mbr.S revision 1.2
1 /* $NetBSD: mbr.S,v 1.2 2003/04/29 10:24:24 dsl Exp $ */ 2 3 /* 4 * Copyright (c) 1999-2003 The NetBSD Foundation, Inc. 5 * All rights reserved. 6 * 7 * This code is derived from software contributed to The NetBSD Foundation 8 * by Frank van der Linden. 9 * Major surgery performed by David Laight. 10 * 11 * Redistribution and use in source and binary forms, with or without 12 * modification, are permitted provided that the following conditions 13 * are met: 14 * 1. Redistributions of source code must retain the above copyright 15 * notice, this list of conditions and the following disclaimer. 16 * 2. Redistributions in binary form must reproduce the above copyright 17 * notice, this list of conditions and the following disclaimer in the 18 * documentation and/or other materials provided with the distribution. 19 * 3. All advertising materials mentioning features or use of this software 20 * must display the following acknowledgement: 21 * This product includes software developed by TooLs GmbH. 22 * 4. The name of TooLs GmbH may not be used to endorse or promote products 23 * derived from this software without specific prior written permission. 24 * 25 * THIS SOFTWARE IS PROVIDED BY TOOLS GMBH ``AS IS'' AND ANY EXPRESS OR 26 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 27 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 28 * IN NO EVENT SHALL TOOLS GMBH BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 29 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, 30 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; 31 * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, 32 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR 33 * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF 34 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 35 */ 36 37 /* 38 * i386 master boot code 39 */ 40 41 /* Compile options: 42 * BOOTSEL - bootselector code 43 * BOOT_EXTENDED - scan extended partition list (LBA reads) 44 * TERSE_ERROR - terse error messages 45 * NO_CHS - all reads are LBA 46 * NO_LBA_CHECK - no check if bios supports LBA reads 47 */ 48 49 #include <machine/asm.h> 50 #include <sys/disklabel_mbr.h> 51 52 #define BOOTADDR 0x7c00 53 #define LOADADDR 0x0600 /* address were are linked to */ 54 55 #define TABENTRYSIZE (PARTNAMESIZE + 1) 56 #define NAMETABSIZE (NMBRPART * TABENTRYSIZE) 57 58 /* 59 * Minimum and maximum drive number that is considered to be valid. 60 */ 61 #define MINDRV 0x80 62 #define MAXDRV 0x87 63 64 #ifdef TERSE_ERROR 65 /* 66 * Error codes. Done this way to save space. 67 */ 68 #define ERR_INVPART '1' /* Invalid partition table */ 69 #define ERR_READ '2' /* Read error */ 70 #define ERR_NOOS '3' /* Magic no. check failed for part. */ 71 #define ERR_KEY '?' /* unknown key press */ 72 #define ERR_NO_LBA 'L' /* sector above chs limit */ 73 74 #define set_err(err) movb $err, %al 75 76 #else 77 #define set_err(err) mov $err, %ax 78 #endif 79 80 .text 81 .code16 82 /* 83 * Move ourselves out of the way first. 