mbr.S revision 1.20
1 /* $NetBSD: mbr.S,v 1.20 2008/11/28 18:18:16 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 * 20 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 21 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 22 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 23 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 24 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 25 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 26 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 27 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 28 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 29 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 30 * POSSIBILITY OF SUCH DAMAGE. 31 */ 32 33 /* 34 * i386 master boot code 35 */ 36 37 /* Compile options: 38 * BOOTSEL - bootselector code 39 * BOOT_EXTENDED - scan extended partition list (LBA reads) 40 * COM_PORT - do serial io to specified port number 41 * 0..3 => bios port, otherwise actual io_addr 42 * COM_BAUD - initialise serial port baud rate 43 * 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 * NO_BANNER - do not output title line 'banner' 48 */ 49 50 #ifdef BOOT_EXTENDED 51 #define NO_CHS 1 52 #define BOOTSEL 1 53 #endif 54 55 #ifdef COM_PORT 56 #if COM_PORT < 4 57 /* The first 4 items in the 40:xx segment are the serial port base addresses */ 58 #define COM_PORT_VAL (0x400 + (COM_PORT * 2)) 59 #else 60 #define COM_PORT_VAL $COM_PORT 61 #endif 62 63 #if !defined(COM_FREQ) 64 #define COM_FREQ 1843200 65 #endif 66 #endif 67 68 #ifdef BOOTSEL 69 #define TERSE_ERROR 1 70 #endif 71 72 #include <machine/asm.h> 73 #include <sys/bootblock.h> 74 75 #define BOOTADDR 0x7c00 76 #define LOADADDR 0x0600 /* address were are linked to */ 77 78 #define TABENTRYSIZE (MBR_BS_PARTNAMESIZE + 1) 79 #define NAMETABSIZE (MBR_PART_COUNT * TABENTRYSIZE) 80 81 #ifdef COM_PORT 82 /* ASCII values for the keys */ 83 #define KEY_ACTIVE '\r' 84 #define KEY_DISK1 'a' 85 #define KEY_PTN1 '1' 86 #else 87 /* Scan values for the various keys we use, as returned by the BIOS */ 88 #define SCAN_ENTER 0x1c 89 #define SCAN_F1 0x3b 90 #define SCAN_1 0x2 91 92 #define KEY_ACTIVE SCAN_ENTER 93 #define KEY_DISK1 SCAN_F1 94 #define KEY_PTN1 SCAN_1 95 #endif 96 97 /* 98 * Minimum and maximum drive number that is considered to be valid. 99 */ 100 #define MINDRV 0x80 101 #define MAXDRV 0x8f 102 103 #ifdef TERSE_ERROR 104 /* 105 * Error codes. Done this way to save space. 106 */ 107 #define ERR_INVPART '1' /* Invalid partition table */ 108 #define ERR_READ '2' /* Read error */ 109 #define ERR_NOOS '3' /* Magic no. check failed for part. */ 110 #define ERR_KEY '?' /* unknown key press */ 111 #define ERR_NO_LBA 'L' /* sector above chs limit */ 112 113 #define set_err(err) movb $err, %al 114 115 #else 116 #define set_err(err) mov $err, %ax 117 #endif 118 119 .text 120 .code16 121 /* 122 * Move ourselves out of the way first. 123 * (to the address we are linked at - 0x600) 124 * and zero our bss 125 */ 126 ENTRY(start) 127 xor %ax, %ax 128 mov %ax, %ss 129 movw $BOOTADDR, %sp 130 mov %ax, %es 131 mov %ax, %ds 132 movw $mbr, %di 133 mov $mbr - LOADADDR + BOOTADDR, %si 134 push %ax /* zero for %cs of lret */ 135 push %di 136 movw $(bss_start - mbr), %cx 137 rep 138 movsb /* relocate code */ 139 mov $(bss_end - bss_start + 511)/512, %ch 140 rep 141 stosw /* zero bss */ 142 lret /* Ensures %cs == 0 */ 143 144 /* 145 * Sanity check the drive number passed by the BIOS. Some BIOSs may not 146 * do this and pass garbage. 147 */ 148 mbr: 149 cmpb $MAXDRV, %dl /* relies on MINDRV being 0x80 */ 150 jle 1f 151 movb $MINDRV, %dl /* garbage in, boot disk 0 */ 152 1: 153 push %dx /* save drive number */ 154 push %dx /* twice - for err_msg loop */ 155 156 #if defined(COM_PORT) && defined(COM_BAUD) 157 mov $com_args, %si 158 mov $num_com_args, %cl /* %ch is zero from above */ 159 mov COM_PORT_VAL, %dx 160 1: lodsw 161 add %ah, %dl 162 outb %dx 163 loop 1b 164 #endif 165 166 #ifndef NO_BANNER 167 mov $banner, %si 168 call message_crlf 169 #endif 170 171 /* 172 * Walk through the selector (name) table printing used entries. 173 * 174 * Register use: 175 * %ax temp 176 * %bx nametab[] boot seletor menu 177 * %ecx base of 'extended' partition 178 * %edx next extended partition 179 * %si message ptr (etc) 180 * %edi sector number of this partition 181 * %bp parttab[] mbr partition table 182 */ 183 bootsel_menu: 184 movw $nametab, %bx 185 #ifdef BOOT_EXTENDED 186 xorl %ecx, %ecx /* base of extended partition */ 187 next_extended: 188 xorl %edx, %edx /* for next extended partition */ 189 #endif 190 lea parttab - nametab(%bx), %bp 191 next_ptn: 192 movb 4(%bp), %al /* partition type */ 193 #ifdef NO_CHS 194 movl 8(%bp), %edi /* partition sector number */ 195 #ifdef BOOT_EXTENDED 196 cmpb $MBR_PTYPE_EXT, %al /* Extended partition */ 197 je 1f 198 cmpb $MBR_PTYPE_EXT_LBA, %al /* Extended LBA partition */ 199 je 1f 200 cmpb $MBR_PTYPE_EXT_LNX, %al /* Linux extended partition */ 201 jne 2f 202 1: movl %edi, %edx /* save next extended ptn */ 203 jmp 4f 204 2: 205 #endif 206 addl lba_sector, %edi /* add in extended ptn base */ 207 #endif 208 test %al, %al /* undefined partition */ 209 je 4f 210 cmpb $0x80, (%bp) /* check for active partition */ 211 jne 3f /* jump if not... */ 212 #define ACTIVE (4 * ((KEY_ACTIVE - KEY_DISK1) & 0xff)) 213 #ifdef NO_CHS 214 movl %edi, ptn_list + ACTIVE /* save location of active ptn */ 215 #else 216 mov %bp, ptn_list + ACTIVE 217 #endif 218 #undef ACTIVE 219 3: 220 #ifdef BOOTSEL 221 cmpb $0, (%bx) /* check for prompt */ 222 jz 4f 223 /* output menu item */ 224 movw $prefix, %si 225 incb (%si) 226 call message /* menu number */ 227 mov (%si), %si /* ':' << 8 | '1' + count */ 228 shl $2, %si /* const + count * 4 */ 229 #define CONST (4 * ((':' << 8) + '1' - ((KEY_PTN1 - KEY_DISK1) & 0xff))) 230 #ifdef NO_CHS 231 movl %edi, ptn_list - CONST(%si) /* sector to read */ 232 #else 233 mov %bp, ptn_list - CONST(%si) /* partition info */ 234 #endif 235 #undef CONST 236 mov %bx, %si 237 call message_crlf /* prompt */ 238 #endif 239 4: 240 add $0x10, %bp 241 add $TABENTRYSIZE, %bx 242 cmpb $(nametab - start - 0x100) + 4 * TABENTRYSIZE, %bl 243 jne next_ptn 244 245 #ifdef BOOT_EXTENDED 246 /* 247 * Now check extended partition chain 248 */ 249 testl %edx, %edx 250 je wait_key 251 testl %ecx, %ecx 252 jne 1f 253 xchg %ecx, %edx /* save base of ext ptn chain */ 254 1: addl %ecx, %edx /* sector to read */ 255 movl %edx, lba_sector 256 movw $lba_info, %si 257 movb $0x42, %ah 258 pop %dx /* recover drive # */ 259 push %dx /* save drive */ 260 int $0x13 261 movw $nametab - LOADADDR + BOOTADDR, %bx 262 jnc next_extended /* abort menu on read fail */ 263 #endif 264 265 /* 266 * The non-bootsel code traverses this code path, it needs the 267 * correct keycode to select the active partition. 268 */ 269 270 #ifndef BOOTSEL 271 mov $(KEY_ACTIVE - KEY_DISK1) & 0xff, %ax 272 #else 273 /* 274 * Get the initial time value for the timeout comparison. It is returned 275 * by int 1a in cx:dx. We do sums modulo 2^16 so it doesn't matter if 276 * the counter wraps (which it does every hour) - so we can safely 277 * ignore 'cx'. 