Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)
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FreeBSD/Linux Kernel Cross Reference
sys/boot/bootblock.s
Version:
- FREEBSD - FREEBSD-13-STABLE - FREEBSD-13-0 - FREEBSD-12-STABLE - FREEBSD-12-0 - FREEBSD-11-STABLE - FREEBSD-11-0 - FREEBSD-10-STABLE - FREEBSD-10-0 - FREEBSD-9-STABLE - FREEBSD-9-0 - FREEBSD-8-STABLE - FREEBSD-8-0 - FREEBSD-7-STABLE - FREEBSD-7-0 - FREEBSD-6-STABLE - FREEBSD-6-0 - FREEBSD-5-STABLE - FREEBSD-5-0 - FREEBSD-4-STABLE - FREEBSD-3-STABLE - FREEBSD22 - l41 - OPENBSD - linux-2.6 - MK84 - PLAN9 - xnu-8792
SearchContext: - none - 3 - 10
SearchContext: - none - 3 - 10
1 ! Bootblock 1.5 - Minix boot block. Author: Kees J. Bot 2 ! 21 Dec 1991 3 ! 4 ! When the PC is powered on, it will try to read the first sector of floppy 5 ! disk 0 at address 0x7C00. If this fails due to the absence of flexible 6 ! magnetic media, it will read the master boot record from the first sector 7 ! of the hard disk. This sector not only contains executable code, but also 8 ! the partition table of the hard disk. When executed, it will select the 9 ! active partition and load the first sector of that at address 0x7C00. 10 ! This file contains the code that is eventually read from either the floppy 11 ! disk, or the hard disk partition. It is just smart enough to load /boot 12 ! from the boot device into memory at address 0x10000 and execute that. The 13 ! disk addresses for /boot are patched into this code by installboot as 24-bit 14 ! sector numbers and 8-bit sector counts above enddata upwards. /boot is in 15 ! turn smart enough to load the different parts of the Minix kernel into 16 ! memory and execute them to finally get Minix started. 17 ! 18 19 LOADOFF = 0x7C00 ! 0x0000:LOADOFF is where this code is loaded 20 BOOTSEG = 0x1000 ! Secondary boot code segment. 21 BOOTOFF = 0x0030 ! Offset into /boot above header 22 BUFFER = 0x0600 ! First free memory 23 LOWSEC = 8 ! Offset of logical first sector in partition 24 ! table 25 26 ! Variables addressed using bp register 27 device = 0 ! The boot device 28 lowsec = 2 ! Offset of boot partition within drive 29 secpcyl = 6 ! Sectors per cylinder = heads * sectors 30 31 .text 32 33 ! Start boot procedure. 34 35 boot: 36 xor ax, ax ! ax = 0x0000, the vector segment 37 mov ds, ax 38 cli ! Ignore interrupts while setting stack 39 mov ss, ax ! ss = ds = vector segment 40 mov sp, #LOADOFF ! Usual place for a bootstrap stack 41 sti 42 43 push ax 44 push ax ! Push a zero lowsec(bp) 45 46 push dx ! Boot device in dl will be device(bp) 47 mov bp, sp ! Using var(bp) is one byte cheaper then var. 48 49 push es 50 push si ! es:si = partition table entry if hard disk 51 52 mov di, #LOADOFF+sectors ! char *di = sectors; 53 54 testb dl, dl ! Winchester disks if dl >= 0x80 55 jge floppy 56 57 winchester: 58 59 ! Get the offset of the first sector of the boot partition from the partition 60 ! table. The table is found at es:si, the lowsec parameter at offset LOWSEC. 61 62 eseg 63 les ax, LOWSEC(si) ! es:ax = LOWSEC+2(si):LOWSEC(si) 64 mov lowsec+0(bp), ax ! Low 16 bits of partition's first sector 65 mov lowsec+2(bp), es ! High 16 bits of partition's first sector 66 67 ! Get the drive parameters, the number of sectors is bluntly written into the 68 ! floppy disk sectors/track array. 69 70 movb ah, #0x08 ! Code for drive parameters 71 int 0x13 ! dl still contains drive 72 andb cl, #0x3F ! cl = max sector number (1-origin) 73 movb (di), cl ! Number of sectors per track 74 incb dh ! dh = 1 + max head number (0-origin) 75 jmp loadboot 76 77 ! Floppy: 78 ! Execute three read tests to determine the drive type. Test for each floppy 79 ! type by reading the last sector on the first track. If it fails, try a type 80 ! that has less sectors. Therefore we start with 1.44M (18 sectors) then 1.2M 81 ! (15 sectors) ending with 720K/360K (both 9 sectors). 82 83 next: inc di ! Next number of sectors per track 84 85 floppy: xorb ah, ah ! Reset drive 86 int 0x13 87 88 movb cl, (di) ! cl = number of last sector on track 89 90 cmpb cl, #9 ! No need to do the last 720K/360K test 91 je success 92 93 ! Try to read the last sector on track 0 94 95 mov es, lowsec(bp) ! es = vector segment (lowsec = 0) 96 mov bx, #BUFFER ! es:bx buffer = 0x0000:0x0600 97 mov ax, #0x0201 ! Read sector, #sectors = 1 98 xorb ch, ch ! Track 0, last sector 99 xorb dh, dh ! Drive dl, head 0 100 int 0x13 101 jc next ! Error, try the next floppy type 102 103 success:movb dh, #2 ! Load number of heads for multiply 104 105 loadboot: 106 ! Load /boot from the boot device 107 108 movb al, (di) ! al = (di) = sectors per track 109 mulb dh ! dh = heads, ax = heads * sectors 110 mov secpcyl(bp), ax ! Sectors per cylinder = heads * sectors 111 112 mov ax, #BOOTSEG ! Segment to load /boot into 113 mov es, ax 114 xor bx, bx ! Load first sector at es:bx = BOOTSEG:0x0000 115 mov si, #LOADOFF+addresses ! Start of the boot code addresses 116 load: 117 mov ax, 1(si) ! Get next sector number: low 16 bits 118 movb dl, 3(si) ! Bits 16-23 for your up to 8GB partition 119 xorb dh, dh ! dx:ax = sector within partition 120 add ax, lowsec+0(bp) 121 adc dx, lowsec+2(bp)! dx:ax = sector within drive 122 cmp dx, #[1024*255*63-255]>>16 ! Near 8G limit? 123 jae bigdisk 124 div secpcyl(bp) ! ax = cylinder, dx = sector within cylinder 125 xchg ax, dx ! ax = sector within cylinder, dx = cylinder 126 movb ch, dl ! ch = low 8 bits of cylinder 127 divb (di) ! al = head, ah = sector (0-origin) 128 xorb dl, dl ! About to shift bits 8-9 of cylinder into dl 129 shr dx, #1 130 shr dx, #1 ! dl[6..7] = high cylinder 131 orb dl, ah ! dl[0..5] = sector (0-origin) 132 movb cl, dl ! cl[0..5] = sector, cl[6..7] = high cyl 133 incb cl ! cl[0..5] = sector (1-origin) 134 movb dh, al ! dh = al = head 135 movb dl, device(bp) ! dl = device to read 136 movb al, (di) ! Sectors per track - Sector number (0-origin) 137 subb al, ah ! = Sectors left on this track 138 cmpb al, (si) ! Compare with # sectors to read 139 jbe read ! Can't read past the end of a cylinder? 140 movb al, (si) ! (si) < sectors left on this track 141 read: push ax ! Save al = sectors to read 142 movb ah, #0x02 ! Code for disk read (all registers in use now!) 143 int 0x13 ! Call the BIOS for a read 144 pop cx ! Restore al in cl 145 jmp rdeval 146 bigdisk: 147 movb cl, (si) ! Number of sectors to read 148 push si ! Save si 149 mov si, #LOADOFF+ext_rw ! si = extended read/write parameter packet 150 movb 2(si), cl ! Fill in # blocks to transfer 151 mov 4(si), bx ! Buffer address 152 mov 8(si), ax ! Starting block number = dx:ax 153 mov 10(si), dx 154 movb dl, device(bp) ! dl = device to read 155 movb ah, #0x42 ! Extended read 156 int 0x13 157 pop si ! Restore si to point to the addresses array 158 !jmp rdeval 159 rdeval: 160 jc error ! Jump on disk read error 161 movb al, cl ! Restore al = sectors read 162 addb bh, al ! bx += 2 * al * 256 (add bytes read) 163 addb bh, al ! es:bx = where next sector must be read 164 add 1(si), ax ! Update address by sectors read 165 adcb 3(si), ah ! Don't forget bits 16-23 (add ah = 0) 166 subb (si), al ! Decrement sector count by sectors read 167 jnz load ! Not all sectors have been read 168 add si, #4 ! Next (address, count) pair 169 cmpb ah, (si) ! Done when no sectors to read 170 jnz load ! Read next chunk of /boot 171 172 done: 173 174 ! Call /boot, assuming a long a.out header (48 bytes). The a.out header is 175 ! usually short (32 bytes), but to be sure /boot has two entry points: 176 ! One at offset 0 for the long, and one at offset 16 for the short header. 177 ! Parameters passed in registers are: 178 ! 179 ! dl = Boot-device. 180 ! es:si = Partition table entry if hard disk. 181 ! 182 pop si ! Restore es:si = partition table entry 183 pop es ! dl is still loaded 184 jmpf BOOTOFF, BOOTSEG ! jmp to sec. boot (skipping header). 185 186 ! Read error: print message, hang forever 187 error: 188 mov si, #LOADOFF+errno+1 189 prnum: movb al, ah ! Error number in ah 190 andb al, #0x0F ! Low 4 bits 191 cmpb al, #10 ! A-F? 192 jb digit ! 0-9! 193 addb al, #7 ! 'A' - ':' 194 digit: addb (si), al ! Modify '0' in string 195 dec si 196 movb cl, #4 ! Next 4 bits 197 shrb ah, cl 198 jnz prnum ! Again if digit > 0 199 200 mov si, #LOADOFF+rderr ! String to print 201 print: lodsb ! al = *si++ is char to be printed 202 testb al, al ! Null byte marks end 203 hang: jz hang ! Hang forever waiting for CTRL-ALT-DEL 204 movb ah, #0x0E ! Print character in teletype mode 205 mov bx, #0x0001 ! Page 0, foreground color 206 int 0x10 ! Call BIOS VIDEO_IO 207 jmp print 208 209 .data 210 rderr: .ascii "Read error " 211 errno: .ascii "00 \0" 212 errend: 213 214 ! Floppy disk sectors per track for the 1.44M, 1.2M and 360K/720K types: 215 sectors: 216 .data1 18, 15, 9 217 218 ! Extended read/write commands require a parameter packet. 219 ext_rw: 220 .data1 0x10 ! Length of extended r/w packet 221 .data1 0 ! Reserved 222 .data2 0 ! Blocks to transfer (to be filled in) 223 .data2 0 ! Buffer address offset (tbfi) 224 .data2 BOOTSEG ! Buffer address segment 225 .data4 0 ! Starting block number low 32 bits (tbfi) 226 .data4 0 ! Starting block number high 32 bits 227 228 .align 2 229 addresses: 230 ! The space below this is for disk addresses for a 38K /boot program (worst 231 ! case, i.e. file is completely fragmented). It should be enough.
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