84 * (to the address we are linked at - 0x600) 85 * and zero our bss 86 */ 87 ENTRY(start) 88 xor %ax, %ax 89 mov %ax, %ss 90 movw $BOOTADDR, %sp 91 mov %ax, %es 92 mov %ax, %ds 93 mov %sp, %si 94 movw $start, %di 95 movw $(bss_start - start)/2, %cx 96 rep 97 movsw 98 mov $(bss_end - bss_start + 1)/2, %cx 99 rep 100 stosw 101 ljmp $0, $mbr /* leap into copy of code */ 102 103 /* 104 * Sanity check the drive number passed by the BIOS. Some BIOSs may not 105 * do this and pass garbage. 106 */ 107 mbr: 108 cmpb $MAXDRV, %dl /* relies on MINDRV being 0x80 */ 109 jle 1f 110 movb $MINDRV, %dl /* garbage in, boot disk 0 */ 111 1: 112 push %dx /* save drive number */ 113 push %dx /* twice - for err_msg loop */ 114 115 #ifdef BOOTSEL 116 /* 117 * Walk through the selector (name) table printing used entries. 118 */ 119 bootsel_menu: 120 movw $nametab, %bx 121 #ifdef BOOT_EXTENDED 122 xorl %ecx, %ecx /* base of extended partition */ 123 next_extended: 124 xorl %edx, %edx /* for next extended partition */ 125 #endif 126 lea parttab - nametab(%bx), %bp 127 next_ptn: 128 movb 4(%bp), %al /* partition type */ 129 #ifdef BOOT_EXTENDED 130 movl 8(%bp), %edi /* partition sector number */ 131 cmpb $MBR_PTYPE_EXT, %al /* Extended partition */ 132 je 1f 133 cmpb $MBR_PTYPE_EXT_LBA, %al /* Extended LBA partition */ 134 je 1f 135 cmpb $MBR_PTYPE_EXT_LNX, %al /* Linux extended partition */ 136 jne 2f 137 1: movl %edi, %edx /* save next extended ptn */ 138 jmp 3f 139 2: 140 #endif 141 test %al, %al /* undefined partition */ 142 je 3f 143 cmpb $0, (%bx) /* check for prompt */ 144 jz 3f 145 146 /* output menu item */ 147 movw $prefix, %si 148 incb (%si) 149 call message /* menu number */ 150 mov (%si), %si /* ':' << 8 | '1' + count */ 151 shl $2, %si /* const + count * 4 */ 152 #define CONST (4 * ((':' << 8) + '1' - ((SCAN_1 - SCAN_F1) & 0xff))) 153 #ifdef NO_CHS 154 addl lba_sector, %edi 155 movl %edi, ptn_list - CONST(%si) /* sector to read */ 156 #else 157 mov %bp, ptn_list - CONST(%si) /* partition info */ 158 #endif 159 #undef CONST 160 mov %bx, %si 161 call message /* prompt */ 162 movw $crlf, %si 163 call message 164 3: 165 add $0x10, %bp 166 add $TABENTRYSIZE, %bx 167 cmpb $(nametab - start - 0x100) + 4 * TABENTRYSIZE, %bl 168 jne next_ptn 169 170 #ifdef BOOT_EXTENDED 171 /* 172 * Now check extended partition chain 173 */ 174 testl %edx, %edx 175 je wait_key 176 testl %ecx, %ecx 177 jne 1f 178 xchg %ecx, %edx /* save base of ext ptn chain */ 179 1: addl %ecx, %edx /* sector to read */ 180 movl %edx, lba_sector 181 movw $lba_info, %si 182 movb $0x42, %ah 183 pop %dx /* recover drive # */ 184 push %dx /* save drive */ 185 int $0x13 186 jc wait_key /* abort menu on read fail */ 187 cmpw $MBR_MAGIC, LOADADDR + MBR_MAGICOFF 188 movw $nametab - LOADADDR + BOOTADDR, %bx 189 je next_extended 190 #endif 191 192 /* 193 * Get the initial time value for the timeout comparison. It is returned 194 * by int 1a in cx:dx. We do sums modulo 2^16 so it doesn't matter if 195 * the counter wraps (which it does every hour) - so we can safely 196 * ignore 'cx'. 197 * 198 * Loop around checking for a keypress until we have one, or timeout is 199 * reached. 200 */ 201 wait_key: 202 xorb %ah, %ah 203 int $0x1a 204 mov %dx, %di /* start time to di */ 205 3: 206 movb $1, %ah /* looks to see if a */ 207 int $0x16 /* key has been pressed */ 208 jnz get_key 209 xorb %ah, %ah 210 int $0x1a /* current time to cx:dx */ 211 sub %di, %dx 212 movw timeout, %ax 213 cmp %ax, %dx /* always wait for 1 tick... */ 214 jbe 3b /* 0xffff means never timeout */ 215 def_key: 216 movb defkey, %al /* timedout - pick default key */ 217 jmp check_key 218 get_key: 219 xorb %ah, %ah 220 int $0x16 /* 'read key', code ah, ascii al */ 221 shr $8, %ax /* code in %al, %ah zero */ 222 223 /* 224 * We have a keycode, see what it means. 225 * If we don't know we generate error '?' and go ask again 226 */ 227 check_key: 228 /* 229 * <enter> -> boot active partition. 230 */ 231 cmpb $SCAN_ENTER, %al 232 jne boot_disk 233 #endif /* BOOTSEL */ 234 235 /* 236 * Scan MBR for first active partition, and boot it. 237 */ 238 mov $NMBRPART, %cx 239 mov $parttab, %si 240 1: 241 cmpb $0x80, (%si) 242 #ifdef NO_CHS 243 jne 10f /* not active */ 244 movl 8(%si), %ebp /* sector number of ptn */ 245 jmp boot_lba 246 10: 247 #else 248 je boot_si 249 #endif 250 add $0x10, %si 251 loop 1b 252 set_err(ERR_INVPART) 253 jmp err_msg 254 255 #ifdef BOOTSEL 256 /* 257 * F1-F10 -> boot disk 0-9. Check if the requested disk isn't above 258 * the number of disks actually in the system as stored in 0:0475 by 259 * the BIOS. 260 * If we trust loc 475, we needn't check the upper bound on the keystroke 261 * This is always sector 0, so always read using chs. 262 */ 263 boot_disk: 264 subb $SCAN_F1, %al 265 cmpb 0x0475, %al 266 jae boot_ptn 267 addb $0x80, %al 268 pop %dx /* dump saved drive # */ 269 push %ax /* replace with new */ 270 #ifdef NO_CHS 271 xorl %ebp, %ebp /* read sector number 0 */ 272 jmp boot_lba 273 #else 274 movw $chs_zero, %si /* chs read sector zero info */ 275 jmp read_chs 276 #endif 277 278 /* 279 * Boot requested partition. 280 * Use keycode to index the table we generated when we scanned the mbr 281 * while generating the menu. 282 * 283 * We very carfully saved the values in the correct part of the table. 284 */ 285 286 boot_ptn: 287 shl $2, %ax 288 movw %ax, %si 289 #ifdef NO_CHS 290 movl ptn_list(%si), %ebp 291 testl %ebp, %ebp 292 jnz boot_lba 293 #else 294 mov ptn_list(%si), %si 295 test %si, %si 296 jnz boot_si 297 #endif 298 set_err(ERR_KEY) 299 /* jmp err_msg */ 300 #endif /* BOOTSEL */ 301 302 /* Something went wrong... 303 * Output error code, 304 * reset disk subsystem - needed after read failure, 305 * and wait for user key 306 */ 307 err_msg: 308 #ifdef TERSE_ERROR 309 movb %al, errcod 310 movw $errtxt, %si 311 call message 312 #else 313 push %ax 314 movw $errtxt, %si 315 call message 316 pop %si 317 call message 318 movw $crlf, %si 319 call message 320 #endif 321 pop %dx /* drive we errored on */ 322 xor %ax,%ax /* only need %ah = 0 */ 323 int $0x13 /* reset disk subsystem */ 324 #ifdef BOOTSEL 325 pop %dx /* original drive number */ 326 push %dx 327 push %dx 328 jmp get_key 329 #else 330 int $0x18 /* BIOS might ask for a key */ 331 /* press and retry boot seq. */ 332 1: sti 333 hlt 334 jmp 1b 335 #endif 336 337 #ifndef NO_CHS 338 /* 339 * Active partition pointed to by si. 340 * Read the first sector. 