mbr.S revision 1.24
1 /* $NetBSD: mbr.S,v 1.24 2023/08/01 21:26:27 andvar 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 #else 67 #undef COM_BAUD 68 #endif 69 70 #ifdef BOOTSEL 71 #define TERSE_ERROR 1 72 #endif 73 74 #include <machine/asm.h> 75 #include <sys/bootblock.h> 76 77 #define BOOTADDR 0x7c00 /* where we get loaded to */ 78 79 #define TABENTRYSIZE (MBR_BS_PARTNAMESIZE + 1) 80 #define NAMETABSIZE (MBR_PART_COUNT * TABENTRYSIZE) 81 82 #ifdef COM_PORT 83 /* ASCII values for the keys */ 84 #define KEY_ACTIVE '\r' 85 #define KEY_DISK1 'a' 86 #define KEY_PTN1 '1' 87 #else 88 /* Scan values for the various keys we use, as returned by the BIOS */ 89 #define SCAN_ENTER 0x1c 90 #define SCAN_F1 0x3b 91 #define SCAN_1 0x2 92 93 #define KEY_ACTIVE SCAN_ENTER 94 #define KEY_DISK1 SCAN_F1 95 #define KEY_PTN1 SCAN_1 96 #endif 97 98 /* 99 * Minimum and maximum drive number that is considered to be valid. 100 */ 101 #define MINDRV 0x80 102 #define MAXDRV 0x8f 103 104 #ifdef TERSE_ERROR 105 /* 106 * Error codes. Done this way to save space. 107 */ 108 #define ERR_INVPART '1' /* Invalid partition table */ 109 #define ERR_READ '2' /* Read error */ 110 #define ERR_NOOS '3' /* Magic no. check failed for part. */ 111 #define ERR_KEY '?' /* unknown key press */ 112 #define ERR_NO_LBA 'L' /* sector above chs limit */ 113 114 #define set_err(err) movb $err, %al 115 116 #else 117 #define set_err(err) mov $err, %ax 118 #endif 119 120 .text 121 .code16 122 /* 123 * Move ourselves out of the way first. 124 * (to the address we are linked at) 125 * and zero our bss 126 */ 127 ENTRY(start) 128 xor %ax, %ax 129 mov %ax, %ss 130 movw $BOOTADDR, %sp 131 mov %ax, %es 132 mov %ax, %ds 133 movw $mbr, %di 134 mov $BOOTADDR + (mbr - start), %si 135 push %ax /* zero for %cs of lret */ 136 push %di 137 movw $(bss_start - mbr), %cx 138 rep 139 movsb /* relocate code */ 140 mov $(bss_end - bss_start + 511)/512, %ch 141 rep 142 stosw /* zero bss */ 143 lret /* Ensures %cs == 0 */ 144 145 /* 146 * Sanity check the drive number passed by the BIOS. Some BIOSs may not 147 * do this and pass garbage. 148 */ 149 mbr: 150 cmpb $MAXDRV, %dl /* relies on MINDRV being 0x80 */ 151 jle 1f 152 movb $MINDRV, %dl /* garbage in, boot disk 0 */ 153 1: 154 push %dx /* save drive number */ 155 push %dx /* twice - for err_msg loop */ 156 157 #if defined(COM_BAUD) 158 mov $com_args, %si 159 mov $num_com_args, %cl /* %ch is zero from above */ 160 mov COM_PORT_VAL, %dx 161 1: lodsw 162 add %ah, %dl 163 outb %dx 164 loop 1b 165 #endif 166 167 #ifndef NO_BANNER 168 mov $banner, %si 169 call message_crlf 170 #endif 171 172 /* 173 * Walk through the selector (name) table printing used entries. 174 * 175 * Register use: 176 * %ax temp 177 * %bx nametab[] boot selector menu 178 * %ecx base of 'extended' partition 179 * %edx next extended partition 180 * %si message ptr (etc) 181 * %edi sector number of this partition 182 * %bp parttab[] mbr partition table 183 */ 184 bootsel_menu: 185 movw $nametab, %bx 186 #ifdef BOOT_EXTENDED 187 xorl %ecx, %ecx /* base of extended partition */ 188 next_extended: 189 xorl %edx, %edx /* for next extended partition */ 190 #endif 191 lea parttab - nametab(%bx), %bp 192 next_ptn: 193 movb 4(%bp), %al /* partition type */ 194 #ifdef NO_CHS 195 movl 8(%bp), %edi /* partition sector