linux学习(一)--启动文件bootsect.s
这是linux由BIOS加载后执行的第一段的启动程序代码,即文件 boot/bootsect.s
首先附图，简单介绍一下从开机加电到第一段linux代码执行的简要过程
1 .globl begtext, begdata, begbss, endtext, enddata, endbss
2 .text
3 begtext:
4 .data
5 begdata:
6 .bss
7 begbss:
8 .text
9
10 //规划内存，由BIOS执行
11 SETUPLEN = 4 ! nr of setup-sectors
12 BOOTSEG = 0x07c0 ! original address of boot-sector
13 INITSEG = 0x9000 ! we move boot here - out of the way
14 SETUPSEG = 0x9020 ! setup starts here
15 SYSSEG = 0x1000 ! system loaded at 0x10000 (65536).
16 ENDSEG = SYSSEG + SYSSIZE ! where to stop loading
17
18 ! ROOT_DEV: 0x000 - same type of floppy as boot.
19 ! 0x301 - first partition on first drive etc
20 ROOT_DEV = 0x306
21
22 //复制自身到指定地址
23 entry _start
24 _start:
25 mov ax,#BOOTSEG ;0x07c0,启动代码所在位置放入ax
26 mov ds,ax ;将启动代码与ds寄存器关联
27 mov ax,#INITSEG ;启动代码要被复制到的目的地址
28 mov es,ax ;将目的地址与es寄存器关联
29 mov cx,#256 ;循环控制字节,512字节
30 sub si,si ;si清零,ds:si即0x07c00
31 sub di,di ;di清零,es:si即0x90000
32 rep ;循环直到cx==0
33 movw ;将ds:si复制到es:di
34 jmpi go,INITSEG;跳转到go标志处
35
36 //由于启动代码复制到了新位置，需要更改相应寄存器的值
37 go: mov ax,cs ;将当前的cs值赋值给各寄存器,方便接下来程序继续执行
38 mov ds,ax
39 mov es,ax
40 ! put stack at 0x9ff00.
41 mov ss,ax ;开始引入栈
42 mov sp,#0xFF00 ;栈空间的起始地址为0x9ff00
43
44 //开始加载setup块
45 load_setup:
46 mov dx,#0x0000 ;为后面进入中断处理传入相应信息
47 mov cx,#0x0002
48 mov bx,#0x0200
49 mov ax,#0x0200+SETUPLEN
50 int 0x13 ;进入中断服务程序,将setup.s对应的程序加载至内存指定地址
51 jnc ok_load_setup;cf标志寄存器为0就跳转至ok_load_setup块
52 mov dx,#0x0000
53 mov ax,#0x0000 ;cf!=０则重新设置传入信息,进入中断
54 int 0x13
55 j load_setup
56
57 //取磁盘驱动器参数
58 ok_load_setup:
59 mov dl,#0x00
60 mov ax,#0x0800 ;磁盘参数
61 int 0x13
62 mov ch,#0x00
63 seg cs ;下一条语句的操作数在cs所指段中
64 mov sectors,cx ;保存每磁道扇区数
65 mov ax,#INITSEG
66 mov es,ax
67
68 //由于加载代码量庞大，这时在屏幕上输出 "Loding system..."
69 mov ah,#0x03 ;读光标位置
70 xor bh,bh
71 int 0x10
72
73 mov cx,#24 ;共24个字符
74 mov bx,#0x0007 ;! page 0, attribute 7 (normal)
75 mov bp,#msg1 ;指向要显示字符串的地址
76 mov ax,#0x1301 ;! write string, move cursor
77 int 0x10
78
79 //加载第三批代码，即剩余内核代码，时间较长
80 mov ax,#SYSSEG ;内核代码被加载到的地址
81 mov es,ax ;! segment of 0x010000
82 call read_it ;读取磁盘上的system模块
83 call kill_motor;关闭驱动器
84
85 //确定使用哪个根文件系统设备,若指定了设备(开始的ax!=0)，就直接用给定的设备
86 seg cs
87 mov ax,root_dev
88 cmp ax,#0 ;比较ax是否为0
89 jne root_defined ;ax!=0跳转
90 seg cs
91 mov bx,sectors ;取磁道扇区数，如果sectors==15,则说明是1.2Mb驱动器
92 ;如果sectors==18,则说明是1.44Mb驱动器
93 mov ax,#0x0208 ;! /dev/ps0 - 1.2Mb
94 cmp bx,#15 ;判断磁道扇区数是否为15
95 je root_defined
96 mov ax,#0x021c ;! /dev/PS0 - 1.44Mb
97 cmp bx,#18
98 je root_defined
99 undef_root: ;如果都不是，死循环
100 jmp undef_root
101 root_defined:
102 seg cs
103 mov root_dev,ax ;保存设备号到数据区
104
105 //本程序执行完毕，跳转到已经加载在内存的setup处继续执行
106 jmpi 0,SETUPSEG
107
108 //以下是被调用的块的详细代码,以及显示在屏幕的文字信息的数据安排
109 sread: .word 1+SETUPLEN ! sectors read of current track
110 head: .word 0 ! current head
111 track: .word 0 ! current track
112
113 read_it:
114 mov ax,es
115 test ax,#0x0fff
116 die: jne die ! es must be at 64kB boundary
117 xor bx,bx ! bx is starting address within segment
118 rp_read:
119 mov ax,es
120 cmp ax,#ENDSEG ! have we loaded all yet?
121 jb ok1_read
122 ret
123 ok1_read:
124 seg cs
125 mov ax,sectors
126 sub ax,sread
127 mov cx,ax
128 shl cx,#9
129 add cx,bx
130 jnc ok2_read
131 je ok2_read
132 xor ax,ax
133 sub ax,bx
134 shr ax,#9
135 ok2_read:
136 call read_track
137 mov cx,ax
138 add ax,sread
139 seg cs
140 cmp ax,sectors
141 jne ok3_read
142 mov ax,#1
143 sub ax,head
144 jne ok4_read
145 inc track
146 ok4_read:
147 mov head,ax
148 xor ax,ax
149 ok3_read:
150 mov sread,ax
151 shl cx,#9
152 add bx,cx
153 jnc rp_read
154 mov ax,es
155 add ax,#0x1000
156 mov es,ax
157 xor bx,bx
158 jmp rp_read
159
160 read_track:
161 push ax
162 push bx
163 push cx
164 push dx
165 mov dx,track
166 mov cx,sread
167 inc cx
168 mov ch,dl
169 mov dx,head
170 mov dh,dl
171 mov dl,#0
172 and dx,#0x0100
173 mov ah,#2
174 int 0x13
175 jc bad_rt
176 pop dx
177 pop cx
178 pop bx
179 pop ax
180 ret
181 bad_rt: mov ax,#0
182 mov dx,#0
183 int 0x13
184 pop dx
185 pop cx
186 pop bx
187 pop ax
188 jmp read_track
189
190 !/*
191 ! * This procedure turns off the floppy drive motor, so
192 ! * that we enter the kernel in a known state, and
193 ! * don't have to worry about it later.
194 ! */
195 kill_motor:
196 push dx
197 mov dx,#0x3f2
198 mov al,#0
199 outb
200 pop dx
201 ret
202
203 sectors:
204 .word 0
205
206 msg1:
207 .byte 13,10
208 .ascii "Loading system ..."
209 .byte 13,10,13,10
210
211 .org 508
212 root_dev:
213 .word ROOT_DEV
214 boot_flag:
215 .word 0xAA55
216
217 .text
218 endtext:
219 .data
220 enddata:
221 .bss
222 endbss:
启动代码短小精悍，却又十分高效，利用效率让人惊叹！
多处借鉴，重在学习
如有错误，欢迎指正
