use of outportb(0x20,0x20);

this is used to reset interrupts after servicing ISR

eplain in detail ( bit wise)

 

You don't give any details of the platform you are using. If it is an x86 type board with an Intel 8259 Interrupt Controller or equivalent you can dig up the datasheet for the details of telling the interrupt controller that the service routine is finished and that it can drop its priority level to a lower value.

 

i use this code for communicating between 2 pc........can u now say the use of outportb(0x20,0x20);

#include <dos.h>
#include <stdio.h>
#include <conio.h>

#define PORT1 0x3F8 /* Port Address Goes Here */
#define INTVECT 0x0C /* Com Port's IRQ here (Must also change PIC setting) */

/* Defines Serial Ports Base Address */
/* COM1 0x3F8 */
/* COM2 0x2F8 */
/* COM3 0x3E8 */
/* COM4 0x2E8 */

int bufferin = 0;
int bufferout = 0;
char ch;
char buffer[1025];

void interrupt (*oldport1isr)();

void interrupt PORT1INT() /* Interrupt Service Routine (ISR) for PORT1 */
{
int c;
do { c = inportb(PORT1 + 5);
if (c & 1) {buffer[bufferin] = inportb(PORT1);
bufferin++;
if (bufferin == 1024) {bufferin = 0;}}
}while (c & 1);
outportb(0x20,0x20);
}

void main(void)
{
int c;
outportb(PORT1 + 1 , 0); /* Turn off interrupts - Port1 */

oldport1isr = getvect(INTVECT); /* Save old Interrupt Vector of later
recovery */

setvect(INTVECT, PORT1INT); /* Set Interrupt Vector Entry */
/* COM1 - 0x0C */
/* COM2 - 0x0B */
/* COM3 - 0x0C */
/* COM4 - 0x0B */

/* PORT 1 - Communication Settings */

outportb(PORT1 + 3 , 0x80); /* SET DLAB ON */
outportb(PORT1 + 0 , 0x0C); /* Set Baud rate - Divisor Latch Low Byte */
/* Default 0x03 = 38,400 BPS */
/* 0x01 = 115,200 BPS */
/* 0x02 = 57,600 BPS */
/* 0x06 = 19,200 BPS */
/* 0x0C = 9,600 BPS */
/* 0x18 = 4,800 BPS */
/* 0x30 = 2,400 BPS */
outportb(PORT1 + 1 , 0x00); /* Set Baud rate - Divisor Latch High Byte */
outportb(PORT1 + 3 , 0x03); /* 8 Bits, No Parity, 1 Stop Bit */
outportb(PORT1 + 2 , 0xC7); /* FIFO Control Register */
outportb(PORT1 + 4 , 0x0B); /* Turn on DTR, RTS, and OUT2 */

outportb(0x21,(inportb(0x21) & 0xEF)); /* Set Programmable Interrupt Controller */
/* COM1 (IRQ4) - 0xEF */
/* COM2 (IRQ3) - 0xF7 */
/* COM3 (IRQ4) - 0xEF */
/* COM4 (IRQ3) - 0xF7 */

outportb(PORT1 + 1 , 0x01); /* Interrupt when data received */

printf("\nSample Comm's Program. Press ESC to quit \n");

do {

if (bufferin != bufferout){ch = buffer[bufferout];
bufferout++;
if (bufferout == 1024) {bufferout = 0;}
printf("%c",ch);}

if (kbhit()){c = getch();
outportb(PORT1, c);}

} while (c !=27);

outportb(PORT1 + 1 , 0); /* Turn off interrupts - Port1 */
outportb(0x21,(inportb(0x21) | 0x10)); /* MASK IRQ using PIC */
/* COM1 (IRQ4) - 0x10 */
/* COM2 (IRQ3) - 0x08 */
/* COM3 (IRQ4) - 0x10 */
/* COM4 (IRQ3) - 0x08 */
setvect(INTVECT, oldport1isr); /* Restore old interrupt vector */

}

 

I already did. It sends an EOI (End of Interrupt) signal to the Interrupt Controller. To understand the operation of the Interrupt Controller dig up a data sheet for an Intel 8259A Programmable Interrupt Controller. The chip itself is now obsolete but people include the design in the combo chips they put on PC motherboards as companions to the x86 processors