i am facing a problem in interrupt driven serial i/o. follow is my code

#include<AT89X51.h>

unsigned char data SEND=0;

UART_ISR() interrupt 4

{ if(TI==1)TI=0;}

main()

{

TMOD=0x20h;

TH1=0xFD; //9600 baud rate

SCON=0x50;

TR1=1;

ES=1;

EA=1;

while(1)

{

SBUF=SEND;

while(TI!=1){;}

SEND++

}

}

though theorectically there is no problem. if i write code as follow than my requiremwnt is fulfilled. I comment ISR and disable Serial interrupt.

#include<AT89X51.h>

unsigned char data SEND=0;

// UART_ISR() interrupt 4 { if(TI==1)TI=0;}

main()

{

TMOD=0x20h;

TH1=0xFD; //9600 baud rate

SCON=0x50;

TR1=1;

//ES=1;

//EA=1;

while(1)

{

SBUF=SEND;

while(TI!=1){;}

TI=0;

SEND++

}

}

pls give the appropriate reason of it.