linux内核代码注释赵炯第三章引导启动程序

linux内核代码注释

第三章引导启动程序

boot目录中的三个汇编代码文件 bootsect.s和setup.s采用近似intel的汇编语法，需要8086汇编器连接器as86和ld86

head.s用GNU的汇编程序格式并且运行在保护模式下，需要GNU的as进行编译。为AT&T语法的汇编语言程序。GNU只支持386后的cpu 不支持实模式下的程序

总体功能

pc加电-》cpu进入实模式-》从0xfff0开始执行程序代码（bios地址），进行系统检测，并在物理地址0处开始初始化中断向量--》启动设备的第一个扇区引导扇区 512字节，读入到内存0x7c00处，并且跳转到此。

linux系统最前面的部分就是boot/bootsect.s 由bios读入到内存绝对地址0x7c00 （31k）--》当此程序执行时，会将自己移动到绝对地址0x90000 576k处--》将启动设备中后2k字节代码boot/setup.s读入到内存0x90200处，而内处的其他部分 system模块则被读入到从地址0x10000开始处，，后面的setup程序会把system模块移动到内存起始处。看上图

整个系统从地址0x10000移动到0x0000处，进入保护模式并跳转到系统的鱼虾部分在0x0000处。此时所有的32位运行方式的设置启动完成，IDT GDT 以及LDT被加载，cpu和协处理器已经确认，分页机制也已经准备就绪

然后嗲用init/main.c程序

一bootsect.s 程序

（1）

磁盘引导块程序驻留在磁盘的一个扇区中引导扇区，0磁道柱面，0磁头，第1个扇区。pc机加电rom-bios自检后，引导扇区由bios加载到内存0x7c00处，然后将自己移动到内存0x90000处。

作用：

1.将setup模块（由setup.s编译而成）从磁盘加载到内存，紧接着bootsect的后面位置0x90200、

2.利用BIOS中断0x13取磁盘参数表中当前启动引导盘的参数

3.在屏幕上显示loading system... 字符串

4.将system模块从磁盘加载到内存0x10000开始处，

5.确定根文件系统的设备号若未指定，则根据所保存的引导盘的每磁道扇区数判别处盘的类型和种类并保存其设备号于root_dev 引导块0x508地址处，最后长跳转到setup程序的开始处 0x90200 执行setup程序。

