一、总体功能介绍
这是关于Linux-kernel-0.11中boot文件夹下bootsect.s源文件的说明，其中涉及到了一些基础知识可以参考这两篇文章。
bootsect.s 代码是磁盘引导块程序，存储在磁盘的第一个扇区中(0面0道1扇区)，在计算机上电BIOS自检后，BIOS 会吧引导扇区代码bootsect加载到内存0x90000处开并运行。
bootsect代码主要完成以下几项工作：
加载从磁盘第二个扇区开始的4个扇区的内容（由setup.s编译而成）到内存紧接着bootsect后面的0x90200处。
利用BIOS中断 int 13h 获取磁盘参数表中当前启动引导盘的参数。
在屏幕上显示
"Loading system..."字符串。把磁盘上setup模块后面的system模块加载到内存0x10000开始的地方。
确定根文件系统的设备号，若没有指定，则根据所保存的引导盘的每磁道扇区数判别出磁盘的类型和种类(1.2MB 或 1.44MB 软盘)，并将其设备号保存在root_dev（引导扇区的508地址出，即第一个扇区的第509个字节处）。
长跳转到setup程序开始处(0x90200)执行setup程序。
在磁盘上，引导块bootsect、setup模块和system模块的扇区位置和大小如下图所示：
二、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! SYS_SIZE 是要加载的系统模块的长度，单位是节 1节 = 16字节， 0x3000 字节 = 196kB。！这里定义了system模块长度，所以makefile中的规则就失效了！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 或 .global 用于定义随后的标识符是外部的或全局的，并且即使不使用也强制引入。! .text .data .bss 分别定义当前代码段、数据段和未初始化数据段。.globl begtext, begdata, begbss, endtext, enddata, endbss !定义了6个全局标识符.text !文本段begtext:.data !数据段begdata:.bss !未初始化数据段begbss:.textSETUPLEN = 4 ! nr of setup-sectors setup程序的扇区(setup-sectors)值BOOTSEG = 0x07c0 ! original address of boot-sector bootsect的原始值(是段地址) INITSEG = 0x9000 ! we move boot here - out of the way 将bootsect移到这里SETUPSEG = 0x9020 ! setup starts here setup程序从这里开始SYSSEG = 0x1000 ! system loaded at 0x10000 (65536). system模块加载到0x10000(64KB)处ENDSEG = SYSSEG + SYSSIZE ! where to stop loading 停止加载的段地址! ROOT_DEV: 0x000 - same type of floppy as boot. 根文件系统设备与引导使用同样的软驱设备! 0x301 - first partition on first drive etc 根文件系统设备在第一个硬盘的第一个分区上。ROOT_DEV = 0x306entry start !告诉链接程序，程序从标号start开始执行start: mov ax,#BOOTSEG !设置ds为0x07c0(段地址) mov ds,ax mov ax,#INITSEG !设置es为0x9000(段地址) mov es,ax mov cx,#256 !移动计数值256个字 sub si,si !源地址 ds:si = 0x07c0:0x0000 sub di,di !目的地址 es:di = 0x9000:0x0000 rep !重复执行并cx的值，直到cx = 0 movw !串传送指令，从[si]移动cx个字到[di]处。从start开始到这里，这段程序将bootsect从0x07c0复制到0x9000处 jmpi go,INITSEG !段间跳转，INITSEG 指跳转到的段地址， 标号go是段内偏移地址。go: mov ax,cs !设置ds，ss和es为复制代码后代码所在的段。 mov ds,ax mov es,ax! put stack at 0x9ff00. mov ss,ax mov sp,#0xFF00 ! arbitrary value >>512 设置栈顶指针，远大于512个字节偏移。! load the setup-sectors directly after the bootblock.! Note that 'es' is already set up.!加载 setup 模块代码数据，注意 es已经指向了 0x9000处，不用再设置。!将setup从磁盘第二个扇区读到0x90200开始处，共读4个扇区。如果读出错，则复位驱动器并重读。load_setup: mov dx,#0x0000 ! drive 0, head 0 对驱动器0进行操作 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 ! j 即jmp指令ok_load_setup:! Get disk drive parameters, specifically nr of sectors/track! 获取磁盘参数，尤其是每道的扇区数量 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 在屏幕上显示msg1指向的字符串，"Loading system ..." 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 ! 写字符串并将光标移动到字符串结尾处。! ok, we've written the message, now 现在开始将system模块加载到 0x10000处(64KB处)! we want to load the system (at 0x10000) mov ax,#SYSSEG mov es,ax ! segment of 0x010000 call read_it ! 读磁盘上的system模块，es为输入参数 call kill_motor ! 关闭驱动马达，这样就可以知道驱动器的状态了。! 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.! 确定选用哪个根文件系统 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 ! 判断每磁道扇区数是否为15 je root_defined mov ax,#0x021c ! /dev/PS0 - 1.44Mb cmp bx,#18 ! 判断每磁道扇区数是否为18 je root_definedundef_root: jmp undef_root 如果根文件系统设备都不对，则死循环。root_defined: seg cs mov root_dev,ax ! 将检查过的设备号保存到 root_dev 中! after that (everyting loaded), we jump to! the setup-routine loaded directly after! the bootblock: jmpi 0,SETUPSEG ! 到这里所有的文件都已经加载完毕，程序跳转到setup文件中去执行。!!!!! bootsect.s 执行到这里就结束了。 下面是两个子程序。! 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 当前磁道中已读扇区数head: .word 0 ! current head 当前磁头号track: .word 0 ! current track 当前磁道号read_it: mov ax,es test ax,#0x0fffdie: jne die ! es must be at 64kB boundary xor bx,bx ! bx is starting address within segmentrp_read: mov ax,es cmp ax,#ENDSEG ! have we loaded all yet? jb ok1_read retok1_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,#9ok2_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 trackok4_read: mov head,ax xor ax,axok3_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_readread_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 retbad_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 !软驱控制卡的数字输出寄存器(DOR)端口，只写 mov al,#0 !A驱动器，关闭FDC，禁止DMA和中断请求，关闭马达。 outb !将al中的内容输出到dx指定的端口。 pop dx retsectors: .word 0 !存放当前启动软盘每磁道的存储扇区数。msg1: !调用BIOS中断显示的信息 .byte 13,10 !回车，换行的ASCⅡ码 .ascii "Loading system ..." .byte 13,10,13,10 !共24个ASCⅡ码字符.org 508 !表示语句从地址508(0x01fc)开始，所以root_dev在启动扇区的第508开始的2个字节中。root_dev: .word ROOT_DEV !这里存放根文件系统所在设备号(init/main.c 中会用到)boot_flag: .word 0xAA55 !启动盘具有有效引导扇区的标志，仅供BIOS中程序加载引导扇区时识别使用。 !它必须位于引导扇区的最后两个字节中。.textendtext:.dataenddata:.bssendbss: