Xref: news2.mv.net comp.os.msdos.djgpp:5325
From: Shawn Hargreaves <slh100 AT york DOT ac DOT uk>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: (Q) Soundcard drivers, DMA, and you.
Date: Mon, 24 Jun 1996 11:18:20 +0100
Organization: The University of York, UK
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DJ-Gateway: from newsgroup comp.os.msdos.djgpp

On 20 Jun 1996, Jonathan D. Feinberg wrote:

> I've got Ensoniq's protected mode driver source for their Soundscape
> card.  I have hopes of perhaps integrating Soundscape support into
> Allegro 2.x.  However, I have never written a driver before, and know
> exactly nothing about the use of interrupts, DMA, the port functions,
> etc.  This is a huge steak to chew on.  I'm hoping that you folks
> can point me to texts, ftp sites, etc. that will lead me gently
> towards enlightenment.

A Soundscape driver would be fantastic, but you are right, it's a lot to 
take on. Good luck with it!

As a starting place, I'd suggest working on the MIDI playback side of 
things. I don't know exactly how the Ensoniq handles MIDI, but if it is 
anything like most other wavetable cards, it can probably be done with 
port writes, avoiding the need for messy stuff like dma and interrupt 
handing. You should probably start by copying one of the existing Allegro 
MIDI drivers (the GUS one would be a good choice, since it doesn't 
contain any code yet) to see where everything has to go. For a MIDI 
driver, you provide functions to detect the soundcard, initialise it, 
turn notes on and off, and alter the volume/pitch of notes while they are 
playing. This can probably all be done with port accesses, in which case 
you just need to #include <pc.h> and write your code. Plus you need to 
lock all the memory the driver touches, since it is called inside the 
timer interrupt handler: this means you need to use Allegro's  
LOCK_VARIABLE() macro on all your global and static data, and 
LOCK_FUNCTION() on all the non-inline functions.

> referring especially to functions that read and write to
> peripheral devices ("outportb", for example).  Any comments?  

outportb() is there in djgpp, along with outportw(), outportl(), and 
inport versions of all three. 

The stuff that differs from other DOS compilers is the interrupt 
handling: djgpp doesn't understand the 'interrupt' keyword and setvect() 
function. Since you are doing this with Allegro, you can use the 
interrupt and dma functions in irq.c and dma.c: look at the SB driver for 
an example of how they work. Basically you write a C function for your 
interrupt handler and then install_irq(int irq_number, int (*function)());
I think that is right: I don't have the source here to check. Your C 
function should return zero or one depending on whether it wants to iret 
or chain to the old handler (for a soundcard interrupt you probably don't 
want to chain), and don't forget an outportb(0x20, 0x20) at the end of 
the interrupt handler to acknowledge it with the interrupt controller.


/*
 *	Shawn Hargreaves.        Why is 'phonetic' spelt with a ph?
 *	Check out Allegro and FED on http://www.york.ac.uk/~slh100/
 */