mbr.S revision 1.17
1 /* $NetBSD: mbr.S,v 1.17 2008/01/25 21:40:58 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 * COM_PORT - do serial io to specified port number 48 * 0..3 => bios port, otherwise actual io_addr 49 * COM_BAUD - initialise serial port baud rate 50 * 51 * TERSE_ERROR - terse error messages 52 * NO_CHS - all reads are LBA 53 * NO_LBA_CHECK - no check if bios supports LBA reads 54 * NO_BANNER - do not output title line 'banner' 55 */ 56 57 #ifdef BOOT_EXTENDED 58 #define NO_CHS 1 59 #define BOOTSEL 1 60 #endif 61 62 #ifdef COM_PORT 63 #if COM_PORT < 4 64 /* The first 4 items in the 40:xx segment are the serial port base addresses */ 65 #define COM_PORT_VAL (0x400 + (COM_PORT * 2)) 66 #else 67 #define COM_PORT_VAL $COM_PORT 68 #endif 69 70 #if !defined(COM_FREQ) 71 #define COM_FREQ 1843200 72 #endif 73 #endif 74 75 #ifdef BOOTSEL 76 #define TERSE_ERROR 1 77 #endif 78 79 #include <machine/asm.h> 80 #include <sys/bootblock.h> 81 82 #define BOOTADDR 0x7c00 83 #define LOADADDR 0x0600 /* address were are linked to */ 84 85 #define TABENTRYSIZE (MBR_BS_PARTNAMESIZE + 1) 86 #define NAMETABSIZE (MBR_PART_COUNT * TABENTRYSIZE) 87 88 #ifdef COM_PORT 89 /* ASCII values for the keys */ 90 #define KEY_ACTIVE '\m' 91 #define KEY_DISK1 'a' 92 #define KEY_PTN1 '1' 93 #else 94 /* Scan values for the various keys we use, as returned by the BIOS */ 95 #define SCAN_ENTER 0x1c 96 #define SCAN_F1 0x3b 97 #define SCAN_1 0x2 98 99 #define KEY_ACTIVE SCAN_ENTER 100 #define KEY_DISK1 SCAN_F1 101 #define KEY_PTN1 SCAN_1 102 #endif 103 104 /* 105 * Minimum and maximum drive number that is considered to be valid. 106 */ 107 #define MINDRV 0x80 108 #define MAXDRV 0x8f 109 110 #ifdef TERSE_ERROR 111 /* 112 * Error codes. Done this way to save space. 113 */ 114 #define ERR_INVPART '1' /* Invalid partition table */ 115 #define ERR_READ '2' /* Read error */ 116 #define ERR_NOOS '3' /* Magic no. check failed for part. */ 117 #define ERR_KEY '?' /* unknown key press */ 118 #define ERR_NO_LBA 'L' /* sector above chs limit */ 119 120 #define set_err(err) movb $err, %al 121 122 #else 123 #define set_err(err) mov $err, %ax 124 #endif 125 126 .text 127 .code16 128 /* 129 * Move ourselves out of the way first. 130 * (to the address we are linked at - 0x600) 131 * and zero our bss 132 */ 133 ENTRY(start) 134 xor %ax, %ax 135 mov %ax, %ss 136 movw $BOOTADDR, %sp 137 mov %ax, %es 138 mov %ax, %ds 139 movw $mbr, %di 140 mov $mbr - LOADADDR + BOOTADDR, %si 141 push %ax /* zero for %cs of lret */ 142 push %di 143 movw $(bss_start - mbr), %cx 144 rep 145 movsb /* relocate code */ 146 mov $(bss_end - bss_start + 1)/2, %cx 147 rep 148 stosw /* zero bss */ 149 lret /* Ensures %cs == 0 */ 150 151 /* 152 * Sanity check the drive number passed by the BIOS. Some BIOSs may not 153 * do this and pass garbage. 154 */ 155 mbr: 156 cmpb $MAXDRV, %dl /* relies on MINDRV being 0x80 */ 157 jle 1f 158 movb $MINDRV, %dl /* garbage in, boot disk 0 */ 159 1: 160 push %dx /* save drive number */ 161 push %dx /* twice - for err_msg loop */ 162 163 #if defined(COM_PORT) && defined(COM_BAUD) 164 mov $com_args, %si 165 mov $num_com_args, %cl /* %ch is zero from above */ 166 mov COM_PORT_VAL, %dx 167 1: lodsw 168 add %ah, %dl 169 outb %dx 170 loop 1b 171 #endif 172 173 #ifndef NO_BANNER 174 mov $banner, %si 175 call message_crlf 176 #endif 177 178 /* 179 * Walk through the selector (name) table printing used entries. 