这是书中的例题，如下所示：
//dualview.c -- 位字段和位运算符
#include<stdio.h>
//位字段常量
//是否透明和是否可见
#define YES 1
#define NO 0
//边框样式
#define SOLID 0
#define DOTTED 1
#define DASHED 2
//三原色
#define BLUUE 4
#define GREEN 2
#define RED 1
//混合颜色
#define BALACK 0
#define YELLOW (RED | GREEN)
#define MAGENTA (RED | BLUE)
#define CYAN (GREEN | BLUE)
#define WHITE (RED | GREEN | BLUE)
//位运算中使用的常量
#define OPAQUE 0x1
#define FILL_BLUE 0x8
#define FILL_GREEN 0x4
#define FILL_RED 0x2
#define FILL_MASK 0xE
#define BORDER 0x100
#define BORDER_BLUE 0x800
#define BORDER_GREEN 0x400
#define BORDER_RED 0x200
#define BORDER_MASK 0xE00
#define B_SOLID 0
#define B_DOTTED 0x1000
#define B_DASHED 0x2000
#define STYLE_MASK 0x3000
const char * colors[8] = {"black","red","green","yellow",
              "blue","magenta","cyan","white"};
struct box_props{
    unsigned int opaque         : 1;
    unsigned int fill_color        : 3;
    unsigned int             : 4;
    unsigned int show_border    : 1;
    unsigned int border_color    : 3;
    unsigned int border_style    : 2;
    unsigned int             : 2;
};
union Views         //把数据看作结构或unsigned short变量
    {
        struct box_props st_view;
        unsigned int ui_view;
    };
void show_settings(const struct box_props * pb);
void show_settings1(unsigned short);
char * itobs(int n, char * ps);        //把short值以二进制字符串的形式显示
int main(void)
{
    //创建Views对象，初始化结构box view
    union Views box = {{YES,YELLOW,YES,GREEN,DASHED}};
    char bin_str[8 * sizeof(unsigned int) + 1];
    printf("Original box settings:\n");
    show_settings(&box.st_view);
    printf("\nBox settings using unsigned int view:\n");
    show_settings1(box.ui_view);
    printf("bits are %s\n",itobs(box.ui_view,bin_str));
    box.ui_view &= ~FILL_MASK;    //把代表填充色的位清0
    box.ui_view |= (FILL_BLUE | FILL_GREEN);    //重置填充色
    box.ui_view ^= OPAQUE;    //转置指示是否透明的位
    box.ui_view |= BORDER_RED;    //错误的方法
    box.ui_view &= ~STYLE_MASK;    //清楚样式位
    box.ui_view |= B_DOTTED;    //把样式设置为点
    printf("\nBox settings using unsigned int view:\n");
    show_settings1(box.ui_view);
    printf("bits are %s\n",itobs(box.ui_view,bin_str));
   
    return 0;
}
void show_settings(const struct box_props * pb)
{
    printf("Box is %s.\n",pb -> opaque == YES? "opaque" : "transparent");
    printf("The fill color is %s.\n",colors[pb->fill_color]);
    printf("Border %s.\n",pb->show_border == YES? "shown" : "not shown");
    printf("The border color is %s.\n",colors[pb->border_color]);
    printf("The border style is ");
    switch(pb->border_style)
    {
        case SOLID : printf("solid.\n"); break;
        case DOTTED : printf("dotten.\n"); break;
        case DASHED : printf("dashed.\n"); break;
        default : printf("unknown type.\n");
    }
}
void show_settings1(unsigned short us)
{
    printf("box is %s.\n",us & OPAQUE == OPAQUE? "opaque" : "transparent");
    printf("The fill color is %s.\n",colors[(us >> 1) & 07]);
    printf("Border %s.\n",us & BORDER == BORDER? "shown" : "not shown");
    printf("The border style is ");
    switch(us & STYLE_MASK)
    {
        case B_SOLID : printf("solid.\n"); break;
        case B_DOTTED : printf("dotted.\n"); break;
        case B_DASHED : printf("dashed.\n"); break;
        default : printf("unknown type.\n");
    }
    printf("The border color is %s.\n",colors[(us >> 9) & 07]);
}
//把int转换为二进制字符串
char * itobs(int n,char * ps)
{
    int i;
    static int size = 8 * sizeof (unsigned int);
    for(i = size - 1;i >= 0;i--,n >>= 1)
        ps[i] = (01 & n) + '0';
    ps[size] = '\0';
    return ps;
}

代码中加粗红色字体部分，看不懂，不明白，对C理解不深，莫拍砖。望各位高手解答。
us & OPAQUE中us的值是什么，我不明白啊