about serial communication!

I can't step over this instruction ( while(TI==0);//  )
the program :

#include "reg51.h"
main()
{
  TCON = 0x50;
  TMOD = 0x20;
  TL1 = 0xFD;
  TH1 = 0xFD;
  TR1 = 1;
  while(1)
  {
   SBUF = 0xFF;
   while(TI==0);//  I can't step over here .
   TI = 0;
  }