linux2.6.11启动代码之bootsect.s

/*

 *     bootsect.S                 Copyright (C) 1991, 1992 Linus Torvalds

 *

 *     modified by Drew Eckhardt

 *     modified by Bruce Evans (bde)

 *     modified by Chris Noe (May 1999) (as86 -> gas)

 *     gutted by H. Peter Anvin (Jan 2003)

 *

 * BIG FAT NOTE: We're in real mode using 64k segments.  Therefore segment

 * addresses must be multiplied by 16 to obtain their respective linear

 * addresses. To avoid confusion, linear addresses are written using leading

 * hex while segment addresses are written as segment:offset.

 *

 */

这段代码首先是有bios系统调用装入的，应该是LILO的一部分。

这部分主要实现了一个显示bugger_off_msg的过程，主要有3个比较重要的bios调用，int 10

Int  16 和int  19，下面详细的讲一下这三个系统调用的意思：

(1)     Int  0x10

所用到的寄存器的解释：

         AH = 0Eh

         AL = 要显示的字符

         BH = 页号（开启页机制的情况下）

         BL = 前景色 (仅在图形模式下用)

Return: nothing

功能：display a character on the screen, advancing the cursor and scrolling

           the screen as necessary

(2)     Int  0x16

Inp.:

AH = 00h

Return: AH = BIOS scan code

AL = ASCII character

(3)     Int  0x19  

This interrupt reboots the system without clearing memory or restoring

  interrupt vectors.  Because interrupt vectors are preserved, this

  interrupt usually causes a system hang if any TSRs have hooked

  vectors from 00h through 1Ch, particularly INT 08.

/***********************************************************************

 

#include <asm/boot.h>

 

SETUPSECTS     = 4                      /* default nr of setup-sectors */

BOOTSEG                   = 0x07C0           /* original address of boot-sector */

INITSEG             = DEF_INITSEG                  /* we move boot here - out of the way   0x9000*/

SETUPSEG         = DEF_SETUPSEG              /*setup starts here     0x9020*/

SYSSEG              = DEF_SYSSEG           /* system loaded at 0x10000 (65536) */

SYSSIZE              = DEF_SYSSIZE          /* system size: # of 16-byte clicks */

                                               /* to be loaded */

ROOT_DEV       = 0                     /* ROOT_DEV is now written by "build" */

SWAP_DEV       = 0                      /* SWAP_DEV is now written by "build" */

 

#ifndef SVGA_MODE

#define SVGA_MODE ASK_VGA         //0xfffd

#endif

 

#ifndef RAMDISK

#define RAMDISK 0

#endif

 

#ifndef ROOT_RDONLY

#define ROOT_RDONLY 1

#endif

 

.code16

.text

 

.global _start

_start:

 

         # Normalize the start address

         jmpl          $BOOTSEG, $start2

 

start2:

         movw       %cs, %ax

         movw       %ax, %ds

         movw       %ax, %es

         movw       %ax, %ss

         movw       $0x7c00, %sp

         sti                                       //设置中断                  

         cld                                       //清除方向标志，cld即告诉程序si，di      

                                             //向前移动，std指令为设置方向，告诉程

//序si，di向后移动 

         movw       $bugger_off_msg, %si               //si指向要显示的字符

/*********************************************

逐字符的显示bugger_off_msg中的内容

**************************************************************/

msg_loop:

         lodsb                  // LODSB的功能是将DS:[si]中的内容取到AL中，然后SI<=SI+/-1

         andb         %al, %al

       jz      die              // je是jump if zero的意思,jz是jump if equal的意思,两者都是结果为

//0(相等),即zF=1时,转移,

         movb        $0xe, %ah         

         movw       $7, %bx

         int    $0x10            //显示

         jmp  msg_loop

 

die:

         # Allow the user to press a key, then reboot

         xorw         %ax, %ax

         int    $0x16              //键盘中断 在ah中接收基本键盘输入功能信号

  int    $0x19             

//int 19一般是不会执行的，因为在int 16之后系统将会跳到用户选择的系统运行，也就//是说如果用户不选择的话，系统将会重启，然后执行下面的跳转代码

 

         # int 0x19 should never return.  In case it does anyway,

         # invoke the BIOS reset code...

         ljmp $0xf000,$0xfff0         //

 

 

bugger_off_msg:

         .ascii         "Direct booting from floppy is no longer supported./r/n"

         .ascii         "Please use a boot loader program instead./r/n"

         .ascii         "/n"

         .ascii         "Remove disk and press any key to reboot . . ./r/n"

         .byte         0

        

 

         # Kernel attributes; used by setup

 

         .org 497

setup_sects:    .byte SETUPSECTS

root_flags:       .word ROOT_RDONLY

syssize:    .word SYSSIZE

swap_dev:        .word SWAP_DEV

ram_size:         .word RAMDISK

vid_mode:        .word SVGA_MODE

root_dev:          .word ROOT_DEV

boot_flag:         .word 0xAA55