I wrote a c code with falling edge interrupt and serail port application. When I use those codes seperately, they work correctly, but when I use together, new code doesnt work.
Would you give me any feedback or suggestion? My code is:

#include <DS89C4xx.h>
#include <stdio.h>

void send(unsigned char x);
void msdelay(unsigned int itime);

bit flag = 0;
void ex0_isr (void) interrupt 1
{
flag =1 ;
}

void main()
{

TMOD = 0x20;
TH1 = 0xFD;
SCON0 =0x50;
TR1=1;

while(1)
{

if (flag == 1)
{
send('A');
}


}
}
void msdelay(unsigned int itime)
{
unsigned int i, j;
for (i=0; i<itime; i++)
for(j=0; j<1275; j++);
}

void send(unsigned char x)
{
SBUF0 = x;
while (TI_0 = 0);
TI_0=0;
}