First of all, thank you members in this forum who gives a lot of helpful suggestion to me.
I am sorry that i want to raise question again.

My problem is about the serial send interrupt problem.
I can send out data through
----------------------------------
SBUF=0xff;
wait:
if (TI==0) goto wait;
TI=0;
------------------------------

However, we i try to send data out through serial interrupt method but can't success.
My testing code is as shown in the following:
any wrong part on my code?

#include <AT89X51.H>

void delay(int x);

void main ()
{
char ind=0;

TMOD=0x20;
TH1=-03;
TR1=1;
SCON=0x50;
IE=0x90;
P2_7=1;
delay(20000);

while(1)
{SBUF='a';}
}
}

void RX() interrupt 4
{ char temp;
if (TI) //clear TI flag
{
TI=0;
}

if (RI)
{
RI=0;
temp=SBUF;
P0=temp;
}
}


void delay(int x)
{ int m;
for (m=0;m<x;m++);
}


//THANK FOR YOUR HELP