linux-kernel - Re: [RFC] Documentation about unaligned memory access

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Date:	Sun, 25 Nov 2007 12:16:08 +0100 (CET)
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Heikki Orsila <shdl@...alwe.fi>
Cc:	Daniel Drake <dsd@...too.org>, linux-kernel@...r.kernel.org,
	davem@...emloft.net, kune@...ne-taler.de, johannes@...solutions.net
Subject: Re: [RFC] Documentation about unaligned memory access

On Fri, 23 Nov 2007, Heikki Orsila wrote:
> On Fri, Nov 23, 2007 at 12:15:53AM +0000, Daniel Drake wrote:
> > Why unaligned access is bad
> > ===========================
> > 
> > Most architectures are unable to perform unaligned memory accesses. Any
> > unaligned access causes a processor exception.
> 
> "Some architectures are unable to perform unaligned memory accesses, 
> either an exception is generated, or the data 
> access is silently invalid. In architectures that allow unaligned 
> access, natural aligned accesses are usually faster than non-aligned."
> 
> > In summary: if your code causes unaligned memory accesses to happen, your code
> > will not work on some platforms, and will perform *very* badly on others.
> 
> *very* -> *slower*
> 
> > Natural alignment
> > =================
> 
> Please move this definition before "Why unaligned access is bad".
> 
> Also, it would be nice to have a table of ISAs:
> 
> ISA		Need		Need
> 		natural		alignment
> 		alignment	by x
> --------------------------------------------
> m68k		No		2

`No' for >= 68020.
`Yes' for < 68020.

> powerpc/ppc	Yes		Word size
> x86		No		No
> x86_64		No		No

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds
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