number */ 196 #ifdef BOOT_EXTENDED 197 cmpb $MBR_PTYPE_EXT, %al /* Extended partition */ 198 je 1f 199 cmpb $MBR_PTYPE_EXT_LBA, %al /* Extended LBA partition */ 200 je 1f 201 cmpb $MBR_PTYPE_EXT_LNX, %al /* Linux extended partition */ 202 jne 2f 203 1: movl %edi, %edx /* save next extended ptn */ 204 jmp 4f 205 2: 206 #endif 207 addl lba_sector, %edi /* add in extended ptn base */ 208 #endif 209 test %al, %al /* undefined partition */ 210 je 4f 211 cmpb $0x80, (%bp) /* check for active partition */ 212 jne 3f /* jump if not... */ 213 #define ACTIVE (4 * ((KEY_ACTIVE - KEY_DISK1) & 0xff)) 214 #ifdef NO_CHS 215 movl %edi, ptn_list + ACTIVE /* save location of active ptn */ 216 #else 217 mov %bp, ptn_list + ACTIVE 218 #endif 219 #undef ACTIVE 220 3: 221 #ifdef BOOTSEL 222 cmpb $0, (%bx) /* check for prompt */ 223 jz 4f 224 /* output menu item */ 225 movw $prefix, %si 226 incb (%si) 227 call message /* menu number */ 228 mov (%si), %si /* ':' << 8 | '1' + count */ 229 shl $2, %si /* const + count * 4 */ 230 #define CONST (4 * ((':' << 8) + '1' - ((KEY_PTN1 - KEY_DISK1) & 0xff))) 231 #ifdef NO_CHS 232 movl %edi, ptn_list - CONST(%si) /* sector to read */ 233 #else 234 mov %bp, ptn_list - CONST(%si) /* partition info */ 235 #endif 236 #undef CONST 237 mov %bx, %si 238 call message_crlf /* prompt */ 239 #endif 240 4: 241 add $0x10, %bp 242 add $TABENTRYSIZE, %bx 243 cmpb $(nametab - start - 0x100) + 4 * TABENTRYSIZE, %bl 244 jne next_ptn 245 246 #ifdef BOOT_EXTENDED 247 /* 248 * Now check extended partition chain 249 */ 250 testl %edx, %edx 251 je wait_key 252 testl %ecx, %ecx 253 jne 1f 254 xchg %ecx, %edx /* save base of ext ptn chain */ 255 1: addl %ecx, %edx /* sector to read */ 256 movl %edx, lba_sector 257 movw $lba_info, %si 258 movb $0x42, %ah 259 pop %dx /* recover drive # */ 260 push %dx /* save drive */ 261 int $0x13 262 movw $BOOTADDR + (nametab - start), %bx 263 jnc next_extended /* abort menu on read fail */ 264 #endif 265 266 /* 267 * The non-bootsel code traverses this code path, it needs the 268 * correct keycode to select the active partition. 269 */ 270 271 #ifndef BOOTSEL 272 mov $(KEY_ACTIVE - KEY_DISK1) & 0xff, %ax 273 #else 274 /* 275 * Get the initial time value for the timeout comparison. It is returned 276 * by int 1a in cx:dx. We do sums modulo 2^16 so it doesn't matter if 277 * the counter wraps (which it does every hour) - so we can safely 278 * ignore 'cx'. 279 * 280 * Loop around checking for a keypress until we have one, or timeout is 281 * reached. 282 */ 283 wait_key: 284 xorb %ah, %ah 285 int $0x1a 286 mov %dx, %di /* start time to di */ 287 3: 288 #ifdef COM_PORT_VAL 289 mov COM_PORT_VAL, %dx 290 push %dx 291 add $5, %dx 292 inb %dx 293 pop %dx 294 test $1, %al 295 jz 1f 296 inb %dx 297 jmp check_key 298 #else 299 movb $1, %ah /* looks to see if a */ 300 int $0x16 /* key has been pressed */ 301 jz 1f 302 get_key: 303 xorb %ah, %ah 304 int $0x16 /* 'read key', code ah, ascii al */ 305 shr $8, %ax /* code in %al, %ah zero */ 306 jmp check_key 307 #endif 308 309 1: xorb %ah, %ah 310 int $0x1a /* current time to cx:dx */ 311 sub %di, %dx 312 cmpw timeout, %dx /* always wait for 1 tick... */ 313 jbe 3b /* 0xffff means never timeout */ 314 def_key: 315 mov defkey, %al /* timedout - we need %ah to still be zero! */ 316 317 /* 318 * We have a keycode, see what it means. 