180 * 181 * Register use: 182 * %ax temp 183 * %bx nametab[] boot seletor menu 184 * %ecx base of 'extended' partition 185 * %edx next extended partition 186 * %si message ptr (etc) 187 * %edi sector number of this partition 188 * %bp parttab[] mbr partition table 189 */ 190 bootsel_menu: 191 movw $nametab, %bx 192 #ifdef BOOT_EXTENDED 193 xorl %ecx, %ecx /* base of extended partition */ 194 next_extended: 195 xorl %edx, %edx /* for next extended partition */ 196 #endif 197 lea parttab - nametab(%bx), %bp 198 next_ptn: 199 movb 4(%bp), %al /* partition type */ 200 #ifdef NO_CHS 201 movl 8(%bp), %edi /* partition sector number */ 202 #ifdef BOOT_EXTENDED 203 cmpb $MBR_PTYPE_EXT, %al /* Extended partition */ 204 je 1f 205 cmpb $MBR_PTYPE_EXT_LBA, %al /* Extended LBA partition */ 206 je 1f 207 cmpb $MBR_PTYPE_EXT_LNX, %al /* Linux extended partition */ 208 jne 2f 209 1: movl %edi, %edx /* save next extended ptn */ 210 jmp 4f 211 2: 212 #endif 213 addl lba_sector, %edi /* add in extended ptn base */ 214 #endif 215 test %al, %al /* undefined partition */ 216 je 4f 217 cmpb $0x80, (%bp) /* check for active partition */ 218 jne 3f /* jump if not... */ 219 #define ACTIVE (4 * ((KEY_ACTIVE - KEY_DISK1) & 0xff)) 220 #ifdef NO_CHS 221 movl %edi, ptn_list + ACTIVE /* save location of active ptn */ 222 #else 223 mov %bp, ptn_list + ACTIVE 224 #endif 225 #undef ENTER 226 3: 227 #ifdef BOOTSEL 228 cmpb $0, (%bx) /* check for prompt */ 229 jz 4f 230 /* output menu item */ 231 movw $prefix, %si 232 incb (%si) 233 call message /* menu number */ 234 mov (%si), %si /* ':' << 8 | '1' + count */ 235 shl $2, %si /* const + count * 4 */ 236 #define CONST (4 * ((':' << 8) + '1' - ((KEY_PTN1 - KEY_DISK1) & 0xff))) 237 #ifdef NO_CHS 238 movl %edi, ptn_list - CONST(%si) /* sector to read */ 239 #else 240 mov %bp, ptn_list - CONST(%si) /* partition info */ 241 #endif 242 #undef CONST 243 mov %bx, %si 244 call message_crlf /* prompt */ 245 #endif 246 4: 247 add $0x10, %bp 248 add $TABENTRYSIZE, %bx 249 cmpb $(nametab - start - 0x100) + 4 * TABENTRYSIZE, %bl 250 jne next_ptn 251 252 #ifdef BOOT_EXTENDED 253 /* 254 * Now check extended partition chain 255 */ 256 testl %edx, %edx 257 je wait_key 258 testl %ecx, %ecx 259 jne 1f 260 xchg %ecx, %edx /* save base of ext ptn chain */ 261 1: addl %ecx, %edx /* sector to read */ 262 movl %edx, lba_sector 263 movw $lba_info, %si 264 movb $0x42, %ah 265 pop %dx /* recover drive # */ 266 push %dx /* save drive */ 267 int $0x13 268 movw $nametab - LOADADDR + BOOTADDR, %bx 269 jnc next_extended /* abort menu on read fail */ 270 #endif 271 272 /* 273 * The non-bootsel code traverses this code path, it needs the 274 * correct keycode to select the active partition. 275 */ 276 277 #ifndef BOOTSEL 278 mov $(KEY_ACTIVE - KEY_DISK1) & 0xff, %ax 279 #else 280 /* 281 * Get the initial time value for the timeout comparison. It is returned 282 * by int 1a in cx:dx. We do sums modulo 2^16 so it doesn't matter if 283 * the counter wraps (which it does every hour) - so we can safely 284 * ignore 'cx'. 285 * 286 * Loop around checking for a keypress until we have one, or timeout is 287 * reached. 