Linux内核0.11 bootsect文件说明

!
! 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: