boot.s is loaded at 0x7c00 by the bios-startup routines, and moves itself out of the way to address 0x90000, and jumps there.
当PC机启动时，Intel系列的CPU首先进入的是实模式，并开始执行位于地址0xFFF0处的代码，也就是ROM-BIOS起始位置的代码。BIOS先进行一系列的系统自检，然后初始化位于地址0的中断向量表。最后BIOS将启动盘的第一个扇区装入0x7C00(31K；0111,1100,0000,0000)，并开始执行此处的代码。这就是对内核初始化过程的一个最简单的描述。
最初，Linux核心的最开始部分是用8086汇编语言编写的。当开始运行时，核心将自己装入到绝对地址0x90000(576K；1001,0000,0000,0000,0000)，再将其后的2k字节装入到地址0x90200(576.5k；1001,0000,0010,0000,0000)处，最后将核心的其余部分装入到0x10000(64k；1,0000,0000,0000,0000).
It then loads the system at 0x10000, using BIOS interrupts. Thereafter it disables all interrupts, moves the system down to 0x0000, changes to protected mode, and calls the start of system. System then must RE-initialize the protected mode in it's own tables, and enable interrupts as needed.
然后，关掉所有中断，把系统下移到0x0000(0k；0000,0000,0000,0000,0000)处，改变到保护模式，然后开始系统的运行．系统必须重新在保护模式下初始化自己的系统表格，并且打开所需的中断．
NOTE 1! currently system is at most 8*65536(8*64k=512k；1000,0000,0000,0000,0000) 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 - in fact more would mean we'd have to move not just these start-up routines, but also do something about the cache-memory (block IO devices). The area left over in the lower 640 kB(0xA0000；1010,0000,0000,0000,0000) is meant for these. No other memory is assumed to be "physical", ie all memory over 1Mb is demand-paging. All addresses under 1Mb are guaranteed to match their physical addresses.
NOTE1 abouve is no longer valid in it's entirety. cache-memory is allocated above the 1Mb mark as well as below. Otherwise it is mainly correct.
NOTE 2! The boot disk type must be set at compile-time, by setting the following equ. Having the boot-up procedure hunt for the right disk type is severe brain-damage. The loader has been made as simple as possible (had to, to get it in 512 bytes with the code to move to protected mode), 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.
| 1.44Mb disks: sectors = 18
| 1.2Mb disks:
| sectors = 15
| 720kB disks:
| sectors = 9
**********************************************************************************************
.globl begtext, begdata, begbss, endtext, enddata, endbss
.text
begtext:
.data
begdata:
.bss
begbss:
.text
BOOTSEG = 0x07c0
|把第一个扇区装入到此处
INITSEG = 0x9000
|核心装入地址的段地址
SYSSEG = 0x1000
|system loaded at 0x10000 (65536).
ENDSEG = SYSSEG + SYSSIZE
|SYSSIZE在Makefile中定义的 ^_^
entry start
start:
mov ax,#BOOTSEG
| BOOTSEG = 0x07C0；现在应仍处在REAL MODE下.
mov ds,ax
| 移动自身从BOOTSEG:0000到INITSEG:0000
mov ax,#INITSEG
| 共512字节.
mov es,ax
| 那么BOOT.S处在0x90000-0x90200.
mov cx,#256
sub si,si
| 寄存器清零
sub di,di
| 寄存器清零
rep
movw
| 将由SI作为指针的源串中的一个字节或字或双字传送到由DI作为指针的目的串中，并根据标志DF值自动修
| 改这两个指针以指向串中下一项。若带有前缀REP，则重复执行这一传送，直到CX寄存器等于零为止。
| 源地址：DS=0x07C0(31K；0111,1100,0000,0000)；
| 目的地址：ES=0x9000(576K；1001,0000,0000,0000,0000)
| 小结：BIOS将启动盘的第一个扇区调入到0x07C00处，然后系统把自己从0x07C00处移动到0x90000-
| 0x90200处.
jmpi go,INITSEG
go: mov ax,cs
mov ds,ax
mov es,ax
| 将DS,ES,SS均设为0x9000,所有数据都以
| 0x9000为段偏移.
mov ss,ax
| 堆栈偏移0x9000
mov sp,#0x400
| 栈顶指针0x9000:0x0400,堆栈空间512bytes??
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
| 显示Loading System ...
mov ax,#0x1301
| write string, move cursor
int 0x10
| ok, we've written the message, now
| we want to load the system (at 0x10000)
| 我们已经完成了“Loading...”的在屏幕上显示，
| 以下我们将完成把核心从0x10000(64k)移到0x01000(4k)处.
mov ax,#SYSSEG
| SYSSEG = 0x1000
mov es,ax
| segment of 0x010000
call read_it
| 读内核到0x10000
call kill_motor
| 杀了软驱!? ^_^
| if the read went well we get current cursor position ans save it for
| posterity.
mov ah,#0x03
| read cursor pos
xor bh,bh
int 0x10
| save it in known place, con_init fetches
mov [510],dx
| it from 0x90510(1001,0000,0101,0001,0000).
| 功能03H，读取光标位置和类型。AH=03H，BH=页号。
| 返回：DH=当前字符行号； DL=当前字符列号
| CH=光标的起始光栅线；CL=光标的终止光栅线
| now we want to move to protected mode ...
cli
| 关掉所有中断
| first we move the system to it's rightful place
mov ax,#0x0000
cld
| 'direction'=0, movs moves forward
do_move:
mov es,ax
| ES=0x0000；destination segment
add ax,#0x1000
cmp ax,#0x9000
jz end_move
mov ds,ax
| DS=0x1000；source segment
sub di,di
| 置零,地址为0x1000:0000
sub si,si
| 置零,地址为0x9000:0000
mov cx,#0x8000
| cx的作用是计数器,共0x8000=32k
rep
movsw
j do_move
| 将位于低端0x1000:0000的内核移到内存
| 高端0x9000:0000,覆盖了boot.S !?
| then we load the segment descriptors
end_move:
mov ax,cs
| right, forgot this at first. didn't work :-)
mov ds,ax
lidt idt_48
| idt_48和gdt_48都是一个3个word长的数据结构
lgdt gdt_48
| 第一个字说明(Global || Interrupt) Descriptor
| Table有多长,因为每个Table是四个字长,所以
| 可以得出整个DescriptorTable的entries.(见下)
| 后两个字指出DT的具体位置.
| idt_48是0,0,0;应表示没有中断描述符entries.
| gdt_48有256个入口,第一个是个空入口,然后
| 定义了一个code段和一个data段.基址都是
| 0x00000000, !?那里是什么东西???
| *** 0x00000000 != 0x0000:0000 ***
| 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.
| 初始化中断处理器8259i
| 初始化顺序为: 1. 向主8259A写ICW1, 0x20
| 2. 向第二块8259A写ICW1, 0xA0
| 3. 向主8259A写ICW2, 0x21
| 4. 向第二块8259A写ICW2, 0xA1
| 5. 如果ICW1指示有级联中断处理器,则初始化Master&Slave
| (在下例中只有IR2有级联8259A), 0x21, 0xA1
| 6. 向两块8259写ICW4,指定工作模式.
| 输入了适当的初始化命令之后, 8259已经准备好接收中断请求.
| 现在向他输入工作
| 命令字以规定其工作方式. 8259A共有三个工作命令字,但下例中只用过OCW1.
| OCW1将所有的中断都屏蔽掉, OCW2&OCW3也就没什么意义了.
| ** ICW stands for Initialization Command Word;
| OCW for Operation Command Word.
1. mov al,#0x11
out #0x20,al
.word 0x00eb,0x00eb | jmp $+2, jmp $+2
2. out #0xA0,al | and to 8259A-2
.word 0x00eb,0x00eb
3. mov al,#0x20 | 向主8259A写入ICW2.
out #0x21,al | 硬件中断入口地址0x20, 并由ICW1
| 得知中断向量长度 = 8 bytes.
.word 0x00eb,0x00eb
4. mov al,#0x28 | start of hardware int's 2 (0x28)
out #0xA1,al | 第二块8259A的中断入口是0x28.
.word 0x00eb,0x00eb
5. mov al,#0x04 | 8259-1 is master
out #0x21,al | Interrupt Request 2有级联处理.
.word 0x00eb,0x00eb
mov al,#0x02 | 8259-2 is slave
out #0xA1,al | 于上面对应,告诉大家我就是IR2对应
| 级联处理器.
.word 0x00eb,0x00eb
6. 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!
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
*********************************************************************************
| 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)
|
| This routine has to be recompiled to fit another drive type,
| just change the "sectors" variable at the start of the file
| (originally 18, for a 1.44Mb drive)
|
sread: .word 1 | sectors read of current track
head: .word 0 | current head
track: .word 0 | current track
**read-it子函数********************************************************************
read_it:
mov ax,es
| ES当前应0x1000，对，是这样的！
test ax,#0x0fff
| 目的操作数与源操作数进行逻辑与操作，结果只反映在标志位上，对两个操作数无影响
| 必需确保ES处在64KB段边界上,即0x?000:XXXX.
| 要不你就会收到一个"DMA..."什么什么的ERR.
die: jne die
| jne:不相等/不等于零时转移，ZF=0
| es must be at 64kB boundary
xor bx,bx
| bx is starting address within segment
rp_read:
| **** 循
