[mini244]cpu'll dead in the uart_print's vsprintf from nand boot

I write a program to make a function named uart_printf,just as the printf
in C.I use keil MDK,if I download it to nor flash or SDRAM,it's OK,but if I
download it to nand flash,it'll dead.
MY CODE are as the attachment.
Here's some pic

MY SETTING
http://oi35.tinypic.com/wb6jwx.jpg

THE simulation
http://oi38.tinypic.com/34q8hes.jpg
http://oi34.tinypic.com/kbet0w.jpg
http://oi37.tinypic.com/2hmewoz.jpg

HERE's MY uart_ptintf function
..................
void uart0_printf(char *fmt,...)
{
  va_list ap;
  char string[500];
  
  va_start(ap,fmt);
  vsprintf(string,fmt,ap);
  uart0_sendstring(string);
  va_end(ap);
}
..................