341 * 342 * We can either do a CHS (Cylinder Head Sector) or an LBA (Logical 343 * Block Address) read. Always doing the LBA one 344 * would be nice - unfortunately not all systems support it. 345 * Also some may contain a separate (eg SCSI) bios that doesn't 346 * support it even when the main bios does. 347 * 348 * There is also the additional problem that the CHS values may be wrong 349 * (eg if fdisk was run on a different system that used different BIOS 350 * geometry). We convert the CHS value to a LBA sector number using 351 * the geometry from the BIOS, if the number matches we do a CHS read. 352 */ 353 boot_si: 354 movl 8(%si), %ebp /* get sector # */ 355 356 testb $BFL_READ_LBA, flags 357 jnz boot_lba /* fdisk forced LBA read */ 358 359 pop %dx /* collect saved drive... */ 360 push %dx /* ...number to dl */ 361 movb $8, %ah 362 int $0x13 /* chs info */ 363 364 /* 365 * Validate geometry, if the CHS sector number doesn't match the LBA one 366 * we'll do an LBA read. 367 * calc: (cylinder * number_of_heads + head) * number_of_sectors + sector 368 * and compare against LBA sector number. 369 * Take a slight 'flier' and assume we can just check 16bits (very likely 370 * to be true because the number of sectors per track is 63). 371 */ 372 movw 2(%si), %ax /* cylinder + sector */ 373 push %ax /* save for sector */ 374 shr $6, %al 375 xchgb %al, %ah /* 10 bit cylinder number */ 376 shr $8, %dx /* last head */ 377 inc %dx /* number of heads */ 378 mul %dx 379 mov 1(%si), %dl /* head we want */ 380 add %dx, %ax 381 and $0x3f, %cx /* number of sectors */ 382 mul %cx 383 pop %dx /* recover sector we want */ 384 and $0x3f, %dx 385 add %dx, %ax 386 dec %ax 387 388 cmp %bp, %ax 389 je read_chs 390 391 #ifndef NO_LBA_CHECK 392 /* 393 * Determine whether we have int13-extensions, by calling int 13, function 41. 394 * Check for the magic number returned, and the disk packet capability. 395 */ 396 movw $0x55aa, %bx 397 movb $0x41, %ah 398 pop %dx 399 push %dx 400 int $0x13 401 jc 1f /* no int13 extensions */ 402 cmpw $0xaa55, %bx 403 jnz 1f 404 testb $1, %cl 405 jnz boot_lba 406 1: set_err(ERR_NO_LBA) 407 jmp err_msg 408 #endif /* NO_LBA_CHECK */ 409 #endif /* NO_CHS */ 410 411 /* 412 * Save sector number (passed in %ebp) into lba parameter block, 413 * read the sector and leap into it. 414 */ 415 boot_lba: 416 movl %ebp, lba_sector /* save sector number */ 417 movw $lba_info, %si 418 movb $0x42, %ah 419 pop %dx /* recover drive # */ 420 do_read: 421 push %dx /* save drive */ 422 int $0x13 423 424 set_err(ERR_READ) 425 jc err_msg 426 427 /* 428 * Check signature for valid bootcode 429 */ 430 movb BOOTADDR, %al /* first byte non-zero */ 431 test %al, %al 432 jz 1f 433 movw BOOTADDR + MBR_MAGICOFF, %ax 434 1: cmp $MBR_MAGIC, %ax 435 set_err(ERR_NOOS) 436 jnz err_msg 437 438 /* We pass the sector number through to the next stage boot. 439 * It doesn't have to use it (indeed no other mbr code will generate) it, 440 * but it does let us have a NetBSD pbr that can identify where it was 441 * read from! This lets us use this code to select between two 442 * NetBSD system on the same physical driver. 