linux-kernel - Re: [x86 setup 13/33] Header file to produce 16-bit code with gcc

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Date:	Tue, 10 Jul 2007 20:26:18 -0400
From:	Brian Gerst <bgerst@...ntduck.org>
To:	Pawel Dziepak <hryssta@...il.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	Andi Kleen <ak@...e.de>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [x86 setup 13/33] Header file to produce 16-bit code with gcc

Pawel Dziepak wrote:
> On 7/10/07, Segher Boessenkool <segher@...nel.crashing.org> wrote:
>> > The alternative, of course, is to compile to an .s file and insert
>> > .code16gcc into the .s file.  This makes the Makefile uglier, but
>> > would
>> > be more resilient against oddball gcc changes.
>>
>> This would be even more fragile.  The exact format of GCC's
>> assembler code output isn't defined at all, so in principle
>> this is a hopeless task.  In practice just putting the
>> .code16gcc directive on the first line would likely work
>> though, GCC never generates a .code32 AFAIK, but it isn't
>> guaranteed that this will work (or will keep working).
> 
> Unfortunately, .code16gcc is still experimental (at least binutils'
> website says that). What is worse, it says that it is possible that
> 16bit code produced on GCC won't work on pre-80386 processors (before
> switching to protected mode you have to think about cpu as a
> pre-80386).

What .code16gcc does is produce 32-bit instructions that are meant to be
run from a 16-bit code segment (real mode or 16-bit protected) by using
address and data size override prefixes.  This means that pre-386
processors cannot run the code because they cannot understand 32-bit
instructions.

--
				Brian Gerst
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