From: Brian Dean <brdean AT unx DOT sas DOT com>
Newsgroups: comp.os.msdos.djgpp,gnu.gcc.help
Subject: Re: embedded 16 bit, real mode, x86 target for gcc?
Date: Wed, 03 May 2000 14:15:44 -0400
Organization: The SAS Institue Inc.
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

Karl Rasmussen wrote:
> 
> I am writing some low level IO code (essentially BIOS equivalent) for an
> embedded project. If possible, C would be prefered to assembly language
> (goal: 90% C, 10% assembly or better). We need some code to be 16 bit
> compatible for legacy interface requirements. For other parts, 32 bit is
> fine, though not required.
> 
> I was hoping to find a gcc back end for this. If it would look like a 16
> bit x86 cross compiler, that would be great. Our development environment
> is 32 bit x86, Linux or Win 9x, NT 4.
> 
> I was excited to find DJGPP, but from the FAQ I get the message this is
> not what DJGPP is. Nor did any of the other gcc builds or compilers seem
> to claim to do this. Did I miss something? Is there any gcc based
> solution? Or do I need to get an old Borland or MS 16 bit compiler?
> 
> The DJGPP solution with dpmi services is quite clever, but I can't use
> that trick. The code I'm writing is already at the level that is
> supposed to be implementing those functions for real.
> 
> (Yes, I tried to push back the 16 bit requirement and see if we could go
> all 32 bit, and we are stuck with the 16 bit requirement.)
> 
> Any pointers or ideas will be appreciated.
> 
> Thanks,
> 
> Karl Rasmussen
> karl AT vvm DOT com

Perhaps I'm oversimplifying your need, but I just use:

	asm(".code16");

at the top of my C files.  My boot loader code, for instance, uses
this.  I believe that 32 bit code is still generated, but the opcodes
are properly prefixed to run in real mode.

I am currently using gcc-2.95.2 on FreeBSD, but I've used it with other
versions of gcc as well.

-Brian