288 */ 289 wait_key: 290 xorb %ah, %ah 291 int $0x1a 292 mov %dx, %di /* start time to di */ 293 3: 294 #ifdef COM_PORT 295 mov COM_PORT_VAL, %dx 296 push %dx 297 add $5, %dx 298 inb %dx 299 pop %dx 300 test $1, %al 301 jz 1f 302 inb %dx 303 jmp check_key 304 #else 305 movb $1, %ah /* looks to see if a */ 306 int $0x16 /* key has been pressed */ 307 jz 1f 308 get_key: 309 xorb %ah, %ah 310 int $0x16 /* 'read key', code ah, ascii al */ 311 shr $8, %ax /* code in %al, %ah zero */ 312 jmp check_key 313 #endif 314 315 1: xorb %ah, %ah 316 int $0x1a /* current time to cx:dx */ 317 sub %di, %dx 318 cmpw timeout, %dx /* always wait for 1 tick... */ 319 jbe 3b /* 0xffff means never timeout */ 320 def_key: 321 mov defkey - 1, %ax /* timedout - get default key to %ah */ 322 323 /* 324 * We have a keycode, see what it means. 325 * If we don't know we generate error '?' and go ask again 326 */ 327 check_key: 328 /* 329 * F1-F10 -> boot disk 0-9. Check if the requested disk isn't above 330 * the number of disks actually in the system as stored in 0:0475 by 331 * the BIOS. 332 * If we trust loc 475, we needn't check the upper bound on the keystroke 333 * This is always sector 0, so always read using chs. 334 */ 335 subb $KEY_DISK1, %al 336 cmpb 0x0475, %al 337 jae boot_ptn 338 addb $0x80, %al 339 pop %dx /* dump saved drive # */ 340 push %ax /* replace with new */ 341 #ifdef NO_CHS 342 xorl %ebp, %ebp /* read sector number 0 */ 343 jmp boot_lba 344 #else 345 movw $chs_zero, %si /* chs read sector zero info */ 346 jmp read_chs 347 #endif 348 #endif /* BOOTSEL */ 349 350 /* 351 * Boot requested partition. 352 * Use keycode to index the table we generated when we scanned the mbr 353 * while generating the menu. 354 * 355 * We very carfully saved the values in the correct part of the table. 356 */ 357 358 boot_ptn: 359 shl $2, %ax 360 movw %ax, %si 361 #ifdef NO_CHS 362 movl ptn_list(%si), %ebp 363 testl %ebp, %ebp 364 jnz boot_lba 365 #else 366 mov ptn_list(%si), %si 367 test %si, %si 368 jnz boot_si 369 #endif 370 #ifdef BOOTSEL 371 set_err(ERR_KEY) 372 #else 373 set_err(ERR_INVPART) 374 #endif 375 /* jmp err_msg */ 376 377 /* Something went wrong... 378 * Output error code, 379 * reset disk subsystem - needed after read failure, 380 * and wait for user key 381 */ 382 err_msg: 383 #ifdef TERSE_ERROR 384 movb %al, errcod 385 movw $errtxt, %si 386 call message 387 #else 388 push %ax 389 movw $errtxt, %si 390 call message 391 pop %si 392 call message_crlf 393 #endif 394 pop %dx /* drive we errored on */ 395 xor %ax,%ax /* only need %ah = 0 */ 396 int $0x13 /* reset disk subsystem */ 397 #ifdef BOOTSEL 398 pop %dx /* original drive number */ 399 push %dx 400 push %dx 401 #ifdef COM_PORT 402 jmp wait_key /* Read with timeout (again) */ 403 #else 404 jmp get_key /* Blocking read */ 405 #endif 406 #else 407 int $0x18 /* BIOS might ask for a key */ 408 /* press and retry boot seq. */ 409 1: sti 410 hlt 411 jmp 1b 412 #endif 413 414 #ifndef NO_CHS 415 /* 416 * Active partition pointed to by si. 417 * Read the first sector. 418 * 419 * We can either do a CHS (Cylinder Head Sector) or an LBA (Logical 420 * Block Address) read. Always doing the LBA one 421 * would be nice - unfortunately not all systems support it. 422 * Also some may contain a separate (eg SCSI) bios that doesn't 423 * support it even when the main bios does. 424 * 425 * There is also the additional problem that the CHS values may be wrong 426 * (eg if fdisk was run on a different system that used different BIOS 427 * geometry). We convert the CHS value to a LBA sector number using 428 * the geometry from the BIOS, if the number matches we do a CHS read. 429 */ 430 boot_si: 431 movl 8(%si), %ebp /* get sector # */ 432 433 testb $MBR_BS_READ_LBA, flags 434 jnz boot_lba /* fdisk forced LBA read */ 435 436 pop %dx /* collect saved drive... */ 437 push %dx /* ...number to dl */ 438 movb $8, %ah 439 int $0x13 /* chs info */ 440 441 /* 442 * Validate geometry, if the CHS sector number doesn't match the LBA one 443 * we'll do an LBA read. 444 * calc: (cylinder * number_of_heads + head) * number_of_sectors + sector 445 * and compare against LBA sector number. 446 * Take a slight 'flier' and assume we can just check 16bits (very likely 447 * to be true because the number of sectors per track is 63). 448 */ 449 movw 2(%si), %ax /* cylinder + sector */ 450 push %ax /* save for sector */ 451 shr $6, %al 452 xchgb %al, %ah /* 10 bit cylinder number */ 453 shr $8, %dx /* last head */ 454 inc %dx /* number of heads */ 455 mul %dx 456 mov 1(%si), %dl /* head we want */ 457 add %dx, %ax 458 and $0x3f, %cx /* number of sectors */ 459 mul %cx 460 pop %dx /* recover sector we want */ 461 and $0x3f, %dx 462 add %dx, %ax 463 dec %ax 464 465 cmp %bp, %ax 466 je read_chs 467 468 #ifndef NO_LBA_CHECK 469 /* 470 * Determine whether we have int13-extensions, by calling int 13, function 41. 471 * Check for the magic number returned, and the disk packet capability. 472 */ 473 movw $0x55aa, %bx 474 movb $0x41, %ah 475 pop %dx 476 push %dx 477 int $0x13 478 set_err(ERR_NO_LBA) 479 jc err_msg /* no int13 extensions */ 480 cmpw $0xaa55, %bx 481 jnz err_msg 482 testb $1, %cl 483 jz err_msg 484 #endif /* NO_LBA_CHECK */ 485 #endif /* NO_CHS */ 486 487 /* 488 * Save sector number (passed in %ebp) into lba parameter block, 489 * read the sector and leap into it. 490 */ 491 boot_lba: 492 movl %ebp, lba_sector /* save sector number */ 493 movw $lba_info, %si 494 movb $0x42, %ah 495 pop %dx /* recover drive # */ 496 do_read: 497 push %dx /* save drive */ 498 int $0x13 499 500 set_err(ERR_READ) 501 jc err_msg 502 503 /* 504 * Check signature for valid bootcode 505 */ 506 movb BOOTADDR, %al /* first byte non-zero */ 507 test %al, %al 508 jz 1f 509 movw BOOTADDR + MBR_MAGIC_OFFSET, %ax 510 1: cmp $MBR_MAGIC, %ax 511 set_err(ERR_NOOS) 512 jnz err_msg 513 514 /* We pass the sector number through to the next stage boot. 515 * It doesn't have to use it (indeed no other mbr code will generate) it, 516 * but it does let us have a NetBSD pbr that can identify where it was 517 * read from! This lets us use this code to select between two 518 * NetBSD system on the same physical driver. 519 * (If we've read the mbr of a different disk, it gets a random number 520 * - but it wasn't expecting anything...) 521 */ 522 movl %ebp, %esi 523 pop %dx /* recover drive # */ 524 jmp start - LOADADDR + BOOTADDR 525 526 527 #ifndef NO_CHS 528 /* 529 * Sector below CHS limit 530 * Do a cylinder-head-sector read instead. 531 */ 532 read_chs: 533 pop %dx /* recover drive # */ 534 movb 1(%si), %dh /* head */ 535 movw 2(%si), %cx /* ch=cyl, cl=sect */ 536 movw $BOOTADDR, %bx /* es:bx is buffer */ 537 movw $0x201, %ax /* command 2, 1 sector */ 538 jmp do_read 539 #endif 540 541 /* 542 * Control block for int-13 LBA read. 543 * We need a xx, 00, 01, 00 somewhere to load chs for sector zero, 544 * by a complete fluke there is one here! 545 */ 546 chs_zero: 547 lba_info: 548 .word 0x10 /* control block length */ 549 .word 1 /* sector count */ 550 .word BOOTADDR /* offset in segment */ 551 .word 0 /* segment */ 552 lba_sector: 553 .long 0x0000 /* sector # goes here... */ 554 .long 0x0000 555 556 errtxt: .ascii "Error " /* runs into crlf if errcod set */ 557 errcod: .byte 0 558 crlf: .asciz "\r\n" 559 560 #ifndef NO_BANNER 561 #ifdef BOOTSEL 562 #ifdef COM_PORT 563 banner: .asciz "a: disk" 564 #else 565 banner: .asciz "Fn: diskn" 566 #endif 567 #else 568 banner: .asciz "NetBSD MBR boot" 569 #endif 570 #endif 571 572 #ifdef BOOTSEL 573 prefix: .asciz "0: " 574 #endif 575 576 #ifndef TERSE_ERROR 577 ERR_INVPART: .asciz "No active partition" 578 ERR_READ: .asciz "Disk read error" 579 ERR_NOOS: .asciz "No operating system" 580 #ifndef NO_LBA_CHECK 581 ERR_NO_LBA: .asciz "Invalid CHS read" 582 #endif 583 #ifdef BOOTSEL 584 ERR_KEY: .asciz "bad key" 585 #endif 586 #endif 587 588 #if defined(COM_PORT) && defined(COM_BAUD) 589 #define COM_DIVISOR (((COM_FREQ / COM_BAUD) + 8) / 16) 590 com_args: 591 .byte 0x80 /* divisor latch enable */ 592 .byte +3 /* io_port + 3 */ 593 .byte COM_DIVISOR & 0xff 594 .byte -3 /* io_port */ 595 .byte COM_DIVISOR >> 8 /* high baud */ 596 .byte +1 /* io_port + 1 */ 597 .byte 0x03 /* 8 bit no parity */ 598 .byte +2 /* io_port + 3 */ 599 num_com_args = (. - com_args)/2 600 #endif 601 602 /* 603 * I hate #including source files, but the stuff below has to be at 604 * the correct absolute address. 605 * Clearly this could be done with a linker script. 606 */ 607 608 message_crlf: 609 call message 610 movw $crlf, %si 611 #ifdef COM_PORT 612 message: 613 pusha 614 message_1: 615 lodsb 616 test %al, %al 617 jz 3f 618 mov COM_PORT_VAL, %dx 619 outb %al, %dx 620 add $5, %dl 621 2: inb %dx 622 test $0x40, %al 623 jz 2b 624 jmp message_1 625 3: popa 626 ret 627 #else 628 #include <message.S> 629 #endif 630 631 #if 0 632 #include <dump_eax.S> 633 #endif 634 635 /* 636 * Stuff from here on is overwritten by fdisk - the offset must not change... 637 * 638 * Get amount of space to makefile can report it. 639 * (Unfortunately I can't seem to get the value reported when it is -ve) 640 */ 641 mbr_space = defkey - . 642 . = start + MBR_BS_OFFSET 643 /* 644 * Default action, as a keyvalue we'd normally read from the BIOS. 645 */ 646 defkey: 647 .byte KEY_ACTIVE /* ps/2 code */ 648 #ifndef BOOTSEL_FLAGS 649 #define BOOTSEL_FLAGS 0 650 #endif 651 flags: .byte MBR_BS_NEWMBR | BOOTSEL_FLAGS 652 /* 653 * Timeout value. ~65536 ticks per hour, which is ~18.2 times per second. 654 * 0xffff means never timeout. 655 */ 656 timeout: 657 .word 182 /* default to 10 seconds */ 658 /* 659 * mbr_bootsel 660 */ 661 nametab: 662 .fill MBR_PART_COUNT * (MBR_BS_PARTNAMESIZE + 1), 0x01, 0x00 663 664 /* space for mbr_dsn */ 665 . = start + MBR_DSN_OFFSET 666 .long 0 667 668 /* mbr_bootsel_magic */ 669 . = start + MBR_BS_MAGIC_OFFSET 670 .word MBR_BS_MAGIC 671 672 /* 673 * MBR partition table 674 */ 675 . = start + MBR_PART_OFFSET 676 parttab: 677 .fill 0x40, 0x01, 0x00 678 679 . = start + MBR_MAGIC_OFFSET 680 .word MBR_MAGIC 681 682 /* zeroed data space */ 683 bss_off = 0 684 bss_start = . 685 #define BSS(name, size) name = bss_start + bss_off; bss_off = bss_off + size 686 BSS(ptn_list, 256 * 4) /* long[]: boot sector numbers */ 687 BSS(dump_eax_buff, 16) 688 BSS(bss_end, 0) 689
Indexes created Sun Oct 12 09:09:55 GMT 2025