Date: Thu, 12 Jan 2006 17:34:13 +0000
From: Russell King <rmk+lkml@arm.linux.org.uk>
To: David Vrabel <dvrabel@arcom.com>
Cc: Mikael Pettersson <mikpe@csd.uu.se>, linux-kernel@vger.kernel.org,
       linux-usb-devel@lists.sourceforge.net, oliver@neukum.org
Subject: Re: [linux-usb-devel] Re: need for packed attribute
Message-ID: <20060112173413.GD9288@flint.arm.linux.org.uk>
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	Mikael Pettersson <mikpe@csd.uu.se>, linux-kernel@vger.kernel.org,
	linux-usb-devel@lists.sourceforge.net, oliver@neukum.org
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On Thu, Jan 12, 2006 at 05:22:47PM +0000, David Vrabel wrote:
> Russell King wrote:
> > BTW, it's worth noting that the new EABI stuff has it's own set of
> > problems.  We have r0 to r6 to pass 32-bit or 64-bit arguments.
> > With EABI, 64-bit arguments will be aligned to an _even_ numbered
> > register.
> 
> Is there a reason for this alignment requirement?

I think it comes from the 64-bit accessing instructions (ldrd/strd)
having the restriction that they only take an even numbered 32-bit
register.  The immediately consecutive higher numbered 32-bit 
egister is used as the other half of the number.

Think about it as the x86 32-bit eax register being made up of
16-bit ah and al registers.  Only we call then r0, r1 etc not
eax, ah and al (and they're twice the size.)

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core
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