To: dtauritz AT wi DOT leidenuniv DOT nl (D.R.Tauritz)
Cc: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: Re: Interrupt handling 
Date: Tue, 11 Oct 94 08:41:20 +0200
From: "Eli Zaretskii" <eliz AT is DOT elta DOT co DOT il>

> In the file sb.c (djgpp/contrib/sblaster) both protected mode and real mode
> interrupt handlers are installed. Why is this? I thought DJGPP was a
> protected mode only package!

Your program indeed runs in protected mode, but the processor is still
switched to real mode from time to time, e.g. when DOS services are
required (file I/O etc.).  If you only install a protected-mode interrupt
handler, you will lose the interrupts which occur when the CPU is in real
mode, because the mechanism for calling interrupt routine is entirely
different in these two modes.

>                       Anyway, I wrote something similar to the real &
> protected mode combo used in sb.c, alas, without success, my interrupt
> handler installs without giving an error but the installed interrupt routine
> is never called...

Does the original code of sb.c work for you?  If so, the changes you've
introduced are the culprit.  One thing I would check (based on what you
say about IRQ 10/INT 72h) is that you must enable both IRQ 10 in the
second Programmable Interrupt Controller (PIC), at port 0xa0, and IRQ 2
in the first PIC, at port 0x20, during initialization.  Also, at the end
of the interrupt routine you must send ``End-of-interrupt'' (EOI) signal to
both these PICs.  The original code of sb.c only deals with PIC no.1,
because it uses IRQ 7.

Btw, the interrupts which come from the hardware peripherals like timer
chip, SB card etc., are called _hardware_ interrupts.  Software interrupt
is the one invoked by the CPU by issuing the INT XX machine instruction.

	Eli Zaretskii