mbr.S revision 1.14
1 /* $NetBSD: mbr.S,v 1.14 2007/04/10 16:54:34 dsl Exp $ */ 2 3 /* 4 * Copyright (c) 1999-2004 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, based on an earlier work by Wolfgang Solfrank. 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 the NetBSD 22 * Foundation, Inc. and its contributors. 23 * 4. Neither the name of The NetBSD Foundation nor the names of its 24 * contributors may be used to endorse or promote products derived 25 * from this software without specific prior written permission. 26 * 27 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 28 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 29 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 30 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 31 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 32 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 33 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 34 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 35 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 36 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 37 * POSSIBILITY OF SUCH DAMAGE. 38 */ 39 40 /* 41 * i386 master boot code 42 */ 43 44 /* Compile options: 45 * BOOTSEL - bootselector code 46 * BOOT_EXTENDED - scan extended partition list (LBA reads) 47 * TERSE_ERROR - terse error messages 48 * NO_CHS - all reads are LBA 49 * NO_LBA_CHECK - no check if bios supports LBA reads 50 * NO_BANNER - do not output 'banner' 51 */ 52 53 #ifdef BOOT_EXTENDED 54 #define NO_CHS 1 55 #define BOOTSEL 1 56 #endif 57 58 #ifdef BOOTSEL 59 #define TERSE_ERROR 1 60 #endif 61 62 #include <machine/asm.h> 63 #include <sys/bootblock.h> 64 65 #define BOOTADDR 0x7c00 66 #define LOADADDR 0x0600 /* address were are linked to */ 67 68 #define TABENTRYSIZE (MBR_BS_PARTNAMESIZE + 1) 69 #define NAMETABSIZE (MBR_PART_COUNT * TABENTRYSIZE) 70 71 /* Scan values for the various keys we use, as returned by the BIOS */ 72 #define SCAN_ENTER 0x1c 73 #define SCAN_F1 0x3b 74 #define SCAN_1 0x2 75 76 /* 77 * Minimum and maximum drive number that is considered to be valid. 78 */ 79 #define MINDRV 0x80 80 #define MAXDRV 0x8f 81 82 #ifdef TERSE_ERROR 83 /* 84 * Error codes. Done this way to save space. 85 */ 86 #define ERR_INVPART '1' /* Invalid partition table */ 87 #define ERR_READ '2' /* Read error */ 88 #define ERR_NOOS '3' /* Magic no. check failed for part. */ 89 #define ERR_KEY '?' /* unknown key press */ 90 #define ERR_NO_LBA 'L' /* sector above chs limit */ 91 92 #define set_err(err) movb $err, %al 93 94 #else 95 #define set_err(err) mov $err, %ax 96 #endif 97 98 .text 99 .code16 100 /* 101 * Move ourselves out of the way first. 102 * (to the address we are linked at - 0x600) 103 * and zero our bss 104 */ 105 ENTRY(start) 106 xor %ax, %ax 107 mov %ax, %ss 108 movw $BOOTADDR, %sp 109 mov %ax, %es 110 mov %ax, %ds 111 movw $mbr, %di 112 mov $mbr - LOADADDR + BOOTADDR, %si 113 push %ax 114 push %di 115 movw $(bss_start - mbr), %cx 116 rep 117 movsb /* relocate code */ 118 mov $(bss_end - bss_start + 1)/2, %cx 119 rep 120 stosw /* zero bss */ 121 lret /* Ensure %cs == 0 */ 122 123 /* 124 * Sanity check the drive number passed by the BIOS. Some BIOSs may not 125 * do this and pass garbage. 