mbr.S revision 1.12
1 /* $NetBSD: mbr.S,v 1.12 2004/09/12 08:41:47 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 mov %sp, %si 112 movw $start, %di 113 movw $(bss_start - start)/2, %cx 114 rep 115 movsw /* relocate code */ 116 mov $(bss_end - bss_start + 1)/2, %cx 117 rep 118 stosw /* zero bss */ 119 ljmp $0, $mbr /* leap into copy of code */ 120 121 /* 122 * Sanity check the drive number passed by the BIOS. Some BIOSs may not 123 * do this and pass garbage. 124 */ 125 mbr: 126 cmpb $MAXDRV, %dl /* relies on MINDRV being 0x80 */ 127 jle 1f 128 movb $MINDRV, %dl /* garbage in, boot disk 0 */ 129 1: 130 push %dx /* save drive number */ 131 push %dx /* twice - for err_msg loop */ 132 133 #ifndef NO_BANNER 134 mov $banner, %si 135 call message_crlf 136 #endif 137 138 /* 139 * Walk through the selector (name) table printing used entries. 140 */ 141 bootsel_menu: 142 movw $nametab, %bx 143 #ifdef BOOT_EXTENDED 144 xorl %ecx, %ecx /* base of extended partition */ 145 next_extended: 146 xorl %edx, %edx /* for next extended partition */ 147 #endif 148 lea parttab - nametab(%bx), %bp 149 next_ptn: 150 movb 4(%bp), %al /* partition type */ 151 #ifdef NO_CHS 152 movl 8(%bp), %edi /* partition sector number */ 153 #ifdef BOOT_EXTENDED 154 cmpb $MBR_PTYPE_EXT, %al /* Extended partition */ 155 je 1f 156 cmpb $MBR_PTYPE_EXT_LBA, %al /* Extended LBA partition */ 157 je 1f 158 cmpb $MBR_PTYPE_EXT_LNX, %al /* Linux extended partition */ 159 jne 2f 160 1: movl %edi, %edx /* save next extended ptn */ 161 jmp 4f 162 2: 163 #endif 164 addl lba_sector, %edi /* add in extended ptn base */ 165 #endif 166 test %al, %al /* undefined partition */ 167 je 4f 168 cmpb $0x80, (%bp) /* check for active partition */ 169 jne 3f /* jump if not... */ 170 #define ENTER (4 * ((SCAN_ENTER - SCAN_F1) & 0xff)) 171 #ifdef NO_CHS 172 movl %edi, ptn_list + ENTER /* save location of active ptn */ 173 #else 174 mov %bp, ptn_list + ENTER 175 #endif 176 #undef ENTER 177 3: 178 #ifdef BOOTSEL 179 cmpb $0, (%bx) /* check for prompt */ 180 jz 4f 181 /* output menu item */ 182 movw $prefix, %si 183 incb (%si) 184 call message /* menu number */ 185 mov (%si), %si /* ':' << 8 | '1' + count */ 186 shl $2, %si /* const + count * 4 */ 187 #define CONST (4 * ((':' << 8) + '1' - ((SCAN_1 - SCAN_F1) & 0xff))) 188 #ifdef NO_CHS 189 movl %edi, ptn_list - CONST(%si) /* sector to read */ 190 #else 191 mov %bp, ptn_list - CONST(%si) /* partition info */ 192 #endif 193 #undef CONST 194 mov %bx, %si 195 call message_crlf /* prompt */ 196 #endif 197 4: 198 add $0x10, %bp 199 add $TABENTRYSIZE, %bx 200 cmpb $(nametab - start - 0x100) + 4 * TABENTRYSIZE, %bl 201 jne next_ptn 202 203 #ifdef BOOT_EXTENDED 204 /* 205 * Now check extended partition chain 206 */ 207 testl %edx, %edx 208 je wait_key 209 testl %ecx, %ecx 210 jne 1f 211 xchg %ecx, %edx /* save base of ext ptn chain */ 212 1: addl %ecx, %edx /* sector to read */ 213 movl %edx, lba_sector 214 movw $lba_info, %si 215 movb $0x42, %ah 216 pop %dx /* recover drive # */ 217 push %dx /* save drive */ 218 int $0x13 219 jc wait_key /* abort menu on read fail */ 220 cmpw $MBR_MAGIC, LOADADDR + MBR_MAGIC_OFFSET 221 movw $nametab - LOADADDR + BOOTADDR, %bx 222 je next_extended 223 #endif 224 225 /* 226 * The non-bootsel code traverses this code path, it needs the 227 * correct keycode to select the active partition. 228 */ 229 230 #ifndef BOOTSEL 231 mov $(SCAN_ENTER - SCAN_F1) & 0xff, %ax 232 #else 233 /* 234 * Get the initial time value for the timeout comparison. It is returned 235 * by int 1a in cx:dx. We do sums modulo 2^16 so it doesn't matter if 236 * the counter wraps (which it does every hour) - so we can safely 237 * ignore 'cx'. 238 * 239 * Loop around checking for a keypress until we have one, or timeout is 240 * reached. 241 */ 242 wait_key: 243 xorb %ah, %ah 244 int $0x1a 245 mov %dx, %di /* start time to di */ 246 3: 247 movb $1, %ah /* looks to see if a */ 248 int $0x16 /* key has been pressed */ 249 jnz get_key 250 xorb %ah, %ah 251 int $0x1a /* current time to cx:dx */ 252 sub %di, %dx 253 cmpw timeout, %dx /* always wait for 1 tick... */ 254 jbe 3b /* 0xffff means never timeout */ 255 def_key: 256 mov defkey - 1, %ax /* timedout - get default key to %ah */ 257 jmp 4f 258 get_key: 259 xorb %ah, %ah 260 int $0x16 /* 'read key', code ah, ascii al */ 261 4: shr $8, %ax /* code in %al, %ah zero */ 262 263 /* 264 * We have a keycode, see what it means. 265 * If we don't know we generate error '?' and go ask again 266 */ 267 check_key: 268 /* 269 * F1-F10 -> boot disk 0-9. Check if the requested disk isn't above 270 * the number of disks actually in the system as stored in 0:0475 by 271 * the BIOS. 272 * If we trust loc 475, we needn't check the upper bound on the keystroke 273 * This is always sector 0, so always read using chs. 274 */ 275 subb $SCAN_F1, %al 276 cmpb 0x0475, %al 277 jae boot_ptn 278 addb $0x80, %al 279 pop %dx /* dump saved drive # */ 280 push %ax /* replace with new */ 281 #ifdef NO_CHS 282 xorl %ebp, %ebp /* read sector number 0 */ 283 jmp boot_lba 284 #else 285 movw $chs_zero, %si /* chs read sector zero info */ 286 jmp read_chs 287 #endif 288 #endif /* BOOTSEL */ 289 290 /* 291 * Boot requested partition. 292 * Use keycode to index the table we generated when we scanned the mbr 293 * while generating the menu. 294 * 295 * We very carfully saved the values in the correct part of the table. 296 */ 297 298 boot_ptn: 299 shl $2, %ax 300 movw %ax, %si 301 #ifdef NO_CHS 302 movl ptn_list(%si), %ebp 303 testl %ebp, %ebp 304 jnz boot_lba 305 #else 306 mov ptn_list(%si), %si 307 test %si, %si 308 jnz boot_si 309 #endif 310 #ifdef BOOTSEL 311 set_err(ERR_KEY) 312 #else 313 set_err(ERR_INVPART) 314 #endif 315 /* jmp err_msg */ 316 317 /* Something went wrong... 318 * Output error code, 319 * reset disk subsystem - needed after read failure, 320 * and wait for user key 321 */ 322 err_msg: 323 #ifdef TERSE_ERROR 324 movb %al, errcod 325 movw $errtxt, %si 326 call message 327 #else 328 movw $errtxt, %si 329 call message 330 mov %ax, %si 331 call message_crlf 332 #endif 333 pop %dx /* drive we errored on */ 334 xor %ax,%ax /* only need %ah = 0 */ 335 int $0x13 /* reset disk subsystem */ 336 #ifdef BOOTSEL 337 pop %dx /* original drive number */ 338 push %dx 339 push %dx 340 jmp get_key 341 #else 342 int $0x18 /* BIOS might ask for a key */ 343 /* press and retry boot seq. */ 344 1: sti 345 hlt 346 jmp 1b 347 #endif 348 349 #ifndef NO_CHS 350 /* 351 * Active partition pointed to by si. 352 * Read the first sector. 353 * 354 * We can either do a CHS (Cylinder Head Sector) or an LBA (Logical 355 * Block Address) read. Always doing the LBA one 356 * would be nice - unfortunately not all systems support it. 357 * Also some may contain a separate (eg SCSI) bios that doesn't 358 * support it even when the main bios does. 359 * 360 * There is also the additional problem that the CHS values may be wrong 361 * (eg if fdisk was run on a different system that used different BIOS 362 * geometry). We convert the CHS value to a LBA sector number using 363 * the geometry from the BIOS, if the number matches we do a CHS read. 364 */ 365 boot_si: 366 movl 8(%si), %ebp /* get sector # */ 367 368 testb $MBR_BS_READ_LBA, flags 369 jnz boot_lba /* fdisk forced LBA read */ 370 371 pop %dx /* collect saved drive... */ 372 push %dx /* ...number to dl */ 373 movb $8, %ah 374 int $0x13 /* chs info */ 375 376 /* 377 * Validate geometry, if the CHS sector number doesn't match the LBA one 378 * we'll do an LBA read. 379 * calc: (cylinder * number_of_heads + head) * number_of_sectors + sector 380 * and compare against LBA sector number. 381 * Take a slight 'flier' and assume we can just check 16bits (very likely 382 * to be true because the number of sectors per track is 63). 383 */ 384 movw 2(%si), %ax /* cylinder + sector */ 385 push %ax /* save for sector */ 386 shr $6, %al 387 xchgb %al, %ah /* 10 bit cylinder number */ 388 shr $8, %dx /* last head */ 389 inc %dx /* number of heads */ 390 mul %dx 391 mov 1(%si), %dl /* head we want */ 392 add %dx, %ax 393 and $0x3f, %cx /* number of sectors */ 394 mul %cx 395 pop %dx /* recover sector we want */ 396 and $0x3f, %dx 397 add %dx, %ax 398 dec %ax 399 400 cmp %bp, %ax 401 je read_chs 402 403 #ifndef NO_LBA_CHECK 404 /* 405 * Determine whether we have int13-extensions, by calling int 13, function 41. 406 * Check for the magic number returned, and the disk packet capability. 407 */ 408 movw $0x55aa, %bx 409 movb $0x41, %ah 410 pop %dx 411 push %dx 412 int $0x13 413 set_err(ERR_NO_LBA) 414 jc err_msg /* no int13 extensions */ 415 cmpw $0xaa55, %bx 416 jnz err_msg 417 testb $1, %cl 418 jz err_msg 419 #endif /* NO_LBA_CHECK */ 420 #endif /* NO_CHS */ 421 422 /* 423 * Save sector number (passed in %ebp) into lba parameter block, 424 * read the sector and leap into it. 425 */ 426 boot_lba: 427 movl %ebp, lba_sector /* save sector number */ 428 movw $lba_info, %si 429 movb $0x42, %ah 430 pop %dx /* recover drive # */ 431 do_read: 432 push %dx /* save drive */ 433 int $0x13 434 435 set_err(ERR_READ) 436 jc err_msg 437 438 /* 439 * Check signature for valid bootcode 440 */ 441 movb BOOTADDR, %al /* first byte non-zero */ 442 test %al, %al 443 jz 1f 444 movw BOOTADDR + MBR_MAGIC_OFFSET, %ax 445 1: cmp $MBR_MAGIC, %ax 446 set_err(ERR_NOOS) 447 jnz err_msg 448 449 /* We pass the sector number through to the next stage boot. 450 * It doesn't have to use it (indeed no other mbr code will generate) it, 451 * but it does let us have a NetBSD pbr that can identify where it was 452 * read from! This lets us use this code to select between two 453 * NetBSD system on the same physical driver. 454 * (If we've read the mbr of a different disk, it gets a random number 455 * - but it wasn't expecting anything...) 