based use xdate (ADUC 812)

As long as you don't show how those variables are defined, nobody can help you meaningfully.

Better still, show an actual, complete source code example.

/************* Header files ****************************************************/
#include "ADuC812_1.H"                                                // Header file for the ADuC812 MCU
#include <STDIO.H>                                                        // Standard I/O header file
#include <STRING.H>

unsigned char xdata xdata_buffer [16] _at_ 0x0200;
unsigned char data  data_buffer  [16] _at_ 0x60;

int num;
unsigned char interupt1 = 0;


void initialize_system (void);

void ex0_isr (void) interrupt 0 {
        interupt1 = 1;
}

/************* MAIN C Function *************************************************/
void main (void)
{
        //flash_erase_all ();
        initialize_system();    // apply start setting for connect with HyperTerminal

        num = 3;
        memcpy (data_buffer, "DATA Buffer....", sizeof (data_buffer));
        memcpy (xdata_buffer, "XDATA Buffer....", sizeof (xdata_buffer));
        while (1)
        {
                if(interupt1==1){

                        printf("a");
                        num++;
                        printf("%d", num);
                        if(num%2==0){
                                LED ^= -1;
                        }

                        interupt1=0;
                        printf(data_buffer);
                        printf(xdata_buffer);
                }

        }
}


void initialize_system (void)
{
                                                                                        // Initialize the serial port (9600, 8, N, 1) [see page 32 of data sheet]
        PCON &= 0x7F;                                                       // Clear bit 7 of the PCON register (SMOD1 = 0)
        SCON = 0x50;                                                    // 0101,0000 (Mode 1 and RxD enable)
        TMOD |= 0x20;                                                   // Timer #1 in autoreload 8 bit mode
        TCON = 0x40;                                                    // Set Timer #1 to run mode
        TH1 = 0xFA;                                                             // Baud rate is determined by
                                                                                        // Timer #1 overflow rate
                                                                                        // Baud Rate = (Fcpu / 384) / (256 - TH1)
                                                                                        // Fcpu = 11.0592 MHz
                                                                                        // TH1 = FD = 253 -> Baud Rate = 9600
                                                                                        // TH1 = FA = 250 -> Baud Rate = 4800
        TR1 = 1;                                                                // Turn on Timer 1
        TI = 1;                                                                 // Set UART to send first char


        IT0 = 1;   // Configure interrupt 0 for falling edge on /INT0 (P3.2)
        EX0 = 1;   // Enable EX0 Interrupt
        EA = 1;    // Enable Global Interrupt Flag

        return;
}

Terminal Export:
a4DATA Buffer...garbage
a5DATA Buffer...garbage
a6DATA Buffer...garbage
. .
.

Before posting code, be sure to read the instructions for posting code:
http://www.keil.com/forum/tips.asp

Be sure to post the actual code that you compile & test: use copy-and-pase; do not manually re-type it into your post.

Also show what output is produced - just saying "garbage" is not helpful.

Does your board and/or processor require the XDATA to be enabled before it can be used?

Are you sure that your board has XDATA at address 0x0200?

What happens if you remove the _at_ 0x0200 ?

Don't use TABs to lay-out your code - use only spaces.

This is the code now:

/************* Header files ****************************************************/
#include "ADuC812.H" // Header file for the ADuC812 MCU
#include <STDIO.H>     // Standard I/O header file
#include <STRING.H>


sbit  P3_4      = 0xB4;
unsigned char xdata xdata_buffer [16];
unsigned char data  data_buffer  [16] _at_ 0x60;
int num;
unsigned char interupt1 = 0;

#define LED     P3_4 // LED Output

void initialize_system (void);

void ex0_isr (void) interrupt 0 {
        interupt1 = 1;
}

/************* MAIN C Function *************************************************/
void main (void)
{
        //flash_erase_all ();
        initialize_system(); // apply start setting for connect with HyperTerminal

        num = 3;
        memcpy (data_buffer, "DATA Buffer....", sizeof (data_buffer));
        memcpy (xdata_buffer, "XDATA Buffer....", sizeof (xdata_buffer));
        while (1)
        {
                if(interupt1==1){

                        printf("a");
                        num++;
                        printf("%d", num);
                        if(num%2==0){
                                LED ^= -1;
                        }

                        interupt1=0;
                        printf(data_buffer);
                        printf(xdata_buffer);
                }

        }
}


void initialize_system (void)
{
         // Initialize the serial port (9600, 8, N, 1) [see page 32 of data sheet]
        PCON &= 0x7F; // Clear bit 7 of the PCON register (SMOD1 = 0)
        SCON = 0x50;  // 0101,0000 (Mode 1 and RxD enable)
        TMOD |= 0x20; // Timer #1 in autoreload 8 bit mode
        TCON = 0x40;  // Set Timer #1 to run mode
        TH1 = 0xFA;       // Baud rate is determined by
 // Timer #1 overflow rate
 // Baud Rate = (Fcpu / 384) / (256 - TH1)
 // Fcpu = 11.0592 MHz
 // TH1 = FD = 253 -> Baud Rate = 9600
 // TH1 = FA = 250 -> Baud Rate = 4800
        TR1 = 1; // Turn on Timer 1
        TI = 1;  // Set UART to send first char


        IT0 = 1; // Configure interrupt 0 for falling edge on /INT0 (P3.2)
        EX0 = 1; // Enable EX0 Interrupt
        EA = 1;  // Enable Global Interrupt Flag

        return;
}

and this is the full output for 2 external interupts:

a4DATA Buffer....@ABCDFgHIJk
X'!"#$&'()*+,-./p123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_'
abcdefghijklmnopqrstuvwxyz{|}~à‚ƒ„…†‡ˆ‰Š‹ŒŽ''""•–—˜™š›œžŸà¡¢£¤¥¦ç詪«¬­®¯ð
±²³'µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ@
GÈ

PØßà!"#$&g()*+,-./0123456wø9:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_'
abcdefghijklmnopqrstuvwxyz{|}~€‚ƒ„…†‡È‰Š‹ŒŽ''""•–—˜™š›œžŸà¡¢£¤¥¦§è©ª«¬­®¯°
±²³'µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿ@
ABCDFGHIJK

PØà!"#$&'è)*+,-./p123456789:;<=>@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_'
abcdefghijklmnopqrstuvwxyz{|}~€‚ƒ„…†‡È‰Š‹ŒŽÐ''""•–—˜™š›œžßà¡¢£¤¥¦§è©ª«¬­®¯ð
±²³'µ¶·ø¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõö÷øùúûüýþÿa
5DATA Buffer....@ABCDFgHIJk
X'!"#$&'()*+,-./p123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_'
abcdefghijklmnopqrstuvwxyz{|}~à‚ƒ„…†‡ˆ‰Š‹ŒŽ''""•–—˜™š›œžŸà¡¢£¤¥¦ç詪«¬­®¯ð
±²³'µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖ×ØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòó

The core problem here is a buffer overrun. Check how much memory your strings actually consume, and how big your arrays are. Your string written to xdata lacks the terminating '\0'. Bad mojo.

And two more things: don't use printf() where fputs() would suffice, and absolutely don't use it without a format string.

I don't know if I need to change something in my ADUC board to enable the XDATA...

Then what the heck do you think you're doing writing code using xdata memory without having figured that out first?

It's quite probable that you don't have active xdata memory --- the pattern of your output is quite typical for the virtual contents of external memory that's not actually there: the data bytes are the low-byte of their own address, since that was the last thing written to the shared A/D bus lines when the CPU tried to read them.