Linux启动系统详解：bootsect.s, setup.s & head.s关键文件

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本章深入探讨了Linux系统的启动过程，特别是关注于boot/目录下的三个关键Assembly语言源码文件：bootsect.s、setup.s和head.s。这些文件对于理解Linux内核的启动机制至关重要。首先，我们来概述这些文件的作用和编写语言。 **3.1 Linux启动代码基础** - **bootsect.s（5052字节）**: 这是用Intel 8086汇编语言编写的，类似于早期的Intel机器码格式。它在系统启动初期执行，负责加载引导扇区，初始化硬件，以及检测并加载BIOS中的引导记录。由于其基于086汇编，读者需要对8086架构和保护模式编程有一定了解，以便理解其中的指令和内存操作。 - **head.s（5938字节）**: 使用GNU格式的汇编器和AT&T语法编写，这部分代码通常处理早期引导阶段的硬件初始化，包括设置中断向量表、内存管理等。尽管使用不同的语法，但都遵循着相似的系统启动流程。 - **setup.s（5364字节）**: 这是Linux启动过程中的关键文件，负责配置CPU的实模式和保护模式，设置中断处理、加载GDT（全局描述符表）和IDT（中断描述符表），以及引导装载程序（Loader）。这个阶段的工作对后续的系统加载和初始化至关重要。 **3.2 主要功能概览** - **启动执行流程**：Linux启动过程大致分为以下几个步骤： 1. bootsect.s启动引导：初始化硬件，如BIOS查找和加载引导记录。 2. head.s加载和初始化：设置中断处理和内存管理，为进入保护模式做准备。 3. setup.s配置模式转换：从实模式切换到保护模式，为内核加载做准备。 4. Loader（如GRUB或LILO）加载内核映像。 5. 内核初始化：内核运行并执行引导加载脚本，设置初始化硬件、文件系统、进程管理系统等。通过阅读和分析这些源代码，程序员可以深入了解操作系统内核如何与硬件交互，以及启动时的低级操作。掌握这些细节有助于编写更高效和兼容的Linux驱动程序，或者进行底层系统维护和优化。学习这一部分对于任何想深入研究Linux内核的人来说都是必不可少的。

3.3 bootsect.s

105 ! we want to load the system (at 0x10000)

106

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mov ax,#SYSSEG

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mov es,ax ! segment of 0x010000 ! es = seg of system.

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call read_it ! Read system module from disk. es is input parameter.

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call kill_motor ! shut down drive motor, then we know the stats of drive.

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! After that we check which root-device to use. If the device is

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! defined (!= 0), nothing is done and the given device is used.

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! Otherwise, either /dev/PS0 (2,28) or /dev/at0 (2,8), depending

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! on the number of sectors that the BIOS reports currently.

! The meaning of two device file above is:

! The main device nr of floppy is 2, sub nr equals to TYPE*4 + NR. where NR is 0-3 coresponding

! to floppy drive A, B, C or D separatly. TYPE is drive type (2-->1.2Mb or 7-->1.44Mb, etc.)

! As 7*4 + 0 = 28, so /dev/PS0 (2,28) indicates the first drive with 1.44Mb type. Its device

! number is 0x021c. In the same way, /dev/at0 (2,8) indicates that the first drive is a 1.2M

! drive and device number is 0x0208.

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seg cs

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mov ax,root_dev

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cmp ax,#0

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jne root_defined

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seg cs

! Load the sectors/track (SPT) saved at L88 into register. If SPT = 15, it indicates that

! the drive type is a 1.2Mb. If SPT = 18, then it means the drive type is 1.44Mb. The drive

! is the first drive of the machine because it is bootable.

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mov bx,sectors

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mov ax,#0x0208 ! /dev/ps0 - 1.2Mb

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cmp bx,#15

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je root_defined

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mov ax,#0x021c ! /dev/PS0 - 1.44Mb

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cmp bx,#18

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je root_defined

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undef_root: ! If we cann't identify the drive type, we are dead!

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jmp undef_root

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root_defined:

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seg cs

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mov root_dev,ax ! Save the checked root device.

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! after that (everyting loaded), we jump to

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! the setup-routine loaded directly after

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! the bootblock:

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jmpi 0,SETUPSEG ! Jump to 0x9020:0000 (setup.s)

!!!! This ends of the program !!!!

! Here are two subroutines

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! This routine loads the system at address 0x10000, making sure

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! no 64kB boundaries are crossed. We try to load it as fast as

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! possible, loading whole tracks whenever we can.

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! in: es - starting address segment (normally 0x1000)

3.3 bootsect.s

146 !

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sread: .word 1+SETUPLEN ! sectors read of current track. we have already loaded

! 5 sectors. The bootsect occupied 1 sector, setup has 4.

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head: .word 0 ! current head

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track: .word 0 ! current track

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read_it:

! Check the segment value. Data loaded from floppy must be in 64Kb aligment in memory.

! The bx register is used to point at the writing location within current segment

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mov ax,es

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test ax,#0x0fff

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die: jne die ! es must be at 64kB boundary

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xor bx,bx ! bx is starting address within segment

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rp_read:

! Check to see if we have read in all data. Compare the current segment with #ENDSEG. Jump to

! label ok1_read if not equal, else return.

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mov ax,es

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cmp ax,#ENDSEG ! have we loaded all yet? ! 是否已经加载了全部数据？

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jb ok1_read

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ret

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ok1_read:

! Calculate & verify sector number we should get from current floppy track.

! Check the total bytes read with 64Kb segment limitation to see if we can read the sectors

! in current track all together. If exceeded, then do a reverse calculate base on the maximum

! bytes we could read at this time (64Kb - offset in segment).

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seg cs

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mov ax,sectors ! Get sector number per track.

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sub ax,sread ! minus sectors we already read.

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mov cx,ax ! cx = ax = sectors not yet read in current track.

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shl cx,#9 ! cx = cx * 512 bytes.

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add cx,bx ! cx = cx + offset in current segment (bx)

! = total bytes read after the processing of this time.

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jnc ok2_read ! Check 64Kb limit. If not exceed, then jump to ok2_read.

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je ok2_read

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xor ax,ax ! If so, then counting the maximum bytes we can read at this

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sub ax,bx ! time (64Kb - offset in segment) and convert it to sector

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shr ax,#9 ! number.

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ok2_read:

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call read_track

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mov cx,ax ! cx = sector number we have read.

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add ax,sread ! sector number we have read in current track.

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seg cs

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cmp ax,sectors ! If there are unread sectors in current track, then jump

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jne ok3_read ! to label ok3_read.

! Read data under next disk head (head 1). If finished, then goto read the next track.

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mov ax,#1

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sub ax,head ! Check the current head.

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jne ok4_read ! If is head 0, then get data under head 1

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inc track ! else goto next track.

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ok4_read:

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mov head,ax ! Save the current head

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xor ax,ax ! Clear sectors number already read within current track.

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ok3_read:

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