/*----------------------------------------------------------------------------
Software and hardware interrupt handler demo using Turbo C (16-bit)
Chris Giese	<ChrisGiese@SisAndHappy.com>
http://SisAndHappy.com/ChrisGiese

This code is public domain (no copyright).
You can do whatever you want with it.

Release date: June 1, 2004
----------------------------------------------------------------------------*/
#include <stdio.h> /* printf() */
#include <conio.h> /* getch() */
/* getvect(), setvect(), inportb(), outportb() */
#include <dos.h> /* union REGS, int86(), MK_FP() */

#define	SYSCALL_INT	0x32

typedef struct
{
	unsigned int_num;
	char irq_was_enabled_at_8259;
	void interrupt (*old_vector)();
} vector_t;

/* forward declarations */
static void hook_8259(vector_t *v);
static void unhook_8259(vector_t *v);
/*****************************************************************************
*****************************************************************************/
void hook_int(vector_t *v, unsigned int_num, void interrupt (*handler)())
{
	v->int_num = int_num;
/* for hardware interrupts, enable IRQ at 8259 chips */
	hook_8259(v);
/* save address of current interrupt handler
Turbo C++ 1.0 doesn't have _dos_getvect(), or I would've used it here */
	v->old_vector = getvect(int_num);
/* install new handler */
	setvect(int_num, handler);
}
/*****************************************************************************
*****************************************************************************/
void unhook_int(vector_t *v)
{
	unhook_8259(v);
/* restore old interrupt handler */
	setvect(v->int_num, v->old_vector);
}
/*****************************************************************************
save 8259 interrupt enable state for this IRQ,
then enable IRQ at 8259 chips
*****************************************************************************/
static void hook_8259(vector_t *v)
{
	unsigned irq_num, mask = 1, io_adr;

/* INT 8-15 = IRQ 0-7 */
	if(v->int_num >= 8 && v->int_num <= 15)
	{
/* convert int_num to irq_num... */
		irq_num = v->int_num - 8;
/* ...then to enable/disable mask value and 8259 chip I/O address */
		mask <<= irq_num;
		io_adr = 0x21; /* first 8259 chip */
	}
/* INT 0x70-0x77 = IRQ 8-15 */
	else if(v->int_num >= 0x70 && v->int_num < 0x77)
	{
		irq_num = v->int_num - 0x70;
		mask <<= irq_num;
		io_adr = 0xA1; /* second 8259 chip */
	}
	else
		return;
/* save current IRQ enable state
(the interrupt enable bit is ACTIVE LOW) */
	v->irq_was_enabled_at_8259 = ((inportb(io_adr) & mask) == 0);
/* enable IRQ at 8259 chip */
	outportb(io_adr, inportb(io_adr) & ~mask);
/* for IRQ 8-15, enable cascade bit in first 8259 chip */
	if(io_adr == 0xA1)
		outportb(0x21, inportb(0x21) & ~4);
}
/*****************************************************************************
restore 8259 interrupt enable state for this IRQ
*****************************************************************************/
static void unhook_8259(vector_t *v)
{
	unsigned irq_num, mask = 1, io_adr;

	if(v->int_num >= 8 && v->int_num <= 15)
	{
		irq_num = v->int_num - 8;
		mask <<= irq_num;
		io_adr = 0x21;
	}
	else if(v->int_num >= 0x70 && v->int_num < 0x77)
	{
		irq_num = v->int_num - 0x70;
		mask <<= irq_num;
		io_adr = 0xA1;
	}
	else
		return;
/* restore original state of 8259 chip */
	if(v->irq_was_enabled_at_8259)
		outportb(io_adr, inportb(io_adr) & ~mask);
	else
		outportb(io_adr, inportb(io_adr) | mask);
}
/*****************************************************************************
user-defined software interrupt handler
*****************************************************************************/
#pragma argsused
static void interrupt syscall_int(volatile unsigned junk,
	volatile unsigned di, volatile unsigned si, volatile unsigned ds,
	volatile unsigned es, volatile unsigned dx, volatile unsigned cx,
	volatile unsigned bx, volatile unsigned ax, volatile unsigned ip,
	volatile unsigned cs, volatile unsigned flags)
{
	static char farewell[] = "Goodbye";
/**/

/* passing data into software interrupt handler via registers */
	printf("Registers: '%c%c%c%c'    (should be 'BEER')\n",
		ax, bx, cx, dx);
/* passing pointer into software interrupt handler */
	printf("Greeting:  '%s'  (should be 'Hello!')\n", di);
/* passing pointer out of software interrupt handler */
	di = (int)farewell;
}
/*****************************************************************************
hardware interrupt handler
*****************************************************************************/
static void interrupt timer_irq(void)
{
	(*(unsigned char far *)MK_FP(0xB800, 0))++;
	outportb(0x20, 0x20);
}
/*****************************************************************************
*****************************************************************************/
int main(void)
{
	union REGS regs;
	vector_t v;

/* install software interrupt handler and test */
	hook_int(&v, SYSCALL_INT, syscall_int);
	regs.x.ax = 'B';
	regs.x.bx = 'E';
	regs.x.cx = 'E';
	regs.x.dx = 'R';
	regs.x.di = (unsigned)"Hello!";
	int86(SYSCALL_INT, &regs, &regs);
	unhook_int(&v);
	printf("Farewell:  '%s' (should be 'Goodbye')\n", regs.x.di);
/* install hardware interrupt handler and test */
	printf("Timer interrupt test; press Esc to quit\n");
	hook_int(&v, 8, timer_irq);
	while(getch() != 27)
		/* nothing */;
	unhook_int(&v);
	return 0;
}