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: