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Date: Tue, 05 Feb 2008 13:23:27 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Sam Ravnborg <sam@ravnborg.org>
CC: Ingo Molnar <mingo@elte.hu>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org
Subject: Re: [git pull] x86 updates
References: <20080204161219.GA8092@elte.hu> <alpine.LFD.1.00.0802051043550.9374@woody.linux-foundation.org> <20080205192236.GA26426@uranus.ravnborg.org> <20080205210508.GA14708@elte.hu> <47A8D088.7050209@zytor.com> <20080205211854.GB27589@uranus.ravnborg.org>
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Sam Ravnborg wrote:
>>
>> Technically we should also specify @progbits or @nobits on sections, 
>> however, I think @progbits is the default.
> 
> I have on my todo list to do so. But I wanted to investigate a bit
> more since I do not yet fully understand the difference.
> 
> And we have to use %progbits and %nobits as ARM uses '"' to indicate
> comments.
> 

The difference is quite simple:

	a		- this section occupies address space
	w		- this section is writable
	x		- this section is executable
	progbits	- this section has data in the file

In contrast, a nobits section is implicitly zero-filled.

By default:

.bss 	is (aw,nobits).
.data 	is (aw,progbits).
.rodata	is (a,progbits).
.text	is (ax,progbits).

Since .bss is nobits, it doesn't occupy any space in the executable, and 
is zero-filled by the loader.

	-hpa