（2）代码注释

!
! SYS_SIZE is the number of clicks (16 bytes) to be loaded.
! 0x3000 is 0x30000 bytes = 196kB, more than enough for current
! versions of linux
!
SYSSIZE = 0x3000
!
! bootsect.s (C) 1991 Linus Torvalds
!
! bootsect.s is loaded at 0x7c00 by the bios-startup routines, and moves
! iself out of the way to address 0x90000, and jumps there.
!
! It then loads 'setup' directly after itself (0x90200), and the system
! at 0x10000, using BIOS interrupts.
!
! NOTE! currently system is at most 8*65536 bytes long. This should be no
! problem, even in the future. I want to keep it simple. This 512 kB
! kernel size should be enough, especially as this doesn't contain the
! buffer cache as in minix
!
! The loader has been made as simple as possible, and continuos
! read errors will result in a unbreakable loop. Reboot by hand. It
! loads pretty fast by getting whole sectors at a time whenever possible.
.globl begtext, begdata, begbss, endtext, enddata, endbss //段定义
.text
begtext:
.data
begdata:
.bss
begbss:
.text
SETUPLEN = 4 ! nr of setup-sectors 扇区数量定义
BOOTSEG = 0x07c0 ! original address of boot-sector 开始段
INITSEG = 0x9000 ! we move boot here - out of the way 初始段
SETUPSEG = 0x9020 ! setup starts here setup程序所在段
SYSSEG = 0x1000 ! system loaded at 0x10000 (65536). 系统模块段
ENDSEG = SYSSEG + SYSSIZE ! where to stop loading 停止加载的位置
! ROOT_DEV: 0x000 - same type of floppy as boot. 软盘类型
! 0x301 - first partition on first drive etc 硬盘类型
ROOT_DEV = 0x306 启动盘设备类型
entry start 程序入口地址
start:
mov ax,#BOOTSEG
mov ds,ax ds段基地址
mov ax,#INITSEG
mov es,ax es段基地址
mov cx,#256
sub si,si si和di均清0
sub di,di
rep 循环256次共512个字节的内容
movw 传送字将bootseg段的代码移动到es指向的initseg段
jmpi go,INITSEG 跳转到initseg段的go标号位置
go: mov ax,cs cs ds es指向相同的位置
mov ds,ax
mov es,ax
! put stack at 0x9ff00.
mov ss,ax ss堆栈段基址位0x9ff00
mov sp,#0xFF00 ! arbitrary value >>512 sp指针的位置调整ss：sp=0x90000：ff00位置此时位空栈以上为初始化
! load the setup-sectors directly after the bootblock.
! Note that 'es' is already set up.
load_setup: 加载setup程序
mov dx,#0x0000 ! drive 0, head 0 驱动器0 磁头0 dh磁头 dl驱动器号
mov cx,#0x0002 ! sector 2, track 0 扇区2 磁道0 ch磁道号柱面号 cl开始扇区
mov bx,#0x0200 ! address = 512, in INITSEG 512个字节
mov ax,#0x0200+SETUPLEN ! service 2, nr of sectors ah=2 功能号为2 al=4 扇区数量
int 0x13 ! read it 中断13 读磁盘的功能读如到es：bx 数据缓冲区
jnc ok_load_setup ! ok - continue 判断cf标志位 cf=0正常 cf=1 出错
mov dx,#0x0000
mov ax,#0x0000 ! reset the diskette 重设寄存器
int 0x13 调用中断13号复位磁盘
j load_setup 跳转load_setup
ok_load_setup:
! Get disk drive parameters, specifically nr of sectors/track 以下为获取磁盘驱动器的参数
mov dl,#0x00
mov ax,#0x0800 ! AH=8 is get drive parameters
int 0x13 中断13 ah为8调用获取磁盘参数
mov ch,#0x00 ch清0
seg cs 找cs段执行指令下一条指令在cs段
mov sectors,cx 保存每磁道扇区数
mov ax,#INITSEG es指向0x9000的段处
mov es,ax
! Print some inane message
mov ah,#0x03 ! read cursor pos
xor bh,bh
int 0x10 中断读取光标位置
mov cx,#24 共24个字符
mov bx,#0x0007 ! page 0, attribute 7 (normal) bh为页 bl为属性
mov bp,#msg1 es：bp指向要显示的字符串
mov ax,#0x1301 ! write string, move cursor ah为13 显示字符串功能， al为1表示写方式
int 0x10 中断调用
! ok, we've written the message, now
! we want to load the system (at 0x10000)
mov ax,#SYSSEG es指向0x1000处
mov es,ax ! segment of 0x010000
call read_it 调用子程序 134行
call kill_motor 关闭马达 214行
! After that we check which root-device to use. If the device is
! defined (!= 0), nothing is done and the given device is used.
! Otherwise, either /dev/PS0 (2,28) or /dev/at0 (2,8), depending
! on the number of sectors that the BIOS reports currently.
seg cs
mov ax,root_dev
cmp ax,#0
jne root_defined
seg cs
mov bx,sectors
mov ax,#0x0208 ! /dev/ps0 - 1.2Mb
cmp bx,#15
je root_defined
mov ax,#0x021c ! /dev/PS0 - 1.44Mb
cmp bx,#18
je root_defined
undef_root:
jmp undef_root
root_defined:
seg cs
mov root_dev,ax
! after that (everyting loaded), we jump to
! the setup-routine loaded directly after
! the bootblock:
jmpi 0,SETUPSEG
! This routine loads the system at address 0x10000, making sure
! no 64kB boundaries are crossed. We try to load it as fast as
! possible, loading whole tracks whenever we can.
!
! in: es - starting address segment (normally 0x1000)
!
sread: .word 1+SETUPLEN ! sectors read of current track 磁道中已经读取的扇区数开始时已经读取1扇区 bootsect和setup程序所占的扇区数为setuplen
head: .word 0 ! current head
track: .word 0 ! current track
read_it:
mov ax,es
test ax,#0x0fff es必须是64k边界 es实际指向的内存为0x10000共20位相与的是高16位数值结果不保存
die: jne die ! es must be at 64kB boundary
xor bx,bx ! bx is starting address within segment bx指向段基地址
rp_read:
mov ax,es
cmp ax,#ENDSEG ! have we loaded all yet? 判断es是否等于停止加载的段地址
jb ok1_read 小于时跳转到ok1_read标号处继续执行，否则返回
ret
ok1_read:
seg cs cs段的指令执行
mov ax,sectors 取每磁道扇区数
sub ax,sread ax-sread 等于未读取的扇区数结果送ax
mov cx,ax
shl cx,#9 左移9位相当于乘以512 结果为未读扇区的字节数
add cx,bx 加上bx(段内偏移值) 此次读操作后段内读入字节数
jnc ok2_read cf标志位判断是否有进位如果不等于1 则跳转，即没有超过64k
je ok2_read 等于64k则跳转 156行处
xor ax,ax 超过64k 即加上此次将读磁道上所有未读扇区时会超过64k ，则ax清0
sub ax,bx ax-bx 计算此次能读入的字节数 64k-bx（段内读偏移）
shr ax,#9 再次将字节数转换成扇区数右移9位相当于除以512
ok2_read:
call read_track 调用read_track
mov cx,ax
add ax,sread
seg cs
cmp ax,sectors
jne ok3_read
mov ax,#1
sub ax,head
jne ok4_read
inc track
ok4_read:
mov head,ax
xor ax,ax
ok3_read:
mov sread,ax
shl cx,#9
add bx,cx
jnc rp_read
mov ax,es
add ax,#0x1000
mov es,ax
xor bx,bx
jmp rp_read
read_track:
push ax
push bx
push cx
push dx
mov dx,track
mov cx,sread
inc cx
mov ch,dl
mov dx,head
mov dh,dl
mov dl,#0
and dx,#0x0100
mov ah,#2
int 0x13
jc bad_rt
pop dx
pop cx
pop bx
pop ax
ret
bad_rt: mov ax,#0
mov dx,#0
int 0x13
pop dx
pop cx
pop bx
pop ax
jmp read_track
/*
* This procedure turns off the floppy drive motor, so
* that we enter the kernel in a known state, and
* don't have to worry about it later.
*/
kill_motor:
push dx
mov dx,#0x3f2
mov al,#0
outb
pop dx
ret
sectors:
.word 0
msg1:
.byte 13,10
.ascii "Loading system ..."
.byte 13,10,13,10
.org 508
root_dev:
.word ROOT_DEV
boot_flag:
.word 0xAA55
.text
endtext:
.data
enddata:
.bss
endbss:

posted on 2017-11-27 18:59 学习记录园阅读(587) 评论(0) 编辑收藏举报

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