linux-0.11调试教程，con_init函数和右上角的EGAc

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最后4个字母是EGAc对应8个字节0x07470745 0x07630741。

因为是EGAc,所以0xb8000是显存的起始地址，而这时origin 也设置为video_mem_start既0xb8000。

所以可以在0xb8030既第24个字母处（每个字母用2个字节表示），设置badc。

#define ORIG_VIDEO_MODE ((*(unsigned short *)0x90006) & 0xff)

所以，ORIG_VIDEO_MODE 为03不等于7，所以运行 else /* If not, it is color. */

之后的语句。

又因为#define ORIG_VIDEO_EGA_BX (*(unsigned short *)0x9000a)

所以ORIG_VIDEO_EGA_BX 为0x0003，不等于0x10，所以会运行下面的语句。

        if ((ORIG_VIDEO_EGA_BX & 0xff) != 0x10)
       {
           video_type = VIDEO_TYPE_EGAC;
           video_mem_end = 0xbc000;
           display_desc = "EGAc";
       }

相关代码：

void con_init(void)
{
   register unsigned char a;
   char *display_desc = "????";
   char *display_ptr;

   video_num_columns = ORIG_VIDEO_COLS;
   video_size_row = video_num_columns * 2;
   video_num_lines = ORIG_VIDEO_LINES;
   video_page = ORIG_VIDEO_PAGE;
   video_erase_char = 0x0720;

   if (ORIG_VIDEO_MODE == 7)           /* Is this a monochrome display? */
   {
       video_mem_start = 0xb0000;
       video_port_reg = 0x3b4;
       video_port_val = 0x3b5;
       if ((ORIG_VIDEO_EGA_BX & 0xff) != 0x10)
       {
           video_type = VIDEO_TYPE_EGAM;
           video_mem_end = 0xb8000;
           display_desc = "EGAm";
       }
       else
       {
           video_type = VIDEO_TYPE_MDA;
           video_mem_end   = 0xb2000;
           display_desc = "*MDA";
       }
   }
   else                               /* If not, it is color. */
   {
       video_mem_start = 0xb8000;
       video_port_reg   = 0x3d4;
       video_port_val   = 0x3d5;
       if ((ORIG_VIDEO_EGA_BX & 0xff) != 0x10)
       {
           video_type = VIDEO_TYPE_EGAC;
           video_mem_end = 0xbc000;
           display_desc = "EGAc";
       }
       else
       {
           video_type = VIDEO_TYPE_CGA;
           video_mem_end = 0xba000;
           display_desc = "*CGA";
       }
   }

   /* Let the user known what kind of display driver we are using */

   display_ptr = ((char *)video_mem_start) + video_size_row - 8;
   while (*display_desc)
   {
       *display_ptr++ = *display_desc++;
       display_ptr++;
   }

   /* Initialize the variables used for scrolling (mostly EGA/VGA)   */

   origin   = video_mem_start;
   scr_end   = video_mem_start + video_num_lines * video_size_row;
   top   = 0;
   bottom   = video_num_lines;

   gotoxy(ORIG_X,ORIG_Y);
   set_trap_gate(0x21,&keyboard_interrupt);
   outb_p(inb_p(0x21)&0xfd,0x21);
   a=inb_p(0x61);
   outb_p(a|0x80,0x61);
   outb(a,0x61);
}