打开电源,计算机执行的第一句指令是什么?
(1) x86PC刚开机时CPU处于实模式;
(2) 开机时, CS=0xFFFF, IP=0x0000;
(3) 寻址0xFFFF0(ROM BIOS映射区);
(4) 检查RAM, 键盘, 显示器, 软硬磁盘;
(5) 将软盘0面0道1扇区(主引导扇区, 对应操作系统第一段代码,即主引导程序.Linux 0.11源码对应文件为boot/bootsect.s)读入0x7c00处;
(6) 设置cs=0x07c0, ip=0x0000.
bootsect.s
bootsect.s （1个扇区）把自身从0x7c000处搬到0x90000处，再把setup.s（4个扇区）搬到0x90200处，然后调用0x13号中断在屏幕上打印“Loading system…”，再把system模块加载到0x10000处。之后确定根文件系统的设备号，若没指定，则根据所保存的引导盘的每磁道扇区数判定盘的类型（是1.44M A盘吗？）并保存其设备号于root_dev。最后139行跳转到setup.s执行。
140行~227行代码用来加载system模块，由于采用比较原始的CHS模式读取软盘比较复杂，不再细看，暂且当成一个黑箱吧，日后若研究细节再做分析。
setup.s
代码105行之前，利用 ROM BIOS 中断例程获取系统参数（如光标位置、内存大小等），并写入 0x900000-x901FF 处（覆盖原来的 bootsect.s 区域）。然后把 system 模块从 0x10000-0x8ffff （当时认为 system 模块不会超过512K）搬到 0x0000 处（0x00000-0x003ff 的256个中断号的中断向量表被覆盖，之后再用int就不再是查0x00000处的中断向量表了，而是查IDT）。然后开启 A20 地址线并设置 8259A 中断控制芯片，再设置 CR0 的 PE 位，进入 32 位保护模式，转到 system 模块的第一个代码 head.s 执行。
执行191行之后，跳到0x00000000。那么该处对应哪个代码文件呢？
Makefile
要让操作系统正常运行，必须通过Makefile控制各个代码文件加载顺序、如何组合等工作。Makefile是make工具的配置文件，相当于批处理文件，在含有Makefile的当前目录里执行make命令，就会根据Makefile中的信息对源代码进行编译、链接等工作。
根据Makefile的配置要求，0x00000000处就是system模块的head.s，下面要进入head.s执行。
head.s
本程序重新设置GDT和IDT，并做一些校验工作（如A20地址线是否启动等），然后设置并开启分页（分页相关的内容在内存管理再说），然后跳入main函数执行。
源代码（个人适当增加注释）
bootsect.s
!
! 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 ! 设备号=主设备*256+次设备号，指定根文件系统是第2个硬盘的第1个分区
! （主设备号：1-内存/2-软盘/3-硬盘/4-ttyx/5-tty/6-并行口/7-非命名管道）
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，从0x90000开始的5个扇区是bootsect.s和setup.s，
! sp > 5*0x200 + 堆栈大小
! 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，参考BIOS中断手册，查看0x13号详情对比阅读
jnc ok_load_setup ! ok - continue
mov dx,#0x0000
mov ax,#0x0000 ! reset the diskette
int 0x13
j load_setup ! 加载不成功，重新加载
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 ! 87行只影响下一行即88行，87和88行等价于mov cs:[sectors],cx
mov sectors,cx ! 每道的扇区数
mov ax,#INITSEG
mov es,ax
! Print some inane message
mov ah,#0x03 ! read cursor pos
xor bh,bh
int 0x10
mov cx,#24 ! 打印到屏幕的字节数
mov bx,#0x0007 ! page 0, attribute 7 (normal)
mov bp,#msg1 ! 加载完setup.s，就打印msg1处的信息到屏幕
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 ! 关闭驱动马达，这样就可以获取驱动器的状态信息了
! 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.
! 软驱主设备号是2，次设备号是type*4+nr，nr为0-3对应软驱A/B/C/D
seg cs ! type是软驱类型，2-1.2M，7-1.44M，7*4+0=28（/dev/PS0，设备号0x021c）
mov ax,root_dev ! 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:
setup.s
!
! setup.s (C) 1991 Linus Torvalds
!
! setup.s is responsible for getting the system data from the BIOS,
! and putting them into the appropriate places in system memory.
! both setup.s and system has been loaded by the bootblock.
!
! This code asks the bios for memory/disk/other parameters, and
! puts them in a "safe" place: 0x90000-0x901FF, ie where the
! boot-block used to be. It is then up to the protected mode
! system to read them from there before the area is overwritten
! for buffer-blocks.
!
! NOTE! These had better be the same as in bootsect.s!
INITSEG = 0x9000 ! we move boot here - out of the way
SYSSEG = 0x1000 ! system loaded at 0x10000 (65536).
SETUPSEG = 0x9020 ! this is the current segment
.globl begtext, begdata, begbss, endtext, enddata, endbss
.text
begtext:
.data
begdata:
.bss
begbss:
.text
entry start
start:
! ok, the read went well so we get current cursor position and save it for
! posterity.
mov ax,#INITSEG ! this is done in bootsect already, but...
mov ds,ax
mov ah,#0x03 ! read cursor pos
xor bh,bh
int 0x10 ! save it in known place, con_init fetches
mov [0],dx ! it from 0x90000.