456 */ 457 movl %ebp, %esi 458 pop %dx /* recover drive # */ 459 jmp start - LOADADDR + BOOTADDR 460 461 462 #ifndef NO_CHS 463 /* 464 * Sector below CHS limit 465 * Do a cylinder-head-sector read instead. 466 */ 467 read_chs: 468 pop %dx /* recover drive # */ 469 movb 1(%si), %dh /* head */ 470 movw 2(%si), %cx /* ch=cyl, cl=sect */ 471 movw $BOOTADDR, %bx /* es:bx is buffer */ 472 movw $0x201, %ax /* command 2, 1 sector */ 473 jmp do_read 474 #endif 475 476 /* 477 * Control block for int-13 LBA read. 478 * We need a xx, 00, 01, 00 somewhere to load chs for sector zero, 479 * by a complete fluke there is one here! 480 */ 481 chs_zero: 482 lba_info: 483 .word 0x10 /* control block length */ 484 .word 1 /* sector count */ 485 .word BOOTADDR /* offset in segment */ 486 .word 0 /* segment */ 487 lba_sector: 488 .long 0x0000 /* sector # goes here... */ 489 .long 0x0000 490 491 errtxt: .ascii "Error " /* runs into crlf if errcod set */ 492 errcod: .byte 0 493 crlf: .asciz "\r\n" 494 495 #ifndef NO_BANNER 496 #ifdef BOOTSEL 497 banner: .asciz "Fn: diskn" 498 #else 499 banner: .asciz "NetBSD MBR boot" 500 #endif 501 #endif 502 503 #ifdef BOOTSEL 504 prefix: .asciz "0: " 505 #endif 506 507 #ifndef TERSE_ERROR 508 ERR_INVPART: .asciz "No active partition" 509 ERR_READ: .asciz "Disk read error" 510 ERR_NOOS: .asciz "No operating system" 511 #ifndef NO_LBA_CHECK 512 ERR_NO_LBA: .asciz "Invalid CHS read" 513 #endif 514 #ifdef BOOTSEL 515 ERR_KEY: .asciz "bad key" 516 #endif 517 #endif 518 519 /* 520 * I hate #including source files, but the stuff below has to be at 521 * the correct absolute address. 522 * Clearly this could be done with a linker script. 523 */ 524 525 message_crlf: 526 call message 527 movw $crlf, %si 528 #include <message.S> 529 #if 0 530 #include <dump_eax.S> 531 #endif 532 533 /* 534 * Stuff from here on is overwritten by fdisk - the offset must not change... 535 * 536 * Get amount of space to makefile can report it. 537 * (Unfortunately I can't seem to get the value reported when it is -ve) 538 */ 539 mbr_space = defkey - . 540 . = start + MBR_BS_OFFSET 541 /* 542 * Default action, as a keyvalue we'd normally read from the BIOS. 543 */ 544 defkey: 545 .byte SCAN_ENTER /* ps/2 code */ 546 #ifndef BOOTSEL_FLAGS 547 #define BOOTSEL_FLAGS 0 548 #endif 549 flags: .byte MBR_BS_NEWMBR | BOOTSEL_FLAGS 550 /* 551 * Timeout value. ~65536 ticks per hour, which is ~18.2 times per second. 552 * 0xffff means never timeout. 553 */ 554 timeout: 555 .word 182 /* default to 10 seconds */ 556 /* 557 * mbr_bootsel 558 */ 559 nametab: 560 .fill MBR_PART_COUNT * (MBR_BS_PARTNAMESIZE + 1), 0x01, 0x00 561 562 /* space for mbr_dsn */ 563 . = start + MBR_DSN_OFFSET 564 .long 0 565 566 /* mbr_bootsel_magic */ 567 . = start + MBR_BS_MAGIC_OFFSET 568 .word MBR_BS_MAGIC 569 570 /* 571 * MBR partition table 572 */ 573 . = start + MBR_PART_OFFSET 574 parttab: 575 .fill 0x40, 0x01, 0x00 576 577 . = start + MBR_MAGIC_OFFSET 578 .word MBR_MAGIC 579 580 /* zeroed data space */ 581 bss_off = 0 582 bss_start = . 583 #define BSS(name, size) name = bss_start + bss_off; bss_off = bss_off + size 584 BSS(ptn_list, 256 * 4) /* long[]: boot sector numbers */ 585 BSS(bss_end, 0) 586
Indexes created Sun Oct 12 09:09:55 GMT 2025