278 * 279 * Loop around checking for a keypress until we have one, or timeout is 280 * reached. 281 */ 282 wait_key: 283 xorb %ah, %ah 284 int $0x1a 285 mov %dx, %di /* start time to di */ 286 3: 287 #ifdef COM_PORT 288 mov COM_PORT_VAL, %dx 289 push %dx 290 add $5, %dx 291 inb %dx 292 pop %dx 293 test $1, %al 294 jz 1f 295 inb %dx 296 jmp check_key 297 #else 298 movb $1, %ah /* looks to see if a */ 299 int $0x16 /* key has been pressed */ 300 jz 1f 301 get_key: 302 xorb %ah, %ah 303 int $0x16 /* 'read key', code ah, ascii al */ 304 shr $8, %ax /* code in %al, %ah zero */ 305 jmp check_key 306 #endif 307 308 1: xorb %ah, %ah 309 int $0x1a /* current time to cx:dx */ 310 sub %di, %dx 311 cmpw timeout, %dx /* always wait for 1 tick... */ 312 jbe 3b /* 0xffff means never timeout */ 313 def_key: 314 mov defkey, %al /* timedout - we need %ah to still be zero! */ 315 316 /* 317 * We have a keycode, see what it means. 318 * If we don't know we generate error '?' and go ask again 319 */ 320 check_key: 321 /* 322 * F1-F10 -> boot disk 0-9. Check if the requested disk isn't above 323 * the number of disks actually in the system as stored in 0:0475 by 324 * the BIOS. 325 * If we trust loc 475, we needn't check the upper bound on the keystroke 326 * This is always sector 0, so always read using chs. 327 */ 328 subb $KEY_DISK1, %al 329 cmpb 0x0475, %al 330 jae boot_ptn 331 addb $0x80, %al 332 pop %dx /* dump saved drive # */ 333 push %ax /* replace with new */ 334 #ifdef NO_CHS 335 xorl %ebp, %ebp /* read sector number 0 */ 336 jmp boot_lba 337 #else 338 movw $chs_zero, %si /* chs read sector zero info */ 339 jmp read_chs 340 #endif 341 #endif /* BOOTSEL */ 342 343 /* 344 * Boot requested partition. 345 * Use keycode to index the table we generated when we scanned the mbr 346 * while generating the menu. 347 * 348 * We very carfully saved the values in the correct part of the table. 349 */ 350 351 boot_ptn: 352 shl $2, %ax 353 movw %ax, %si 354 #ifdef NO_CHS 355 movl ptn_list(%si), %ebp 356 testl %ebp, %ebp 357 jnz boot_lba 358 #else 359 mov ptn_list(%si), %si 360 test %si, %si 361 jnz boot_si 362 #endif 363 #ifdef BOOTSEL 364 set_err(ERR_KEY) 365 #else 366 set_err(ERR_INVPART) 367 #endif 368 /* jmp err_msg */ 369 370 /* Something went wrong... 371 * Output error code, 372 * reset disk subsystem - needed after read failure, 373 * and wait for user key 374 */ 375 err_msg: 376 #ifdef TERSE_ERROR 377 movb %al, errcod 378 movw $errtxt, %si 379 call message 380 #else 381 push %ax 382 movw $errtxt, %si 383 call message 384 pop %si 385 call message_crlf 386 #endif 387 pop %dx /* drive we errored on */ 388 xor %ax,%ax /* only need %ah = 0 */ 389 int $0x13 /* reset disk subsystem */ 390 #ifdef BOOTSEL 391 pop %dx /* original drive number */ 392 push %dx 393 push %dx 394 #ifdef COM_PORT 395 jmp wait_key /* Read with timeout (again) */ 396 #else 397 jmp get_key /* Blocking read */ 398 #endif 399 #else 400 int $0x18 /* BIOS might ask for a key */ 401 /* press and retry boot seq. */ 402 1: sti 403 hlt 404 jmp 1b 405 #endif 406 407 #ifndef NO_CHS 408 /* 409 * Active partition pointed to by si. 410 * Read the first sector. 