Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!mcsun!inria!laas!news
From: news@laas.laas.fr (USENET News System)
Newsgroups: comp.sys.ibm.pc
Subject: Problem with 15H interrupt handler
Message-ID: <504@laas.laas.fr>
Date: 8 Dec 89 08:56:36 GMT
Reply-To: rlacoste@rigel.laas.fr (Robert Lacoste)
Organization: LAAS-CNRS, Toulouse, France
Lines: 61




	Hi netlanders !

	I have a problem : I am trying to write a small interrupt
handler, in order to get a vga 800x600 screen hardcopy when 
hitting the ALT+SYSRQ keys on my AT-386 keyboard (AMI Bios).

In such a situation, I know the 15H interrupt is called, with 
85H in the AX register. The following program install a small driver, 
executing a delay loop while ALT-SYSRQ is pressed (It's a Turbo-C
and assembler version).

It works fine, but if I increase the delay (WAIT=2000), the system
hangs up on return (The keyboard is no more operant). 

I think the problem is related to some other events occuring while 
the handler is executed (timer ?), but inserting a STI instruction
before the loop in order to re-enable interrupts doesn't work...


	Any idea ?	Thanks !


/*--------------------- START OF LISTING -----------------------*/

	#include <dos.h>
	#define WAIT 20			/* FAIL IF 2000... */

	/*****************************************************/
	/* 0x15 INTERRUPT HANDLER, CALLED WHEN SYSRQ PRESSED */
	/*****************************************************/

	void interrupt my_driver(void)
	{
		asm cmp ah,85H		/* ALT-SYSRQ ? */
		asm jne end

		asm mov cx,WAIT
	q1:
		asm mov ax,WAIT		/* IF PRESSED, */
	q2:				/* WAIT FOR A WHILE */
		asm dex ax
		asm jne q2
		asm loop q1

	end:
		asm nop			/* AND RETURN TO CALLER */
	}


	/*****************************************************/
	/* MAIN PROGRAM: INSTALL HANDLER AND STAY RESIDENT   */
	/*****************************************************/

	main()
	{
		setvect(0x15,my_driver);
		keep(0,100);
	}