一, bootsect.s程序功能简要分析
下面这段代码就是bootsect的源程序, 这是Linus Torvalds在1991年写的
! ! 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 .text begtext: .data begdata: .bss begbss: .text SETUPLEN = 4 ! nr of setup-sectors BOOTSEG = 0x07c0 ! original address of boot-sector INITSEG = 0x9000 ! we move boot here - out of the way SETUPSEG = 0x9020 ! setup starts here SYSSEG = 0x1000 ! system loaded at 0x10000 (65536). ENDSEG = SYSSEG + SYSSIZE ! where to stop loading ! ROOT_DEV: 0x000 - same type of floppy as boot. ! 0x301 - first partition on first drive etc ROOT_DEV = 0x306 entry _start _start: mov ax,#BOOTSEG mov ds,ax mov ax,#INITSEG mov es,ax mov cx,#256 sub si,si sub di,di rep movw jmpi go,INITSEG go: mov ax,cs mov ds,ax mov es,ax ! put stack at 0x9ff00. mov ss,ax mov sp,#0xFF00 ! arbitrary value >>512 ! load the setup-sectors directly after the bootblock. ! Note that ‘es‘ is already set up. load_setup: mov dx,#0x0000 ! drive 0, head 0 mov cx,#0x0002 ! sector 2, track 0 mov bx,#0x0200 ! address = 512, in INITSEG mov ax,#0x0200+SETUPLEN ! service 2, nr of sectors int 0x13 ! read it jnc ok_load_setup ! ok - continue mov dx,#0x0000 mov ax,#0x0000 ! reset the diskette int 0x13 j load_setup ok_load_setup: ! Get disk drive parameters, specifically nr of sectors/track mov dl,#0x00 mov ax,#0x0800 ! AH=8 is get drive parameters int 0x13 mov ch,#0x00 seg cs mov sectors,cx mov ax,#INITSEG mov es,ax ! Print some inane message mov ah,#0x03 ! read cursor pos xor bh,bh int 0x10 mov cx,#24 mov bx,#0x0007 ! page 0, attribute 7 (normal) mov bp,#msg1 mov ax,#0x1301 ! write string, move cursor int 0x10 ! ok, we‘ve written the message, now ! we want to load the system (at 0x10000) mov ax,#SYSSEG mov es,ax ! segment of 0x010000 call read_it call kill_motor ! 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 head: .word 0 ! current head track: .word 0 ! current track read_it: mov ax,es test ax,#0x0fff die: jne die ! es must be at 64kB boundary xor bx,bx ! bx is starting address within segment rp_read: mov ax,es cmp ax,#ENDSEG ! have we loaded all yet? jb ok1_read ret ok1_read: seg cs mov ax,sectors sub ax,sread mov cx,ax shl cx,#9 add cx,bx jnc ok2_read je ok2_read xor ax,ax sub ax,bx shr ax,#9 ok2_read: call 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:
程序从entry _start开始执行
1, start标号开始:
首先将寄存器ax的值设置为0x7c00, 将as的值复制到ds段寄存器, 然后用同样的方法将es的值设置为0x9000, 然后设置cx为256, 然后再将0x7c00处的512个字节拷贝到0x9000处, 接着就跳转到go标号处继续执行. (此时bootsect程序本身就全部被复制到了0x9000处, cs:ip指向了"0x9000 + go" 地址处)
2, go标号:
设置ds和es段寄存器指向当前段, 并设置栈段为0x9ff00, 其中栈顶指针sp指向0xFF00
3, load_setup标号:
通过linus的注释, 可以知道这段代码的功能是通过BIOS的13号终端把磁盘的第2, 3, 4, 5扇区的程序读入到内存0x90200处, 如果读入失败, 将再次读取, 一直循环下去. 如果读入成功, 就跳到标号ok_load_setup处
4, ok_load_setup标号:
这里有一个注释: Get disk drive parameters, specifically nr of sectors/track. 意思是: 获取磁盘驱动器的一些信息, 主要是每磁道/扇区的数量.
接下来还有个注释: Print some inane message. 意思是: 打印一些无意义的信息
可以发现这是通过BIOS的10号中断实现的, 其中CX的值是要打印的字符个数, bp的值为标号msg1的地址. 然后看看msg的内容:
可以发现这个字符串和启动系统时显示的字符串是一样的.
然后注释写到: ok, we‘ve written the message, now we want to load the system (at 0x10000) 意思是: 我们已经写了一些信息了, 现在我们想要载入system模块到内存0x10000处
接下来就是载入system模块的代码了
然后注释写到: after that (everyting loaded), we jump to the setup-routine loaded directly after the bootblock
意思是: 所有的程序都加载完毕后, 我们就要跳到setup模块去了. 于是就执行 jmpi 0, SETUPSEG
到此, bootsect的任务就完成了.
二, 总结启动过程
1, 刚开始的时候, CS:ip指向0x7c00处, 也就是bootsect程序的第一条指令处
2, 接下来bootsect将自身复制到0x9000处
3, 载入setup模块
4, 载入system模块到0x10000处
5, 跳转到setup模块执行
三, 修改bootsect.s
1, 目标
修改bootsect.s程序, 不必载入setup和system模块, 删除与这些功能相关的代码, 仅仅让bootsect执行的时候显示一条信息:
AXF OS is bootding, my name is AiXiangfei …
2, 实现
.globl begtext, begdata, begbss, endtext, enddata, endbss
.text
begtext:
.data
begdata:
.bss
begbss:
.text
BOOTSEG = 0x07c0
ROOT_DEV = 0x306
entry _start
_start:
! 打印消息
mov ah,#0x03 ! 读取光标位置
xor bh,bh
int 0x10
mov cx,#50
mov bx,#0x0007 ! page 0, attribute 7 (normal)
mov bp,#msg1
mov ax,#BOOTSEG
mov es,ax
mov ax,#0x1301 ! write string, move cursor
int 0x10 ! 10号中断，打印字符串
s:
mov cx,#1 ！ 无限循环
add cx,#1
loop s
msg1:
.byte 13,10
.ascii "AXF OS is booting, my name is AiXiangfei ... "
.byte 13,10,13,10
.org 508
root_dev:
.word ROOT_DEV
boot_flag:
.word 0xAA55
.text
endtext:
.data
enddata:
.bss
endbss:
3, 运行截图