411 * 412 * We can either do a CHS (Cylinder Head Sector) or an LBA (Logical 413 * Block Address) read. Always doing the LBA one 414 * would be nice - unfortunately not all systems support it. 415 * Also some may contain a separate (eg SCSI) bios that doesn't 416 * support it even when the main bios does. 417 * 418 * There is also the additional problem that the CHS values may be wrong 419 * (eg if fdisk was run on a different system that used different BIOS 420 * geometry). We convert the CHS value to a LBA sector number using 421 * the geometry from the BIOS, if the number matches we do a CHS read. 422 */ 423 boot_si: 424 movl 8(%si), %ebp /* get sector # */ 425 426 testb $MBR_BS_READ_LBA, flags 427 jnz boot_lba /* fdisk forced LBA read */ 428 429 pop %dx /* collect saved drive... */ 430 push %dx /* ...number to dl */ 431 movb $8, %ah 432 int $0x13 /* chs info */ 433 434 /* 435 * Validate geometry, if the CHS sector number doesn't match the LBA one 436 * we'll do an LBA read. 437 * calc: (cylinder * number_of_heads + head) * number_of_sectors + sector 438 * and compare against LBA sector number. 439 * Take a slight 'flier' and assume we can just check 16bits (very likely 440 * to be true because the number of sectors per track is 63). 441 */ 442 movw 2(%si), %ax /* cylinder + sector */ 443 push %ax /* save for sector */ 444 shr $6, %al 445 xchgb %al, %ah /* 10 bit cylinder number */ 446 shr $8, %dx /* last head */ 447 inc %dx /* number of heads */ 448 mul %dx 449 mov 1(%si), %dl /* head we want */ 450 add %dx, %ax 451 and $0x3f, %cx /* number of sectors */ 452 mul %cx 453 pop %dx /* recover sector we want */ 454 and $0x3f, %dx 455 add %dx, %ax 456 dec %ax 457 458 cmp %bp, %ax 459 je read_chs 460 461 #ifndef NO_LBA_CHECK 462 /* 463 * Determine whether we have int13-extensions, by calling int 13, function 41. 464 * Check for the magic number returned, and the disk packet capability. 465 */ 466 movw $0x55aa, %bx 467 movb $0x41, %ah 468 pop %dx 469 push %dx 470 int $0x13 471 set_err(ERR_NO_LBA) 472 jc err_msg /* no int13 extensions */ 473 cmpw $0xaa55, %bx 474 jnz err_msg 475 testb $1, %cl 476 jz err_msg 477 #endif /* NO_LBA_CHECK */ 478 #endif /* NO_CHS */ 479 480 /* 481 * Save sector number (passed in %ebp) into lba parameter block, 482 * read the sector and leap into it. 483 */ 484 boot_lba: 485 movl %ebp, lba_sector /* save sector number */ 486 movw $lba_info, %si 487 movb $0x42, %ah 488 pop %dx /* recover drive # */ 489 do_read: 490 push %dx /* save drive */ 491 int $0x13 492 493 set_err(ERR_READ) 494 jc err_msg 495 496 /* 497 * Check signature for valid bootcode 498 */ 499 movb BOOTADDR, %al /* first byte non-zero */ 500 test %al, %al 501 jz 1f 502 movw BOOTADDR + MBR_MAGIC_OFFSET, %ax 503 1: cmp $MBR_MAGIC, %ax 504 set_err(ERR_NOOS) 505 jnz err_msg 506 507 /* We pass the sector number through to the next stage boot. 508 * It doesn't have to use it (indeed no other mbr code will generate) it, 509 * but it does let us have a NetBSD pbr that can identify where it was 510 * read from! This lets us use this code to select between two 511 * NetBSD system on the same physical driver. 512 * (If we've read the mbr of a different disk, it gets a random number 513 * - but it wasn't expecting anything...) 514 */ 515 movl %ebp, %esi 516 pop %dx /* recover drive # */ 517 jmp start - LOADADDR + BOOTADDR 518 519 520 #ifndef NO_CHS 521 /* 522 * Sector below CHS limit 523 * Do a cylinder-head-sector read instead. 