126 */ 127 mbr: 128 cmpb $MAXDRV, %dl /* relies on MINDRV being 0x80 */ 129 jle 1f 130 movb $MINDRV, %dl /* garbage in, boot disk 0 */ 131 1: 132 push %dx /* save drive number */ 133 push %dx /* twice - for err_msg loop */ 134 135 #ifndef NO_BANNER 136 mov $banner, %si 137 call message_crlf 138 #endif 139 140 /* 141 * Walk through the selector (name) table printing used entries. 142 */ 143 bootsel_menu: 144 movw $nametab, %bx 145 #ifdef BOOT_EXTENDED 146 xorl %ecx, %ecx /* base of extended partition */ 147 next_extended: 148 xorl %edx, %edx /* for next extended partition */ 149 #endif 150 lea parttab - nametab(%bx), %bp 151 next_ptn: 152 movb 4(%bp), %al /* partition type */ 153 #ifdef NO_CHS 154 movl 8(%bp), %edi /* partition sector number */ 155 #ifdef BOOT_EXTENDED 156 cmpb $MBR_PTYPE_EXT, %al /* Extended partition */ 157 je 1f 158 cmpb $MBR_PTYPE_EXT_LBA, %al /* Extended LBA partition */ 159 je 1f 160 cmpb $MBR_PTYPE_EXT_LNX, %al /* Linux extended partition */ 161 jne 2f 162 1: movl %edi, %edx /* save next extended ptn */ 163 jmp 4f 164 2: 165 #endif 166 addl lba_sector, %edi /* add in extended ptn base */ 167 #endif 168 test %al, %al /* undefined partition */ 169 je 4f 170 cmpb $0x80, (%bp) /* check for active partition */ 171 jne 3f /* jump if not... */ 172 #define ENTER (4 * ((SCAN_ENTER - SCAN_F1) & 0xff)) 173 #ifdef NO_CHS 174 movl %edi, ptn_list + ENTER /* save location of active ptn */ 175 #else 176 mov %bp, ptn_list + ENTER 177 #endif 178 #undef ENTER 179 3: 180 #ifdef BOOTSEL 181 cmpb $0, (%bx) /* check for prompt */ 182 jz 4f 183 /* output menu item */ 184 movw $prefix, %si 185 incb (%si) 186 call message /* menu number */ 187 mov (%si), %si /* ':' << 8 | '1' + count */ 188 shl $2, %si /* const + count * 4 */ 189 #define CONST (4 * ((':' << 8) + '1' - ((SCAN_1 - SCAN_F1) & 0xff))) 190 #ifdef NO_CHS 191 movl %edi, ptn_list - CONST(%si) /* sector to read */ 192 #else 193 mov %bp, ptn_list - CONST(%si) /* partition info */ 194 #endif 195 #undef CONST 196 mov %bx, %si 197 call message_crlf /* prompt */ 198 #endif 199 4: 200 add $0x10, %bp 201 add $TABENTRYSIZE, %bx 202 cmpb $(nametab - start - 0x100) + 4 * TABENTRYSIZE, %bl 203 jne next_ptn 204 205 #ifdef BOOT_EXTENDED 206 /* 207 * Now check extended partition chain 208 */ 209 testl %edx, %edx 210 je wait_key 211 testl %ecx, %ecx 212 jne 1f 213 xchg %ecx, %edx /* save base of ext ptn chain */ 214 1: addl %ecx, %edx /* sector to read */ 215 movl %edx, lba_sector 216 movw $lba_info, %si 217 movb $0x42, %ah 218 pop %dx /* recover drive # */ 219 push %dx /* save drive */ 220 int $0x13 221 jc wait_key /* abort menu on read fail */ 222 cmpw $MBR_MAGIC, LOADADDR + MBR_MAGIC_OFFSET 223 movw $nametab - LOADADDR + BOOTADDR, %bx 224 je next_extended 225 #endif 226 227 /* 228 * The non-bootsel code traverses this code path, it needs the 229 * correct keycode to select the active partition. 230 */ 231 232 #ifndef BOOTSEL 233 mov $(SCAN_ENTER - SCAN_F1) & 0xff, %ax 234 #else 235 /* 236 * Get the initial time value for the timeout comparison. It is returned 237 * by int 1a in cx:dx. We do sums modulo 2^16 so it doesn't matter if 238 * the counter wraps (which it does every hour) - so we can safely 239 * ignore 'cx'. 240 * 241 * Loop around checking for a keypress until we have one, or timeout is 242 * reached. 243 */ 244 wait_key: 245 xorb %ah, %ah 246 int $0x1a 247 mov %dx, %di /* start time to di */ 248 3: 249 movb $1, %ah /* looks to see if a */ 250 int $0x16 /* key has been pressed */ 251 jnz get_key 252 xorb %ah, %ah 253 int $0x1a /* current time to cx:dx */ 254 sub %di, %dx 255 cmpw timeout, %dx /* always wait for 1 tick... */ 256 jbe 3b /* 0xffff means never timeout */ 257 def_key: 258 mov defkey - 1, %ax /* timedout - get default key to %ah */ 259 jmp 4f 260 get_key: 261 xorb %ah, %ah 262 int $0x16 /* 'read key', code ah, ascii al */ 263 4: shr $8, %ax /* code in %al, %ah zero */ 264 265 /* 266 * We have a keycode, see what it means. 267 * If we don't know we generate error '?' and go ask again 268 */ 269 check_key: 270 /* 271 * F1-F10 -> boot disk 0-9. Check if the requested disk isn't above 272 * the number of disks actually in the system as stored in 0:0475 by 273 * the BIOS. 274 * If we trust loc 475, we needn't check the upper bound on the keystroke 275 * This is always sector 0, so always read using chs. 276 */ 277 subb $SCAN_F1, %al 278 cmpb 0x0475, %al 279 jae boot_ptn 280 addb $0x80, %al 281 pop %dx /* dump saved drive # */ 282 push %ax /* replace with new */ 283 #ifdef NO_CHS 284 xorl %ebp, %ebp /* read sector number 0 */ 285 jmp boot_lba 286 #else 287 movw $chs_zero, %si /* chs read sector zero info */ 288 jmp read_chs 289 #endif 290 #endif /* BOOTSEL */ 291 292 /* 293 * Boot requested partition. 294 * Use keycode to index the table we generated when we scanned the mbr 295 * while generating the menu. 296 * 297 * We very carfully saved the values in the correct part of the table. 298 */ 299 300 boot_ptn: 301 shl $2, %ax 302 movw %ax, %si 303 #ifdef NO_CHS 304 movl ptn_list(%si), %ebp 305 testl %ebp, %ebp 306 jnz boot_lba 307 #else 308 mov ptn_list(%si), %si 309 test %si, %si 310 jnz boot_si 311 #endif 312 #ifdef BOOTSEL 313 set_err(ERR_KEY) 314 #else 315 set_err(ERR_INVPART) 316 #endif 317 /* jmp err_msg */ 318 319 /* Something went wrong... 320 * Output error code, 321 * reset disk subsystem - needed after read failure, 322 * and wait for user key 323 */ 324 err_msg: 325 #ifdef TERSE_ERROR 326 movb %al, errcod 327 movw $errtxt, %si 328 call message 329 #else 330 movw $errtxt, %si 331 call message 332 mov %ax, %si 333 call message_crlf 334 #endif 335 pop %dx /* drive we errored on */ 336 xor %ax,%ax /* only need %ah = 0 */ 337 int $0x13 /* reset disk subsystem */ 338 #ifdef BOOTSEL 339 pop %dx /* original drive number */ 340 push %dx 341 push %dx 342 jmp get_key 343 #else 344 int $0x18 /* BIOS might ask for a key */ 345 /* press and retry boot seq. */ 346 1: sti 347 hlt 348 jmp 1b 349 #endif 350 351 #ifndef NO_CHS 352 /* 353 * Active partition pointed to by si. 354 * Read the first sector. 355 * 356 * We can either do a CHS (Cylinder Head Sector) or an LBA (Logical 357 * Block Address) read. Always doing the LBA one 358 * would be nice - unfortunately not all systems support it. 359 * Also some may contain a separate (eg SCSI) bios that doesn't 360 * support it even when the main bios does. 361 * 362 * There is also the additional problem that the CHS values may be wrong 363 * (eg if fdisk was run on a different system that used different BIOS 364 * geometry). We convert the CHS value to a LBA sector number using 365 * the geometry from the BIOS, if the number matches we do a CHS read. 366 */ 367 boot_si: 368 movl 8(%si), %ebp /* get sector # */ 369 370 testb $MBR_BS_READ_LBA, flags 371 jnz boot_lba /* fdisk forced LBA read */ 372 373 pop %dx /* collect saved drive... */ 374 push %dx /* ...number to dl */ 375 movb $8, %ah 376 int $0x13 /* chs info */ 377 378 /* 379 * Validate geometry, if the CHS sector number doesn't match the LBA one 380 * we'll do an LBA read. 381 * calc: (cylinder * number_of_heads + head) * number_of_sectors + sector 382 * and compare against LBA sector number. 383 * Take a slight 'flier' and assume we can just check 16bits (very likely 384 * to be true because the number of sectors per track is 63). 385 */ 386 movw 2(%si), %ax /* cylinder + sector */ 387 push %ax /* save for sector */ 388 shr $6, %al 389 xchgb %al, %ah /* 10 bit cylinder number */ 390 shr $8, %dx /* last head */ 391 inc %dx /* number of heads */ 392 mul %dx 393 mov 1(%si), %dl /* head we want */ 394 add %dx, %ax 395 and $0x3f, %cx /* number of sectors */ 396 mul %cx 397 pop %dx /* recover sector we want */ 398 and $0x3f, %dx 399 add %dx, %ax 400 dec %ax 401 402 cmp %bp, %ax 403 je read_chs 404 405 #ifndef NO_LBA_CHECK 406 /* 407 * Determine whether we have int13-extensions, by calling int 13, function 41. 408 * Check for the magic number returned, and the disk packet capability. 409 */ 410 movw $0x55aa, %bx 411 movb $0x41, %ah 412 pop %dx 413 push %dx 414 int $0x13 415 set_err(ERR_NO_LBA) 416 jc err_msg /* no int13 extensions */ 417 cmpw $0xaa55, %bx 418 jnz err_msg 419 testb $1, %cl 420 jz err_msg 421 #endif /* NO_LBA_CHECK */ 422 #endif /* NO_CHS */ 423 424 /* 425 * Save sector number (passed in %ebp) into lba parameter block, 426 * read the sector and leap into it. 427 */ 428 boot_lba: 429 movl %ebp, lba_sector /* save sector number */ 430 movw $lba_info, %si 431 movb $0x42, %ah 432 pop %dx /* recover drive # */ 433 do_read: 434 push %dx /* save drive */ 435 int $0x13 436 437 set_err(ERR_READ) 438 jc err_msg 439 440 /* 441 * Check signature for valid bootcode 442 */ 443 movb BOOTADDR, %al /* first byte non-zero */ 444 test %al, %al 445 jz 1f 446 movw BOOTADDR + MBR_MAGIC_OFFSET, %ax 447 1: cmp $MBR_MAGIC, %ax 448 set_err(ERR_NOOS) 449 jnz err_msg 450 451 /* We pass the sector number through to the next stage boot. 452 * It doesn't have to use it (indeed no other mbr code will generate) it, 453 * but it does let us have a NetBSD pbr that can identify where it was 454 * read from! This lets us use this code to select between two 455 * NetBSD system on the same physical driver. 456 * (If we've read the mbr of a different disk, it gets a random number 457 * - but it wasn't expecting anything...) 