linux-0.11-操作系统引导

bootsect.s逐字分析

_start 部分

设置数据段（DS）和额外段（ES）寄存器:
```
mov	ax, #BOOTSEG
mov	ds, ax
mov	ax, #INITSEG
mov	es, ax
```
这部分代码将BOOTSEG（0x07c0）值加载到DS寄存器，设置数据段指向引导扇区的原始地址。然后，它将INITSEG（0x9000）值加载到ES寄存器，设置额外段指向内存中的一个新位置，用于临时存储数据。
复制引导扇区:
```
mov	cx, #256
sub	si, si
sub	di, di
rep movsw
```
这里，代码使用rep movsw指令复制引导扇区的内容。CX寄存器设置为256，表示要复制的字（word）数（512字节/2=256字）。SI（源索引）和DI（目的索引）寄存器被清零，指示复制操作的开始位置。rep movsw会重复执行movsw指令，将数据从DS:SI复制到ES:DI，直到CX减到0。
跳转到新位置执行代码:
```
jmpi	go, INITSEG
```
使用远跳转（jmpi）到INITSEG段中的go标签开始执行新的代码段。这个指令实际上是将控制权转移给在INITSEG中更高地址空间的代码，为进一步的操作系统加载做准备。

go 部分

设置段寄存器:
```
go:	mov	ax, cs
mov	ds, ax
mov	es, ax
```
在go标签处，首先将当前代码段（CS）的值加载到AX寄存器，然后将这个值赋给数据段（DS）和额外段（ES），确保数据和额外段指向当前执行的代码段。
设置堆栈段（SS）和堆栈指针（SP）:
```
mov	ss, ax
mov	sp, #0xFF00
```
这里将堆栈段（SS）设置为当前的代码段，并将堆栈指针（SP）设置为0xFF00。这个设置为程序提供了一个堆栈空间，用于存储临时数据和处理函数调用。

在_start部分，程序准备了数据复制的环境并将引导扇区的内容移动到了一个安全的内存位置（INITSEG）。这样做是为了避免后续操作覆盖引导扇区的内容。然后，通过jmpi跳转到go部分，go部分接着对环境进行进一步设置，尤其是为程序的运行配置堆栈环境。这些步骤为加载和执行操作系统的剩余部分做好了准备。

`
!
! 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

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

! 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
	jnc	ok_load_setup		! ok - continue
	mov	dx,#0x0000
	mov	ax,#0x0000		! reset the diskette
	int	0x13
	j	load_setup

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
	mov	sectors,cx
	mov	ax,#INITSEG
	mov	es,ax

! Print some inane message
	
	call read_cursor
	mov	cx,#2
	mov	bx,#0x0007		! page 0, attribute 7 (normal, white color)
	mov	bp,#msg1
	mov	ax,#0x1301		! write string, move cursor
	int	0x10

	call read_cursor
	mov cx, #6
	mov bx, #0x0009		! page 0, attribute 9(bright blue color)
	mov bp, #msg1+2
	mov ax, #0x1301		! write string, move cursor
	int 0x10
	
	call read_cursor
	mov cx, #18
	mov bx, #0x0007		! page 0, attribute 7(normal, white color)
	mov bp, #msg1+8
	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

! 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.

	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
	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

read_cursor:
	push ax
	push bx
	mov	ah,#0x03		! read cursor pos
	xor	bh,bh
	int	0x10
	pop bx
	pop ax
	ret

!/*
! * 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 "Qiunix is loading..."
	.byte 13,10,13,10

.org 508
root_dev:
	.word ROOT_DEV
boot_flag:
	.word 0xAA55

.text
endtext:
.data
enddata:
.bss
endbss:`