[系统底层] 计算机的开机

计算机的开机,说着简单,但也有很多细节问题,大多数人并不是很清楚

不过,大概顺序就是这样:

BIOS程序运行 -> 查找启动区 -> 跳转到系统 -> ......

按顺序讲解, 只是简单说说

1. 加载BIOS

首先, 按下开机键之后, 电源进行初始化, 以确保硬件的供电可靠, 准备好后, 主板上的硬件电路会将一些存在于各硬件ROM里的程序加载到RAM, 然后给CPU发复位(reset)信号

CPU在接到复位信号后, 开始执行, 但RAM内什么都没有, 但硬件工程师们肯定知道这种情况, 所以CPU会直接加载BIOS程序, 通常就在0xFFFF0处, 但这个位置只有16字节的空间, 所以常常只有一个JMP指令

也就是

jmp far f000:e05b ; 跳转到0xfe05b执行

紧接着BIOS会执行开机自检, 以及一系列跟初始化有关的东西。

参考

2. 加载启动区

如果一切完成, BIOS将会扫描每个硬盘设备(其实每个储存设备都行,包括硬盘,U盘,软盘,光盘,或者类似的东西)中0盘0道1扇区的内容,如果这一扇区的最后两字节是0x55 0xaa,BIOS就会认为它是启动区,并复制到内存0x7c00位置

复制完之后, BIOS将直接跳转到0x7c00, 也就是BIOS的使命终结了。

为什么非要是 0x7c00 呢?BIOS开发团队是这样定的,改了会不兼容。

3. 系统内核

经过之前的努力, 终于进入了内核, 不过如果你聪明, 肯定知道启动区里绝对是要跳转的, 因为现代OS肯定大于512字节, 然后就是OS的代码了

OS的操作这里不提, 大概就是 设置中断,GDT,开保护模式,分页,然后规划内存, 然后启动系统各个进程

这里附Linux0.11的启动区bootsert.s (给自己挪个位, 输出Loading system ..., 加载磁盘中剩下的代码)

;
; 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 ; BIOS留的中断, 读文件
        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

        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
        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

/*
 * 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 "Loading system ..."
        .byte 13,10,13,10

.org 508
root_dev:
        .word ROOT_DEV
boot_flag:
        .word 0xAA55 ; 看看这里, 是不是以0xaa55结束?

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

 

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