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:
.text
SETUPLEN = 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 = 0x306
entry 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_defined
undef_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,#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 !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
ret
sectors:
.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。
.text
endtext:
.data
enddata:
.bss
endbss:
