Problem about PIO

Here is my code:

# include <stdio.h># include <system.h># include "altera_avalon_pio_regs.h"# include "alt_types.h"

int main()

{

alt_u8 led = 0x1;

IOWR(0x00000800,0,led);

printf("Hello from Nios II!\n");

return 0;

}

Here, 0x00000800 comes from "#define PIO_0_BASE 0x00000800"

in system.h. But when I run the application in Modelsim. The out_port_from_the_pio_0 (2 bits) is zero all the time (I run it for 800us).

Anybody know why? Thank you very much!