524 */ 525 read_chs: 526 pop %dx /* recover drive # */ 527 movb 1(%si), %dh /* head */ 528 movw 2(%si), %cx /* ch=cyl, cl=sect */ 529 movw $BOOTADDR, %bx /* es:bx is buffer */ 530 movw $0x201, %ax /* command 2, 1 sector */ 531 jmp do_read 532 #endif 533 534 /* 535 * Control block for int-13 LBA read. 536 * We need a xx, 00, 01, 00 somewhere to load chs for sector zero, 537 * by a complete fluke there is one here! 538 */ 539 chs_zero: 540 lba_info: 541 .word 0x10 /* control block length */ 542 .word 1 /* sector count */ 543 .word BOOTADDR /* offset in segment */ 544 .word 0 /* segment */ 545 lba_sector: 546 .long 0x0000 /* sector # goes here... */ 547 .long 0x0000 548 549 errtxt: .ascii "Error " /* runs into crlf if errcod set */ 550 errcod: .byte 0 551 crlf: .asciz "\r\n" 552 553 #ifndef NO_BANNER 554 #ifdef BOOTSEL 555 #ifdef COM_PORT 556 banner: .asciz "a: disk" 557 #else 558 banner: .asciz "Fn: diskn" 559 #endif 560 #else 561 banner: .asciz "NetBSD MBR boot" 562 #endif 563 #endif 564 565 #ifdef BOOTSEL 566 prefix: .asciz "0: " 567 #endif 568 569 #ifndef TERSE_ERROR 570 ERR_INVPART: .asciz "No active partition" 571 ERR_READ: .asciz "Disk read error" 572 ERR_NOOS: .asciz "No operating system" 573 #ifndef NO_LBA_CHECK 574 ERR_NO_LBA: .asciz "Invalid CHS read" 575 #endif 576 #ifdef BOOTSEL 577 ERR_KEY: .asciz "bad key" 578 #endif 579 #endif 580 581 #if defined(COM_PORT) && defined(COM_BAUD) 582 #define COM_DIVISOR (((COM_FREQ / COM_BAUD) + 8) / 16) 583 com_args: 584 .byte 0x80 /* divisor latch enable */ 585 .byte +3 /* io_port + 3 */ 586 .byte COM_DIVISOR & 0xff 587 .byte -3 /* io_port */ 588 .byte COM_DIVISOR >> 8 /* high baud */ 589 .byte +1 /* io_port + 1 */ 590 .byte 0x03 /* 8 bit no parity */ 591 .byte +2 /* io_port + 3 */ 592 num_com_args = (. - com_args)/2 593 #endif 594 595 /* 596 * I hate #including source files, but the stuff below has to be at 597 * the correct absolute address. 598 * Clearly this could be done with a linker script. 599 */ 600 601 message_crlf: 602 call message 603 movw $crlf, %si 604 #ifdef COM_PORT 605 message: 606 pusha 607 message_1: 608 lodsb 609 test %al, %al 610 jz 3f 611 mov COM_PORT_VAL, %dx 612 outb %al, %dx 613 add $5, %dl 614 2: inb %dx 615 test $0x40, %al 616 jz 2b 617 jmp message_1 618 3: popa 619 ret 620 #else 621 #include <message.S> 622 #endif 623 624 #if 0 625 #include <dump_eax.S> 626 #endif 627 628 /* 629 * Stuff from here on is overwritten by fdisk - the offset must not change... 630 * 631 * Get amount of space to makefile can report it. 632 * (Unfortunately I can't seem to get the value reported when it is -ve) 633 */ 634 mbr_space = defkey - . 635 . = start + MBR_BS_OFFSET 636 /* 637 * Default action, as a keyvalue we'd normally read from the BIOS. 638 */ 639 defkey: 640 .byte KEY_ACTIVE /* ps/2 code */ 641 #ifndef BOOTSEL_FLAGS 642 #define BOOTSEL_FLAGS 0 643 #endif 644 flags: .byte MBR_BS_NEWMBR | BOOTSEL_FLAGS 645 /* 646 * Timeout value. ~65536 ticks per hour, which is ~18.2 times per second. 647 * 0xffff means never timeout. 648 */ 649 timeout: 650 .word 182 /* default to 10 seconds */ 651 /* 652 * mbr_bootsel 653 */ 654 nametab: 655 .fill MBR_PART_COUNT * (MBR_BS_PARTNAMESIZE + 1), 0x01, 0x00 656 657 /* space for mbr_dsn */ 658 . = start + MBR_DSN_OFFSET 659 .long 0 660 661 /* mbr_bootsel_magic */ 662 . = start + MBR_BS_MAGIC_OFFSET 663 .word MBR_BS_MAGIC 664 665 /* 666 * MBR partition table 667 */ 668 . = start + MBR_PART_OFFSET 669 parttab: 670 .fill 0x40, 0x01, 0x00 671 672 . = start + MBR_MAGIC_OFFSET 673 .word MBR_MAGIC 674 675 /* zeroed data space */ 676 bss_off = 0 677 bss_start = . 678 #define BSS(name, size) name = bss_start + bss_off; bss_off = bss_off + size 679 BSS(ptn_list, 256 * 4) /* long[]: boot sector numbers */ 680 BSS(dump_eax_buff, 16) 681 BSS(bss_end, 0) 682
Indexes created Sun Oct 12 05:10:08 GMT 2025