458 */ 459 movl %ebp, %esi 460 pop %dx /* recover drive # */ 461 jmp start - LOADADDR + BOOTADDR 462 463 464 #ifndef NO_CHS 465 /* 466 * Sector below CHS limit 467 * Do a cylinder-head-sector read instead. 468 */ 469 read_chs: 470 pop %dx /* recover drive # */ 471 movb 1(%si), %dh /* head */ 472 movw 2(%si), %cx /* ch=cyl, cl=sect */ 473 movw $BOOTADDR, %bx /* es:bx is buffer */ 474 movw $0x201, %ax /* command 2, 1 sector */ 475 jmp do_read 476 #endif 477 478 /* 479 * Control block for int-13 LBA read. 480 * We need a xx, 00, 01, 00 somewhere to load chs for sector zero, 481 * by a complete fluke there is one here! 482 */ 483 chs_zero: 484 lba_info: 485 .word 0x10 /* control block length */ 486 .word 1 /* sector count */ 487 .word BOOTADDR /* offset in segment */ 488 .word 0 /* segment */ 489 lba_sector: 490 .long 0x0000 /* sector # goes here... */ 491 .long 0x0000 492 493 errtxt: .ascii "Error " /* runs into crlf if errcod set */ 494 errcod: .byte 0 495 crlf: .asciz "\r\n" 496 497 #ifndef NO_BANNER 498 #ifdef BOOTSEL 499 banner: .asciz "Fn: diskn" 500 #else 501 banner: .asciz "NetBSD MBR boot" 502 #endif 503 #endif 504 505 #ifdef BOOTSEL 506 prefix: .asciz "0: " 507 #endif 508 509 #ifndef TERSE_ERROR 510 ERR_INVPART: .asciz "No active partition" 511 ERR_READ: .asciz "Disk read error" 512 ERR_NOOS: .asciz "No operating system" 513 #ifndef NO_LBA_CHECK 514 ERR_NO_LBA: .asciz "Invalid CHS read" 515 #endif 516 #ifdef BOOTSEL 517 ERR_KEY: .asciz "bad key" 518 #endif 519 #endif 520 521 /* 522 * I hate #including source files, but the stuff below has to be at 523 * the correct absolute address. 524 * Clearly this could be done with a linker script. 525 */ 526 527 message_crlf: 528 call message 529 movw $crlf, %si 530 #include <message.S> 531 #if 0 532 #include <dump_eax.S> 533 #endif 534 535 /* 536 * Stuff from here on is overwritten by fdisk - the offset must not change... 537 * 538 * Get amount of space to makefile can report it. 539 * (Unfortunately I can't seem to get the value reported when it is -ve) 540 */ 541 mbr_space = defkey - . 542 . = start + MBR_BS_OFFSET 543 /* 544 * Default action, as a keyvalue we'd normally read from the BIOS. 545 */ 546 defkey: 547 .byte SCAN_ENTER /* ps/2 code */ 548 #ifndef BOOTSEL_FLAGS 549 #define BOOTSEL_FLAGS 0 550 #endif 551 flags: .byte MBR_BS_NEWMBR | BOOTSEL_FLAGS 552 /* 553 * Timeout value. ~65536 ticks per hour, which is ~18.2 times per second. 554 * 0xffff means never timeout. 555 */ 556 timeout: 557 .word 182 /* default to 10 seconds */ 558 /* 559 * mbr_bootsel 560 */ 561 nametab: 562 .fill MBR_PART_COUNT * (MBR_BS_PARTNAMESIZE + 1), 0x01, 0x00 563 564 /* space for mbr_dsn */ 565 . = start + MBR_DSN_OFFSET 566 .long 0 567 568 /* mbr_bootsel_magic */ 569 . = start + MBR_BS_MAGIC_OFFSET 570 .word MBR_BS_MAGIC 571 572 /* 573 * MBR partition table 574 */ 575 . = start + MBR_PART_OFFSET 576 parttab: 577 .fill 0x40, 0x01, 0x00 578 579 . = start + MBR_MAGIC_OFFSET 580 .word MBR_MAGIC 581 582 /* zeroed data space */ 583 bss_off = 0 584 bss_start = . 585 #define BSS(name, size) name = bss_start + bss_off; bss_off = bss_off + size 586 BSS(ptn_list, 256 * 4) /* long[]: boot sector numbers */ 587 BSS(bss_end, 0) 588
Indexes created Mon Oct 13 00:09:54 GMT 2025