Date: Wed, 29 May 1996 05:25:44 +1000 From: Bruce Evans <bde@zeta.org.au> To: bde@zeta.org.au, terry@lambert.org Cc: current@FreeBSD.ORG, scrappy@ki.net Subject: Re: memset/memcopy vs bzero/bcopy Message-ID: <199605281925.FAA31591@godzilla.zeta.org.au>
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>If we have a processor where we can turn on unaligned access exceptions,
>and we turn them on, how much of the kernel code starts failing? I would
None, because alignment checking is only done in ring 3 :-). On
Pentiums you can count misaligned data references in counter 11.
>like to see this flag on by default for P5 or better systems... the
>better to encourage writing code that can run on RISC systems.
The library certainly isn't well aligned. The following example dumps
core in printf:
int ef;
main()
{
asm("pushfl; orl $0x40000,(%esp); popfl");
asm("pushfl; popl _ef");
printf("%x", ef);
asm("pushfl; andl $~0x40000,(%esp); popfl");
}
It works if 0 or 1 is printed instead of `ef' but fails if 0x40000 is
printed instead of `ef'
I know of only one compiler bug in this area. The builtin memcpy()
doesn't bother aligning things. The builtin memcpy() is used for
structs that aren't necessarily aligned.
Bruce
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