r = bar2.r;		/* trap */
}
---

Bruce