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: