Linux 0.11/boot/bootsect.s

  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: