Date:	Wed, 24 Feb 1999 11:09:14 +0200 (IST)
From:	Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
X-Sender:	eliz AT is
To:	Dave Bird <dave AT xemu DOT demon DOT co DOT uk>
cc:	djgpp AT delorie DOT com
Subject:	Re: I just wanted to assemble this old program under NASM....
In-Reply-To:	<kCW7FICon102Ew0P@xemu.demon.co.uk>
Message-ID:	<Pine.SUN.3.91.990224110855.22838C-100000@is>
MIME-Version:	1.0
Reply-To:	djgpp AT delorie DOT com

On Wed, 24 Feb 1999, Dave Bird wrote:

> >DJGPP programs *start* in 32-bit mode.  You can't switch to/from real
> >mode the way you're thinking in a djgpp program.
>  You mean "no program which does so can be defined in assembler, 
>  assembled and linked, then fed to GDB......" :-?

Probably not, at least not in the DJGPP port of GDB.

Here's some background on how debugger support works in DJGPP.  Since
there's no way to run two programs (the debugger and the debuggee)
concurrently, and no DOS function to load a protected-mode COFF
executable, DJGPP debuggers cannot use OS services to load the
debuggee and attach the debugger to it, the way e.g. the DOS DEBUG
command of yore does.

Instead, there's a bunch of functions in libdbg.a library, against
which all DJGPP debuggers are linked, which allows any DJGPP program
to do the following magic:

  - load another DJGPP program, in the same way the stub loader does;
  - hook several DOS and DPMI services so that e.g. when the debuggee
    hits a breakpoint and generates SIGTRAP, the control jumps to the
    debugger;
  - resume the debuggee after the control was passed to the debugger
    for some reason (like a breakpoint or a signal);
  - set and unset breakpoints.

The debugging library was written specifically for DJGPP, with nothing
else in mind but debugging DJGPP-style COFF images that run in a DPMI
environment.

From this description, it should become clear that the DJGPP debuggers
are probably not good for debugging anything but DJGPP programs, or
programs that at least look and behave like DJGPP ones.

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