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

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Date:	Mon, 26 Nov 2007 17:12:23 +0000
From:	Ben Dooks <ben@...ff.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, Nov 23, 2007 at 01:43:29PM +0200, 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
> powerpc/ppc	Yes		Word size
> x86		No		No
> x86_64		No		No
arm32		Yes		2 for 16bit data, 4 for 32bit

Note, if the unaligned handler is running, the alignment will be fixed
by the fault handler (at the cost of taking a fault). If the unaligned
handler is turned off, you get a "free" shift of the data instead.

-- 
Ben (ben@...ff.org, http://www.fluff.org/)

  'a smiley only costs 4 bytes'
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