linux-kernel - Re: __LITTLE_ENDIAN vs. __LITTLE_ENDIAN

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Message-Id: <AECA61F9-0EF6-4883-9D37-004C00C57417@cam.ac.uk>
Date:	Fri, 5 Oct 2007 22:06:00 +0100
From:	Anton Altaparmakov <aia21@....ac.uk>
To:	Timur Tabi <timur@...escale.com>
Cc:	Jan Engelhardt <jengelh@...putergmbh.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: __LITTLE_ENDIAN vs. __LITTLE_ENDIAN_BITFIELD

On 5 Oct 2007, at 20:35, Timur Tabi wrote:
> Jan Engelhardt wrote:
>> On Oct 5 2007 13:27, Timur Tabi wrote:
>>> What's the difference between __LITTLE_ENDIAN and  
>>> __LITTLE_ENDIAN_BITFIELD? Can
>>> someone give me an example when __BIG_ENDIAN and  
>>> __LITTLE_ENDIAN_BITFIELD would
>>> both be defined simultaneously?
>> standard x86:
>> ---LSB-- ---2SB-- ---3SB-- ---MSB-- [bytes] LITTLE_ENDIAN
>> M765432L M765432L M765432L M765432L  [bits] ?_BITFIELD
>> (Not sure what bitfield type, but I'd guess BIG_ENDIAN_BITFIELD)
>
> Are you sure?  I would think that all machines would have the same  
> byte and bit endian, otherwise you'd never be able to put a 16-bit  
> value into a shift register.  Your bits will be shifted out like this:
>
> <-- 07 06 05 04 03 02 01 00 15 14 13 12 11 10 09 08
>
> So I think x86 is:
>
> ---LSB-- ---2SB-- ---3SB-- ---MSB-- [bytes] LITTLE_ENDIAN
> L234567M L234567M L234567M L234567M  [bits] LITTLE_ENDIAN_BITFIELD

No it is not.  That makes no sense.  The whole point of little endian  
is that you store LSB, then 2SB, then 3SB, then MSB and then when the  
CPU reads this as a 32-bit word it rotates them all around so that in  
the CPU register you have:

MSB_3SB_2SB_LSB
M765432L_M765432L_M765432L_M765432L

That is what little endian means and that is how shift operations can  
work fine on the CPU.

Best regards,

	Anton
-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer, http://www.linux-ntfs.org/


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