Date:	Wed, 11 Feb 1998 12:21:13 +0200 (IST)
From:	Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
To:	Shawn Hargreaves <Shawn AT talula DOT demon DOT co DOT uk>
cc:	djgpp AT delorie DOT com
Subject:	Re: Allegro: Adding background music to old DOS game
In-Reply-To:	<D$gzrLAWRK40Ewqc@talula.demon.co.uk>
Message-ID:	<Pine.SUN.3.91.980211122051.15677P-100000@is>
MIME-Version:	1.0

On Tue, 10 Feb 1998, Shawn Hargreaves wrote:

> Markus Brenner writes:
> >What I'm trying to do is get some MIDI background music into the old 
> >MS-DOS Ultima games. My plan is to start up Allegro's MIDI player in 
> >the background, then hook the 0x21 interrupt and check for the file-
> >open function.

I don't recommend this course of action.  See below.  To make a long
story short: I'd suggest to look for a different way of solving this
problem, which doesn't involve hooking Int 21h from a DJGPP program.

> I don't think that will be a problem. It would indeed be trick to do
> this from a hardware interrupt, but int 0x21 is a software call made by
> the mainline code, so you can safely call the play_midi() function from
> here.

Int 21h is a software interrupt.  Under DPMI, software interrupts are
not reflected to protected mode by default.  So you will need to
install a real-mode callback, for this to work.

Unfortunately, the real-mode callback facilities in DJGPP have some
real deficiencies.  For starters, there is no provision for chaining
to the previous handler (and you will almost definitely want to chain
to the original Int 21h handler).  So you will probably have to write
some assembly and/or roll your own callback machinery.

Also, the callbacks are non-reentrant.  This means that if you are in
a middle of code called from the callback, and another Int 21h causes
your callback to be called again, you will crash.

(In case you wonder, some of these limitations are there on purpose:
programmer errors in hooking software interrupts can easily wipe your
disk.  It actually happened to some in the past.)

Last, but not least, there are well-known limitations of hooking DOS
interrupt, due to the fact that DOS is non-reentrant.  Getting called
before DOS will solve some, but not all of them (e.g., some DOS
functions are called from within other DOS functions).  Of course,
there are well-documented techniques to work around these limitations,
but be warned that some of them don't work well or don't work at all
in the DPMI environment.  For example, the issue of critical error
handling is very complicated, hooking the DOS Idle interrupt again
bumps into problems with real-mode callbacks, etc.

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