443 * (If we've read the mbr of a different disk, it gets a random number 444 * - but it wasn't expecting anything...) 445 */ 446 movl %ebp, %esi 447 pop %dx /* recover drive # */ 448 jmp start - LOADADDR + BOOTADDR 449 450 451 #ifndef NO_CHS 452 /* 453 * Sector below CHS limit 454 * Do a cylinder-head-sector read instead. 455 */ 456 read_chs: 457 pop %dx /* recover drive # */ 458 movb 1(%si), %dh /* head */ 459 movw 2(%si), %cx /* ch=cyl, cl=sect */ 460 movw $BOOTADDR, %bx /* es:bx is buffer */ 461 movw $0x201, %ax /* command 2, 1 sector */ 462 jmp do_read 463 #endif 464 465 /* 466 * Control block for int-13 LBA read. 467 * We need a xx, 00, 01, 00 somewhere to load chs for sector zero, 468 * by a complete fluke there is one here! 469 */ 470 chs_zero: 471 lba_info: 472 .word 0x10 /* control block length */ 473 .word 1 /* sector count */ 474 .word BOOTADDR /* offset in segment */ 475 .word 0 /* segment */ 476 lba_sector: 477 .long 0x0000 /* sector # goes here... */ 478 .long 0x0000 479 480 errtxt: .ascii "Error " /* runs into crlf if errcod set */ 481 errcod: .byte 0 482 crlf: .asciz "\r\n" 483 484 #ifdef BOOTSEL 485 prefix: .asciz "0: " 486 #endif 487 488 #ifndef TERSE_ERROR 489 ERR_INVPART: .asciz "No active partition" 490 ERR_READ: .asciz "Disk read error" 491 ERR_NOOS: .asciz "No operating system" 492 #ifndef NO_LBA_CHECK 493 ERR_NO_LBA: .asciz "Invalid CHS read" 494 #endif 495 #ifdef BOOTSEL 496 ERR_KEY: .asciz "bad key" 497 #endif 498 #endif 499 500 /* 501 * I hate #including source files, but the stuff below has to be at 502 * the correct absolute address. 503 * Clearly this could be done with a linker script. 504 */ 505 506 #include <message.S> 507 #if 0 508 #include <dump_eax.S> 509 #endif 510 511 /* 512 * Stuff from here on is overwritten by fdisk - the offset must not change... 513 * 514 * Get amount of space to makefile can report it. 515 * (Unfortunately I can't seem to get the value reported when it is -ve) 516 */ 517 mbr_space = defkey - . 518 . = start + MBR_BOOTSELOFF 519 /* 520 * Default action, as a keyvalue we'd normally read from the BIOS. 521 */ 522 defkey: 523 .byte SCAN_ENTER /* ps/2 code */ 524 #ifndef BOOTSEL_FLAGS 525 #define BOOTSEL_FLAGS 0 526 #endif 527 flags: .byte BFL_NEWMBR | BOOTSEL_FLAGS 528 /* 529 * Timeout value. ~65536 ticks per hour, which is ~18.2 times per second. 530 * 0xffff means never timeout. 531 */ 532 timeout: 533 .word 182 /* default to 10 seconds */ 534 /* 535 * Space for name/select table and partition table. 536 */ 537 nametab: 538 .fill NMBRPART * (PARTNAMESIZE + 1), 0x01, 0x00 539 540 . = start + MBR_PARTOFF - 2 541 .word MBR_MAGIC 542 543 . = start + MBR_PARTOFF 544 parttab: 545 .fill 0x40, 0x01, 0x00 546 547 . = start + MBR_MAGICOFF 548 .word MBR_MAGIC 549 550 /* zeroed data space */ 551 bss_off = 0 552 bss_start = . 553 #define BSS(name, size) name = bss_start + bss_off; bss_off = bss_off + size 554 BSS(ptn_list, 256 * 4) /* long[]: boot sector numbers */ 555 BSS(bss_end, 0) 556
Indexes created Sat Oct 11 16:09:52 GMT 2025