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开机过程中干了什么？

注：(5)中读取一个扇区的数据（512B）到内存0x7c00中、0磁道0扇区就是操作系统的引导扇区

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boot_sect -> set up -> system

bootsect.s的工作

先看看代码：可能看不懂，可以直接根据下面解释来看重点！

!! 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.textbegtext:.databegdata:.bssbegbss:.textSETUPLEN = 4				! nr of setup-sectorsBOOTSEG  = 0x07c0			! original address of boot-sectorINITSEG  = 0x9000			! we move boot here - out of the waySETUPSEG = 0x9020			! setup starts hereSYSSEG   = 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 etcROOT_DEV = 0x306entry _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,INITSEGgo:	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_setupok_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	mov	ah,#0x03		! read cursor pos	xor	bh,bh	int	0x10		mov	cx,#21	mov	bx,#0x0007		! page 0, attribute 7 (normal)	mov	bp,#msg1	mov	ax,#0x1301		! write string, move cursor	int	0x10! ok, we've written the message, nowinf_loop:	jmp inf_loop! 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_definedundef_root:	jmp undef_rootroot_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 trackhead:	.word 0			! current headtrack:	.word 0			! current trackread_it:	mov ax,es	test ax,#0x0fffdie:	jne die			! es must be at 64kB boundary	xor bx,bx		! bx is starting address within segmentrp_read:	mov ax,es	cmp ax,#ENDSEG		! have we loaded all yet?	jb ok1_read	retok1_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,#9ok2_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 trackok4_read:	mov head,ax	xor ax,axok3_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_readread_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	retbad_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	retsectors:	.word 0msg1:	.byte 13,10	.ascii "Hello my JYOS !"	.byte 13,10,13,10.org 508root_dev:	.word ROOT_DEVboot_flag:	.word 0xAA55.textendtext:.dataenddata:.bssendbss:

下图是引导扇区的代码，其

先将0x070c：0x0000处的代码，移动到0x9000：0x0000 处 再跳转到go；

之后加载setup模块，如下图中，加载到0x9000后面的 0x9020 读入setup后，在屏幕上打印字符串；使用的是mov bp,msg1 mov ax, #1301 int 0x10 利用0x10中断完成显示（并未运行setup！当前指针并未到9020；当前OK_load_setup最后会先读入system模块，再跳转到执行setup.s如下图所示） 如果想要修改这个字符串：1. 修改msg1 2.修改对应字符串的大小：cx，#24中的24是字符串的长度； 下图为read_it中读入system模块的操作；完成后利用jmpi 给ip和cs赋值，完成跳转到setup，完成bootset；

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setup.s的工作：

读取物理内存大小，并记录，将 system模块移动到0地址处；

在setup的最后，需要完成2件事： 1. 切换寻址模式、 2.跳转到0x0处开始执行system模块的代码；

对于切换寻址模式：16-》32

因为在当前情况下使用的寻址模式是（如下图）cs << 4 + ip:其中cs是一个16位寄存器，其 <<4最多拥有20位地址，也就是1MB的内存大小；

这太小了，如今的计算机至少是4G的，因此需要切换寻址模式，不能使用 cs << 4 + ip；

切换的方法就是：下图中红色代码：使用cr0寄存器改变电路（修改末尾PE从 0—》1），从当前的 实模式 --》》》保护模式

在保护模式下,显然地址翻译不同，增加临时的GDT（global Description Table全局描述表）和IDT（中断描述）表；地址翻译从这个表中完成，得到真正的物理地址；这样上面图中的jmpi, 0 8 才能到0x0处；

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system.s的工作：

system模块相比于前面的bootsect 和 setup来说，要大得多！因为这是 真正用来校验的代码；

他是由一堆文件编译出来的；

1. 操作系统要想正常启动，前面的代码运行顺序是要严格控制的，像上面说的那样，1. bootsect.s 2. setup.s 3.system.s；
   system.s是通过makefile来指定使用哪些文件进行编译的；
   
2. 在head.s中做了啥？（没仔细看）
   重点：做了很多事，包括对于 gdt 和idt的设置；对于系统栈的设置等等；
   
3. 上面跳转到了after_page_tables，这里干了什么呢？
   首先push了一堆，入栈；接着 跳转到 setup_page 进行页表设置； 最后将栈顶出栈也就main函数出栈，开始执行main函数；当main 执行完成 就到了L6中，也就是死循环了；操作系统的main函数不会结束~一直执行；
   
4. main函数中做了什么？
   进行了一堆初始化工作，我们以mem_init内存初始化来看一下~；
   
   可以说是终于看到了一些比较熟悉的东西了，这段代码是什么？
   这就是内存管理中的页式管理！页表的初始化，根据之前在setup.s中保存在0x90002中的本机内存大小，配合我们期望的操作系统内存分页大小（这里是 >> 12右移12位也就是将 总的地址数 / 每页的地址数 = 2^32 / 2^12 = 2^{20），也就是分成2}20个页表；下面while中对每个页表进行初始化为0 ，但是前面不需要初始化，前面是操作系统用到的；

entry _start_start:    mov ah,#0x03    xor bh,bh    int 0x10    mov cx,#36    mov bx,#0x0007    mov bp,#msg1    mov ax,#0x07c0    mov es,ax    mov ax,#0x1301    int 0x10inf_loop:    jmp inf_loopmsg1:    .byte   13,10    .ascii  "Hello OS world, my name is LZJ"    .byte   13,10,13,10.org 510boot_flag:    .word   0xAA55

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