80x86描述符总结及解析描述符的小程序

女爷i 2022-06-03 08:41 165阅读 0赞

# 一、描述符的分类 #

![这里写图片描述][20160109212457811]

# 二、描述符格式 #

## 1、数据段描述符 ##

![这里写图片描述][20160109212506980]

## 2、代码段描述符 ##

![这里写图片描述][20160109212514896]

## 3、LDT段描述符 ##

![这里写图片描述][suFKpM0.png]

## 4、TSS段描述符 ##

![这里写图片描述][SouthEast]

## 5、调用门描述符 ##

![这里写图片描述][SouthEast 1]

## 6、中断门描述符 ##

![这里写图片描述][SouthEast 2]

## 7、陷阱门描述符 ##

![这里写图片描述][SouthEast 3]

## 8、任务门描述符 ##

![这里写图片描述][SouthEast 4]

# 三、解析描述符的小程序 #

#include <stdio.h>
    
    //定义描述符中的低32位
    struct seg_des_low_word
    {
        unsigned int limit_0_15:16;
        unsigned int base_0_15 :16;
    
    };
    //定义描述符中的高32位(通用格式)
    struct seg_des_high_word
    {
        unsigned int base_16_23 :8;
        unsigned int type       :4;
        unsigned int s          :1;
        unsigned int dpl        :2;
        unsigned int p          :1;
        unsigned int limit_16_19:4;
        unsigned int avl        :1;
        unsigned int l          :1;//保留给64位处理器使用
        unsigned int d_b        :1;
        unsigned int g          :1;
        unsigned int base_24_31 :8;
    };
    
    
    void parse_seg_des(unsigned int low_dw, unsigned int hig_dw)
    {
    
        struct seg_des_low_word *pl 
            =(struct seg_des_low_word*)&low_dw;
    
        struct seg_des_high_word *ph
            =(struct seg_des_high_word*)&hig_dw;
    
        unsigned int type = ph->type;
    
        int offset = (hig_dw & 0xFFFF0000) | (low_dw & 0x0000FFFF);
    
    
        unsigned int seg_base = 
            (ph->base_24_31<<24)|(ph->base_16_23<<16)|pl->base_0_15;//拼接基地址字段
    
        unsigned int seg_limit =
            seg_limit = (ph->limit_16_19<<16)|pl->limit_0_15;//拼接段限长字段
    
        int system = ph->s;
    
        //下面的字段输出是不是很方便？这就是位字段的好处之一
        printf("P = %d\n",ph->p);
        printf("DPL = %d\n",ph->dpl); //P和DPL是每个段描述符都有的
    
        if(system == 0) //system segment
        {
            if(type == 9 || type==11 || type==2) //==的优先级高于||
            {
                //print base address,limit,G
                printf("base address = %#X\n",seg_base);
                printf("limit = %#X ，",seg_limit);
    
                if(ph->g == 1)
                    printf("以4KB为单位\n");
                else
                    printf("以B为单位\n");
            }
    
            if(type == 9)
                printf("TSS段，不忙\n");
            else if(type == 11)
                printf("TSS段，忙\n"); 
            else if(type == 5)
                printf("任务门，TSS段选择子 = %#X\n",pl->base_0_15);
            else if(type == 6)
                printf("16位中断门，段选择子 = %#X，偏移 = %#X\n",pl->base_0_15,offset);
            else if(type == 14)
                printf("32位中断门，段选择子 = %#X，偏移 = %#X\n",pl->base_0_15,offset);
            else if(type == 7)
                printf("16位陷阱门，段选择子 = %#X，偏移 = %#X\n",pl->base_0_15,offset);
            else if(type == 15)
                printf("32位陷阱门，段选择子 = %#X，偏移 = %#X\n",pl->base_0_15,offset);
            else if(type == 12)
                printf("调用门，段选择子 = %#X，偏移 = %#X , 参数个数 = %d\n",pl->base_0_15,offset, hig_dw & 0x1F);
            else if(type == 2)
                printf("LDT描述符\n");
        }
        else //data or code segment
        {
            printf("base address = %#X\n",seg_base);
            printf("limit = %#X ，",seg_limit);
    
            if(ph->g == 1)
                printf("以4KB为单位\n");
            else
                printf("以B为单位\n");
    
            if(type & 0x08 ) //代码段，分析C，R，D 
            {
                printf("代码段，");
    
                if(ph->d_b == 1) //D=1
                    printf("默认操作数大小为32位\n");
                else
                    printf("默认操作数大小为16位\n");
    
                if(type & 0x04) //C=1
                    printf("一致性，");
                else
                    printf("非一致性，");
    
                if(type & 0x02) //R=1
                    printf("可读\n");
                else
                    printf("不可读\n");
            }
            else    //数据段，分析B，E，W
            {
                printf("数据段，");
    
                if(ph->d_b == 1) //B=1
                    printf("栈段使用ESP，ESP最大为0xFFFFFFFF\n");
                else
                    printf("栈段使用SP，SP最大为0xFFFF\n");
    
                if(type & 0x04) //E=1
                    printf("向下扩展，");
                else
                    printf("向上扩展，");
    
                if(type & 0x02) //W=1
                    printf("可写\n");
                else
                    printf("不可写\n");
            }
        }
    
    }
    
    int main(void)
    {
        printf("please input the segment descriptor,format: [low dword] [high dword]\n");
        printf("example:\n");
        printf("000003ff 00c0fa00\n");
        printf("The Descriptor:\n");
    
        unsigned int l_dword = 0;
        unsigned int h_dword = 0;
    
        //请求用户输入描述符，先是低32位，再是高32位
        scanf("%x" "%x",&l_dword,&h_dword);
        printf("----------------------\n");
        parse_seg_des(l_dword,h_dword);
    
        return 0;
    }

以上代码编译后运行：

> please input the segment descriptor,format: \[low dword\] \[high dword\]  
> example:  
> 000003ff 00c0fa00  
> The Descriptor:

提示用户输入段描述符，先输入低32位，再输入高32位。注意：中间以空格分开，且以16进制输入，前面不用加“0x”。  
比如，用户输入：

> 80000002 c0920b

回车后得到：

> ——————–  
> P = 1  
> DPL = 0  
> base address = 0XB8000  
> limit = 0X2 ，以4KB为单位  
> 数据段，栈段使用ESP，ESP最大为0xFFFFFFFF  
> 向上扩展，可写

[20160109212457811]: /images/20220603/3fe18b075df34a1eaa581b106aa08156.png
[20160109212506980]: /images/20220603/57fecde354c544e9a3f4a58e1c926da7.png
[20160109212514896]: /images/20220603/2964885a798f44c6938d08c0327eaa19.png
[suFKpM0.png]: http://i.imgur.com/suFKpM0.png
[SouthEast]: /images/20220603/8ffb7e9376864ea5811da8a36af5e5ce.png
[SouthEast 1]: /images/20220603/36314b7f970249f196b2dad0d97f34f2.png
[SouthEast 2]: /images/20220603/12b236235f7749f182573baeba542100.png
[SouthEast 3]: /images/20220603/a3560779e8c64bf8886620e22c45ec38.png
[SouthEast 4]: /images/20220603/6552f37fc60c4c5e88036be97a36ef26.png