how?

#include <reg9e5.h>
#define POWER 3 // 0=min power...3 = max power
#define HFREQ 0 // 0=433MHz, 1=868/915MHz
#define CHANNEL 106 // Channel number: f(MHz) = (422.4+CHANNEL/10)*(1+HFREQ)

unsigned char SpiReadWrite(unsigned char b);
void UartInit(void);
void NrfInit(void);
void Default_Init(void);

#define uchar unsigned char
uchar incoming,outgoing,lastinchar;



void main(void)
{
UartInit();
NrfInit();
while(1);
}


void UartInit(void)
{
TH1 = 243; // 19200@16MHz (when T1M=1 and SMOD=1)
CKCON |= 0x10; // T1M=1 (/4 timer clock)
PCON = 0x80; // SMOD=1 (double baud rate)
SCON = 0x52; // Serial mode1, enable receiver
IE=0x10;
TMOD &= ~0x30;
TMOD |= 0x20; // Timer1 8bit auto reload
TR1 = 1; // Start timer1
P0_ALT |= 0x06; // Alternate functions on pins P0.1-2
P0_DIR |= 0x02; // P0.1 (RxD) is input


}


void NrfInit(void)
{

unsigned char cklf;

SPICLK = 0; // Max SPI clock
SPI_CTRL = 0x02; // Connect SPI controller to Radio

// switch to 16MHz clock:
RACSN = 0;
SpiReadWrite(RRC | 0x09);
cklf = SpiReadWrite(0) | 0x04;
RACSN = 1;
RACSN = 0;
SpiReadWrite(WRC | 0x09);
SpiReadWrite(cklf);
RACSN = 1;

}


void serint(void)
interrupt 4
{
if(RI)
{
incoming=SBUF;
RI=0;

}

if (TI)
{
SBUF=incoming;
TI=0;
}

}


unsigned char SpiReadWrite(unsigned char b)
{
EXIF &= ~0x20; // Clear SPI interrupt
SPI_DATA = b; // Move byte to send to SPI data register
while((EXIF & 0x20) == 0x00) // Wait until SPI hs finished transmitting
;
return SPI_DATA;
}




can help me to solve how to add "interrupt" into this program provided above. So that when i input a ASCII code through serial port, the output is the same ASCII code thanks