Linux 内核解读之Boot-- -bootsect

Linux 内核解读之Boot-- -bootsect

linux bootsect的作用于结构图如下

*以下文章是对linux低版本的理解与分享，参考到来自多方面的文章和自己作为一个coder的心得.

Linux 内核的结构模式是一种操作系统的经典模式，从系统加载开始我们就会用bootsect来把image
加载进内存，其实我们在涉及相关的系统设计时，往往可以看到系统有loader,booter,bootloader之类的东西，
这是一种模式，booter里面可以做很多东西，比如image通过压缩，把文件信息缩小，在加载到memory时
解压可以减少flash的容量需要，也可以加入高级安全之类的验证信息，同时在booter阶段起来就开始做
系统与外部设备的检测也可大量缩减测试时间.

*现在我们开始走进linux的bootsect.s之前

1，需要理解AT&T 汇编

说到汇编，估计80X86的汇编算是比较难的了，相对单片机，ARM MIPS等汇编来说。
而且涉及到AT&T汇编还看起来有点不一样

2，需要理解BIOS

BIOS的原意是"Basic Input Output System“,即基本输入输出系统，其实就是一个主板的片上系统，起机的
时候从ROM里面开始跑起，跑完后再开始跑bootsect

*现在我们开始走进linux的bootsect.s

bootsect程序流程图如下:

(待续)

! ! 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: .textSETUPLEN = 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 = 0x306entry start start: movax,#BOOTSEGmovds,axmovax,#INITSEGmoves,axmovcx,#256subsi,sisubdi,direpmovwjmpigo,INITSEG go:movax,cs movds,axmoves,ax ! put stack at 0x9ff00. movss,axmovsp,#0xFF00! arbitrary value >>512! load the setup-sectors directly after the bootblock. ! Note that 'es' is already set up.load_setup: movdx,#0x0000! drive 0, head 0movcx,#0x0002! sector 2, track 0movbx,#0x0200! address = 512, in INITSEGmovax,#0x0200+SETUPLEN! service 2, nr of sectorsint0x13! read itjncok_load_setup! ok - continuemovdx,#0x0000movax,#0x0000! reset the disketteint0x13jload_setupok_load_setup:! Get disk drive parameters, specifically nr of sectors/trackmovdl,#0x00movax,#0x0800! AH=8 is get drive parametersint0x13movch,#0x00seg csmovsectors,cxmovax,#INITSEGmoves,ax! Print some inane messagemovah,#0x03! read cursor posxorbh,bhint0x10movcx,#24movbx,#0x0007! page 0, attribute 7 (normal)movbp,#msg1movax,#0x1301! write string, move cursorint0x10! ok, we've written the message, now ! we want to load the system (at 0x10000)movax,#SYSSEGmoves,ax! segment of 0x010000callread_itcallkill_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 csmovax,root_devcmpax,#0jneroot_definedseg csmovbx,sectorsmovax,#0x0208! /dev/ps0 - 1.2Mbcmpbx,#15jeroot_definedmovax,#0x021c! /dev/PS0 - 1.44Mbcmpbx,#18jeroot_defined undef_root: jmp undef_root root_defined: seg csmovroot_dev,ax! after that (everyting loaded), we jump to ! the setup-routine loaded directly after ! the bootblock:jmpi0,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 trackread_it: mov ax,estest ax,#0x0fff die:jne die! es must be at 64kB boundary xor bx,bx! bx is starting address within segment rp_read: mov ax,escmp ax,#ENDSEG! have we loaded all yet?jb ok1_readret ok1_read: seg csmov ax,sectorssub ax,sreadmov cx,axshl cx,#9add cx,bxjnc ok2_readje ok2_readxor ax,axsub ax,bxshr ax,#9 ok2_read: call read_trackmov cx,axadd ax,sreadseg cscmp ax,sectorsjne ok3_readmov ax,#1sub ax,headjne ok4_readinc track ok4_read: mov head,axxor ax,ax ok3_read: mov sread,axshl cx,#9add bx,cxjnc rp_readmov ax,esadd ax,#0x1000mov es,axxor bx,bxjmp rp_readread_track: push axpush bxpush cxpush dxmov dx,trackmov cx,sreadinc cxmov ch,dlmov dx,headmov dh,dlmov dl,#0and dx,#0x0100mov ah,#2int 0x13jc bad_rtpop dxpop cxpop bxpop axret bad_rt:mov ax,#0 mov dx,#0int 0x13pop dxpop cxpop bxpop axjmp 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 dxmov dx,#0x3f2mov al,#0outbpop dxretsectors: .word 0msg1: .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: