跟我一起读 linux 源码 01boot

Posted 2021-02-19 闪客sun

计算机启动流程在我的上一个学习计划《自制操作系统》系列中，已经从完全不知道，过渡到了现在的了如指掌了，虽然有些夸张，但整个大体流程已经像过电影一样在我脑海里了，所以在看 linux 源码的这个 boot 部分时，几乎是看到的地方即使自己写不出，也知道它究竟在做什么，以及下一步可能要做什么，真的特别庆幸之前从零开始的折腾。计算机最初的启动原理，可以参考《硬核讲解计算机启动流程》

下好 linux 源码，我们总是想找到 main 函数开始看，但其实 main 函数之前，有三个由汇编语言写的代码才是最先被执行的，分别是 bootsect.s，setup.s，head.s。之后，才执行由 main 函数开始的用 C 语言编写的操作系统内核程序。

大致流程是这样的：

• 第一步，由 Bios 加载 bootsect 到 0x7C00，然后由 bootsect 自己将自己复制到 0x90000
• 第二步，加载 setup 到 0x90200，然后 setup 里做一些准备，进入保护模式
• 第三步，先将 head.s 汇编成目标代码，将用 C 语言编写的内核程序编译成目标代码，然后链接成 system 模块。head 里做了些进入 main 方法前的准备，主要是重建 IDT，GDT，以及建立分页机制

三步走完后，就进入了 main 方法，此时的内存布局是这样的，同时也体现了 boot 的这三个汇编代码做了什么事

 

 

 以下是三个汇编代码的源码，此时我们已经读完了 linux 源码的一个包

  1 ;
  2 ; SYS_SIZE is the number of clicks (16 bytes) to be loaded.
  3 ; 0x3000 is 0x30000 bytes = 196kB, more than enough for current
  4 ; versions of linux
  5 ;
  6 SYSSIZE = 0x3000
  7 ;
  8 ;    bootsect.s        (C) 1991 Linus Torvalds
  9 ;
 10 ; bootsect.s is loaded at 0x7c00 by the bios-startup routines, and moves
 11 ; iself out of the way to address 0x90000, and jumps there.
 12 ;
 13 ; It then loads \'setup\' directly after itself (0x90200), and the system
 14 ; at 0x10000, using BIOS interrupts. 
 15 ;
 16 ; NOTE; currently system is at most 8*65536 bytes long. This should be no
 17 ; problem, even in the future. I want to keep it simple. This 512 kB
 18 ; kernel size should be enough, especially as this doesn\'t contain the
 19 ; buffer cache as in minix
 20 ;
 21 ; The loader has been made as simple as possible, and continuos
 22 ; read errors will result in a unbreakable loop. Reboot by hand. It
 23 ; loads pretty fast by getting whole sectors at a time whenever possible.
 24 
 25 .globl begtext, begdata, begbss, endtext, enddata, endbss
 26 .text
 27 begtext:
 28 .data
 29 begdata:
 30 .bss
 31 begbss:
 32 .text
 33 
 34 SETUPLEN = 4                ; nr of setup-sectors
 35 BOOTSEG  = 0x07c0            ; original address of boot-sector
 36 INITSEG  = 0x9000            ; we move boot here - out of the way
 37 SETUPSEG = 0x9020            ; setup starts here
 38 SYSSEG   = 0x1000            ; system loaded at 0x10000 (65536).
 39 ENDSEG   = SYSSEG + SYSSIZE        ; where to stop loading
 40 
 41 ; ROOT_DEV:    0x000 - same type of floppy as boot.
 42 ;        0x301 - first partition on first drive etc
 43 ROOT_DEV = 0x306
 44 
 45 entry start
 46 start:
 47     mov    ax,#BOOTSEG
 48     mov    ds,ax
 49     mov    ax,#INITSEG
 50     mov    es,ax
 51     mov    cx,#256
 52     sub    si,si
 53     sub    di,di
 54     rep
 55     movw
 56     jmpi    go,INITSEG
 57 go:    mov    ax,cs
 58     mov    ds,ax
 59     mov    es,ax
 60 ; put stack at 0x9ff00.
 61     mov    ss,ax
 62     mov    sp,#0xFF00        ; arbitrary value >>512
 63 
 64 ; load the setup-sectors directly after the bootblock.
 65 ; Note that \'es\' is already set up.
 66 
 67 load_setup:
 68     mov    dx,#0x0000        ; drive 0, head 0
 69     mov    cx,#0x0002        ; sector 2, track 0
 70     mov    bx,#0x0200        ; address = 512, in INITSEG
 71     mov    ax,#0x0200+SETUPLEN    ; service 2, nr of sectors
 72     int    0x13            ; read it
 73     jnc    ok_load_setup        ; ok - continue
 74     mov    dx,#0x0000
 75     mov    ax,#0x0000        ; reset the diskette
 76     int    0x13
 77     j    load_setup
 78 
 79 ok_load_setup:
 80 
 81 ; Get disk drive parameters, specifically nr of sectors/track
 82 
 83     mov    dl,#0x00
 84     mov    ax,#0x0800        ; AH=8 is get drive parameters
 85     int    0x13
 86     mov    ch,#0x00
 87     seg cs
 88     mov    sectors,cx
 89     mov    ax,#INITSEG
 90     mov    es,ax
 91 
 92 ; Print some inane message
 93 
 94     mov    ah,#0x03        ; read cursor pos
 95     xor    bh,bh
 96     int    0x10
 97     
 98     mov    cx,#24
 99     mov    bx,#0x0007        ; page 0, attribute 7 (normal)
100     mov    bp,#msg1
101     mov    ax,#0x1301        ; write string, move cursor
102     int    0x10
103 
104 ; ok, we\'ve written the message, now
105 ; we want to load the system (at 0x10000)
106 
107     mov    ax,#SYSSEG
108     mov    es,ax        ; segment of 0x010000
109     call    read_it
110     call    kill_motor
111 
112 ; After that we check which root-device to use. If the device is
113 ; defined (!= 0), nothing is done and the given device is used.
114 ; Otherwise, either /dev/PS0 (2,28) or /dev/at0 (2,8), depending
115 ; on the number of sectors that the BIOS reports currently.
116 
117     seg cs
118     mov    ax,root_dev
119     cmp    ax,#0
120     jne    root_defined
121     seg cs
122     mov    bx,sectors
123     mov    ax,#0x0208        ; /dev/ps0 - 1.2Mb
124     cmp    bx,#15
125     je    root_defined
126     mov    ax,#0x021c        ; /dev/PS0 - 1.44Mb
127     cmp    bx,#18
128     je    root_defined
129 undef_root:
130     jmp undef_root
131 root_defined:
132     seg cs
133     mov    root_dev,ax
134 
135 ; after that (everyting loaded), we jump to
136 ; the setup-routine loaded directly after
137 ; the bootblock:
138 
139     jmpi    0,SETUPSEG
140 
141 ; This routine loads the system at address 0x10000, making sure
142 ; no 64kB boundaries are crossed. We try to load it as fast as
143 ; possible, loading whole tracks whenever we can.
144 ;
145 ; in:    es - starting address segment (normally 0x1000)
146 ;
147 sread:    .word 1+SETUPLEN    ; sectors read of current track
148 head:    .word 0            ; current head
149 track:    .word 0            ; current track
150 
151 read_it:
152     mov ax,es
153     test ax,#0x0fff
154 die:    jne die            ; es must be at 64kB boundary
155     xor bx,bx        ; bx is starting address within segment
156 rp_read:
157     mov ax,es
158     cmp ax,#ENDSEG        ; have we loaded all yet?
159     jb ok1_read
160     ret
161 ok1_read:
162     seg cs
163     mov ax,sectors
164     sub ax,sread
165     mov cx,ax
166     shl cx,#9
167     add cx,bx
168     jnc ok2_read
169     je ok2_read
170     xor ax,ax
171     sub ax,bx
172     shr ax,#9
173 ok2_read:
174     call read_track
175     mov cx,ax
176     add ax,sread
177     seg cs
178     cmp ax,sectors
179     jne ok3_read
180     mov ax,#1
181     sub ax,head
182     jne ok4_read
183     inc track
184 ok4_read:
185     mov head,ax
186     xor ax,ax
187 ok3_read:
188     mov sread,ax
189     shl cx,#9
190     add bx,cx
191     jnc rp_read
192     mov ax,es
193     add ax,#0x1000
194     mov es,ax
195     xor bx,bx
196     jmp rp_read
197 
198 read_track:
199     push ax
200     push bx
201     push cx
202     push dx
203     mov dx,track
204     mov cx,sread
205     inc cx
206     mov ch,dl
207     mov dx,head
208     mov dh,dl
209     mov dl,#0
210     and dx,#0x0100
211     mov ah,#2
212     int 0x13
213     jc bad_rt
214     pop dx
215     pop cx
216     pop bx
217     pop ax
218     ret
219 bad_rt:    mov ax,#0
220     mov dx,#0
221     int 0x13
222     pop dx
223     pop cx
224     pop bx
225     pop ax
226     jmp read_track
227 
228 /*
229  * This procedure turns off the floppy drive motor, so
230  * that we enter the kernel in a known state, and
231  * don\'t have to worry about it later.
232  */
233 kill_motor:
234     push dx
235     mov dx,#0x3f2
236     mov al,#0
237     outb
238     pop dx
239     ret
240 
241 sectors:
242     .word 0
243 
244 msg1:
245     .byte 13,10
246     .ascii "Loading system ..."
247     .byte 13,10,13,10
248 
249 .org 508
250 root_dev:
251     .word ROOT_DEV
252 boot_flag:
253     .word 0xAA55
254 
255 .text
256 endtext:
257 .data
258 enddata:
259 .bss
260 endbss:

bootsect.s

  1 ;
  2 ;    setup.s        (C) 1991 Linus Torvalds
  3 ;
  4 ; setup.s is responsible for getting the system data from the BIOS,
  5 ; and putting them into the appropriate places in system memory.
  6 ; both setup.s and system has been loaded by the bootblock.
  7 ;
  8 ; This code asks the bios for memory/disk/other parameters, and
  9 ; puts them in a "safe" place: 0x90000-0x901FF, ie where the
 10 ; boot-block used to be. It is then up to the protected mode
 11 ; system to read them from there before the area is overwritten
 12 ; for buffer-blocks.
 13 ;
 14 
 15 ; NOTE; These had better be the same as in bootsect.s;
 16 
 17 INITSEG  = 0x9000    ; we move boot here - out of the way
 18 SYSSEG   = 0x1000    ; system loaded at 0x10000 (65536).
 19 SETUPSEG = 0x9020    ; this is the current segment
 20 
 21 .globl begtext, begdata, begbss, endtext, enddata, endbss
 22 .text
 23 begtext:
 24 .data
 25 begdata:
 26 .bss
 27 begbss:
 28 .text
 29 
 30 entry start
 31 start:
 32 
 33 ; ok, the read went well so we get current cursor position and save it for
 34 ; posterity.
 35 
 36     mov    ax,#INITSEG    ; this is done in bootsect already, but...
 37     mov    ds,ax
 38     mov    ah,#0x03    ; read cursor pos
 39     xor    bh,bh
 40     int    0x10        ; save it in known place, con_init fetches
 41     mov    [0],dx        ; it from 0x90000.
 42 
 43 ; Get memory size (extended mem, kB)
 44 
 45     mov    ah,#0x88
 46     int    0x15
 47     mov    [2],ax
 48 
 49 ; Get video-card data:
 50 
 51     mov    ah,#0x0f
 52     int    0x10
 53     mov    [4],bx        ; bh = display page
 54     mov    [6],ax        ; al = video mode, ah = window width
 55 
 56 ; check for EGA/VGA and some config parameters
 57 
 58     mov    ah,#0x12
 59     mov    bl,#0x10
 60     int    0x10
 61     mov    [8],ax
 62     mov    [10],bx
 63     mov    [12],cx
 64 
 65 ; Get hd0 data
 66 
 67     mov    ax,#0x0000
 68     mov    ds,ax
 69     lds    si,[4*0x41]
 70     mov    ax,#INITSEG
 71     mov    es,ax
 72     mov    di,#0x0080
 73     mov    cx,#0x10
 74     rep
 75     movsb
 76 
 77 ; Get hd1 data
 78 
 79     mov    ax,#0x0000
 80     mov    ds,ax
 81     lds    si,[4*0x46]
 82     mov    ax,#INITSEG
 83     mov    es,ax
 84     mov    di,#0x0090
 85     mov    cx,#0x10
 86     rep
 87     movsb
 88 
 89 ; Check that there IS a hd1 :-)
 90 
 91     mov    ax,#0x01500
 92     mov    dl,#0x81
 93     int    0x13
 94     jc    no_disk1
 95     cmp    ah,#3
 96     je    is_disk1
 97 no_disk1:
 98     mov    ax,#INITSEG
 99     mov    es,ax
100     mov    di,#0x0090
101     mov    cx,#0x10
102     mov    ax,#0x00
103     rep
104     stosb
105 is_disk1:
106 
107 ; now we want to move to protected mode ...
108 
109     cli            ; no interrupts allowed ;
110 
111 ; first we move the system to it\'s rightful place
112 
113     mov    ax,#0x0000
114     cld            ; \'direction\'=0, movs moves forward
115 do_move:
116     mov    es,ax        ; destination segment
117     add    ax,#0x1000
118     cmp    ax,#0x9000
119     jz    end_move
120     mov    ds,ax        ; source segment
121     sub    di,di
122     sub    si,si
123     mov     cx,#0x8000
124     rep
125     movsw
126     jmp    do_move
127 
128 ; then we load the segment descriptors
129 
130 end_move:
131     mov    ax,#SETUPSEG    ; right, forgot this at first. didn\'t work :-)
132     mov    ds,ax
133     lidt    idt_48        ; load idt with 0,0
134     lgdt    gdt_48        ; load gdt with whatever appropriate
135 
136 ; that was painless, now we enable A20
137 
138     call    empty_8042
139     mov    al,#0xD1        ; command write
140     out    #0x64,al
141     call    empty_8042
142     mov    al,#0xDF        ; A20 on
143     out    #0x60,al
144     call    empty_8042
145 
146 ; well, that went ok, I hope. Now we have to reprogram the interrupts :-(
147 ; we put them right after the intel-reserved hardware interrupts, at
148 ; int 0x20-0x2F. There they won\'t mess up anything. Sadly IBM really
149 ; messed this up with the original PC, and they haven\'t been able to
150 ; rectify it afterwards. Thus the bios puts interrupts at 0x08-0x0f,
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