! Get memory size (extended mem, kB)
mov ah,#0x88
int 0x15
mov [2],ax
! Get video-card data:
mov ah,#0x0f
int 0x10
mov [4],bx ! bh = display page
mov [6],ax ! al = video mode, ah = window width
! check for EGA/VGA and some config parameters
mov ah,#0x12
mov bl,#0x10
int 0x10
mov [8],ax
mov [10],bx
mov [12],cx
! Get hd0 data
mov ax,#0x0000
mov ds,ax
lds si,[4*0x41] ! lds reg,操作数，操作数高16位给ds，低16位给reg
mov ax,#INITSEG
mov es,ax
mov di,#0x0080
mov cx,#0x10
rep
movsb
! Get hd1 data
mov ax,#0x0000
mov ds,ax
lds si,[4*0x46]
mov ax,#INITSEG
mov es,ax
mov di,#0x0090
mov cx,#0x10
rep
movsb
! Check that there IS a hd1 :-)
mov ax,#0x01500
mov dl,#0x81
int 0x13
jc no_disk1
cmp ah,#3
je is_disk1
no_disk1:
mov ax,#INITSEG
mov es,ax
mov di,#0x0090
mov cx,#0x10
mov ax,#0x00
rep
stosb
is_disk1:
! now we want to move to protected mode ...
cli ! no interrupts allowed !
! first we move the system to it's rightful place
mov ax,#0x0000
cld ! 'direction'=0, movs moves forward
do_move:
mov es,ax ! destination segment
add ax,#0x1000
cmp ax,#0x9000
jz end_move
mov ds,ax ! source segment
sub di,di
sub si,si
mov cx,#0x8000
rep
movsw
jmp do_move
! then we load the segment descriptors
end_move:
mov ax,#SETUPSEG ! right, forgot this at first. didn't work :-)
mov ds,ax
lidt idt_48 ! load idt with 0,0
lgdt gdt_48 ! load gdt with whatever appropriate
! that was painless, now we enable A20
call empty_8042
mov al,#0xD1 ! command write
out #0x64,al
call empty_8042
mov al,#0xDF ! A20 on
out #0x60,al
call empty_8042
! well, that went ok, I hope. Now we have to reprogram the interrupts :-(
! we put them right after the intel-reserved hardware interrupts, at
! int 0x20-0x2F. There they won't mess up anything. Sadly IBM really
! messed this up with the original PC, and they haven't been able to
! rectify it afterwards. Thus the bios puts interrupts at 0x08-0x0f,
! which is used for the internal hardware interrupts as well. We just
! have to reprogram the 8259's, and it isn't fun.
mov al,#0x11 ! initialization sequence
out #0x20,al ! send it to 8259A-1
.word 0x00eb,0x00eb ! jmp $+2, jmp $+2
out #0xA0,al ! and to 8259A-2
.word 0x00eb,0x00eb
mov al,#0x20 ! start of hardware int's (0x20)
out #0x21,al
.word 0x00eb,0x00eb
mov al,#0x28 ! start of hardware int's 2 (0x28)
out #0xA1,al
.word 0x00eb,0x00eb
mov al,#0x04 ! 8259-1 is master
out #0x21,al
.word 0x00eb,0x00eb
mov al,#0x02 ! 8259-2 is slave
out #0xA1,al
.word 0x00eb,0x00eb
mov al,#0x01 ! 8086 mode for both
out #0x21,al
.word 0x00eb,0x00eb
out #0xA1,al
.word 0x00eb,0x00eb
mov al,#0xFF ! mask off all interrupts for now
out #0x21,al
.word 0x00eb,0x00eb
out #0xA1,al
! well, that certainly wasn't fun :-(. Hopefully it works, and we don't
! need no steenking BIOS anyway (except for the initial loading :-).
! The BIOS-routine wants lots of unnecessary data, and it's less
! "interesting" anyway. This is how REAL programmers do it.
!
! Well, now's the time to actually move into protected mode. To make
! things as simple as possible, we do no register set-up or anything,
! we let the gnu-compiled 32-bit programs do that. We just jump to
! absolute address 0x00000, in 32-bit protected mode.
mov ax,#0x0001 ! protected mode (PE) bit
lmsw ax ! This is it! 加载机器状态字
jmpi 0,8 ! jmp offset 0 of segment 8 (cs)
! This routine checks that the keyboard command queue is empty
! No timeout is used - if this hangs there is something wrong with
! the machine, and we probably couldn't proceed anyway.
empty_8042:
.word 0x00eb,0x00eb
in al,#0x64 ! 8042 status port
test al,#2 ! is input buffer full?
jnz empty_8042 ! yes - loop
ret
gdt:
.word 0,0,0,0 ! dummy
.word 0x07FF ! 8Mb - limit=2047 (2048*4096=8Mb)
.word 0x0000 ! base address=0
.word 0x9A00 ! code read/exec
.word 0x00C0 ! granularity=4096, 386
.word 0x07FF ! 8Mb - limit=2047 (2048*4096=8Mb)
.word 0x0000 ! base address=0
.word 0x9200 ! data read/write
.word 0x00C0 ! granularity=4096, 386
idt_48:
.word 0 ! idt limit=0
.word 0,0 ! idt base=0L
gdt_48:
.word 0x800 ! gdt limit=2048, 256 GDT entries
.word 512+gdt,0x9 ! gdt base = 0X9xxxx 32位线性基地址高16位是0x0009，
! 低16位是512+gdt，对应绝对地址就是0x0009xxxx，更
.text ! 准确的就是0x0009020+gdt，正好就是本代码标号gdt处
endtext:
.data
enddata:
.bss
endbss:
参考资料
[1] 中国大学MOOC《操作系统》李治军 哈尔滨工业大学
[2] 《Linux内核完全注释》赵炯
[3] BIOS中断大全
[4] 汇编语言最全指令表
[5] 《x86汇编语言:从实模式到保护模式》李忠 著
[6] https://blog.youkuaiyun.com/ccnuacmhdu/article/details/104946213
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