位域

看到个很有趣的位域问题，放上来分享下（位域介绍）

#include<stdio.h>

#define STS_TG32        0xC            // 32-bit Call Gate
#define STS_IG32        0xE            // 32-bit Interrupt Gate

typedef unsigned int            uint32_t;


#define SETGATE(gate, istrap, sel, off, dpl) {            \
    (gate).gd_off_15_0 = (uint32_t)(off) & 0xffff;        \
    (gate).gd_ss = (sel);                                \
    (gate).gd_args = 0;                                    \
    (gate).gd_rsv1 = 0;                                    \
    (gate).gd_type = (istrap) ? STS_TG32 : STS_IG32;    \
    (gate).gd_s = 0;                                    \
    (gate).gd_dpl = (dpl);                                \
    (gate).gd_p = 1;                                    \
    (gate).gd_off_31_16 = (uint32_t)(off) >> 16;        \
}

 /* Gate descriptors for interrupts and traps */
 struct gatedesc {
    unsigned gd_off_15_0 : 16;        // low 16 bits of offset in segment
    unsigned gd_ss : 16;            // segment selector
    unsigned gd_args : 5;            // # args, 0 for interrupt/trap gates
    unsigned gd_rsv1 : 3;            // reserved(should be zero I guess)
    unsigned gd_type : 4;            // type(STS_{TG,IG32,TG32})
    unsigned gd_s : 1;                // must be 0 (system)
    unsigned gd_dpl : 2;            // descriptor(meaning new) privilege level
    unsigned gd_p : 1;                // Present
    unsigned gd_off_31_16 : 16;        // high bits of offset in segment
 };


int main()
{
    unsigned intr;
    intr=8;
    SETGATE((*(struct gatedesc*)&intr), 0,1,2,3);
    printf("%x\n",intr);
}

分析intr的值：

　　首先看一下getedesc在内存中是如何存储的：

	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
地址 向下 增大	gd_off_15_0
	gd_ss
	gd_p	gd_dpl		gd_s	gd_type				gd_rsv1			gd_args
	gd_off_31_16

　　unsigned占用4字节，即32位，getedesc占用8字节，即64位。

　　按题目操作，由于X86使用小端存储，intr所占内存对应位置应该是getedesc的低32位，也就是表格的前两列。

　　同样由于X86使用小端存储，所以intr高16位为gd_ss，低16位为gd_off_15_0。

　　所以intr的值应该为 0000 0000 0000 0001 0000 0000 0000 0002 ,即0x10002。

 

知道了intr=0x10002以后，*(&intr+1)=0xee00也就很显而易见了对吧～