static void set_fg_page(int data) {
        sys16_fg_page[0] = data >> 12;
        sys16_fg_page[1] = (data >> 8) & 0xf;
        sys16_fg_page[2] = (data >> 4) & 0xf;
        sys16_fg_page[3] = data & 0xf;
    }