319 * If we don't know we generate error '?' and go ask again 320 */ 321 check_key: 322 /* 323 * F1-F10 -> boot disk 0-9. Check if the requested disk isn't above 324 * the number of disks actually in the system as stored in 0:0475 by 325 * the BIOS. 326 * If we trust loc 475, we needn't check the upper bound on the keystroke 327 * This is always sector 0, so always read using chs. 328 */ 329 subb $KEY_DISK1, %al 330 cmpb 0x0475, %al 331 jae boot_ptn 332 addb $0x80, %al 333 pop %dx /* dump saved drive # */ 334 push %ax /* replace with new */ 335 #ifdef NO_CHS 336 xorl %ebp, %ebp /* read sector number 0 */ 337 jmp boot_lba 338 #else 339 movw $chs_zero, %si /* chs read sector zero info */ 340 jmp read_chs 341 #endif 342 #endif /* BOOTSEL */ 343 344 /* 345 * Boot requested partition. 346 * Use keycode to index the table we generated when we scanned the mbr 347 * while generating the menu. 348 * 349 * We very carfully saved the values in the correct part of the table. 350 */ 351 352 boot_ptn: 353 shl $2, %ax 354 movw %ax, %si 355 #ifdef NO_CHS 356 movl ptn_list(%si), %ebp 357 testl %ebp, %ebp 358 jnz boot_lba 359 #else 360 mov ptn_list(%si), %si 361 test %si, %si 362 jnz boot_si 363 #endif 364 #ifdef BOOTSEL 365 set_err(ERR_KEY) 366 #else 367 set_err(ERR_INVPART) 368 #endif 369 /* jmp err_msg */ 370 371 /* Something went wrong... 372 * Output error code, 373 * reset disk subsystem - needed after read failure, 374 * and wait for user key 375 */ 376 err_msg: 377 #ifdef TERSE_ERROR 378 movb %al, errcod 379 movw $errtxt, %si 380 call message 381 #else 382 push %ax 383 movw $errtxt, %si 384 call message 385 pop %si 386 call message_crlf 387 #endif 388 pop %dx /* drive we errored on */ 389 xor %ax,%ax /* only need %ah = 0 */ 390 int $0x13 /* reset disk subsystem */ 391 #ifdef BOOTSEL 392 pop %dx /* original drive number */ 393 push %dx 394 push %dx 395 #ifdef COM_PORT_VAL 396 jmp wait_key /* Read with timeout (again) */ 397 #else 398 jmp get_key /* Blocking read */ 399 #endif 400 #else 401 int $0x18 /* BIOS might ask for a key */ 402 /* press and retry boot seq. */ 403 1: sti 404 hlt 405 jmp 1b 406 #endif 407 408 #ifndef NO_CHS 409 /* 410 * Active partition pointed to by si. 411 * Read the first sector. 412 * 413 * We can either do a CHS (Cylinder Head Sector) or an LBA (Logical 414 * Block Address) read. Always doing the LBA one 415 * would be nice - unfortunately not all systems support it. 416 * Also some may contain a separate (eg SCSI) bios that doesn't 417 * support it even when the main bios does. 418 * 419 * There is also the additional problem that the CHS values may be wrong 420 * (eg if fdisk was run on a different system that used different BIOS 421 * geometry). We convert the CHS value to a LBA sector number using 422 * the geometry from the BIOS, if the number matches we do a CHS read. 423 */ 424 boot_si: 425 movl 8(%si), %ebp /* get sector # */ 426 427 testb $MBR_BS_READ_LBA, flags 428 jnz boot_lba /* fdisk forced LBA read */ 429 430 pop %dx /* collect saved drive... */ 431 push %dx /* ...number to dl */ 432 movb $8, %ah 433 int $0x13 /* chs info */ 434 435 /* 436 * Validate geometry, if the CHS sector number doesn't match the LBA one 437 * we'll do an LBA read. 438 * calc: (cylinder * number_of_heads + head) * number_of_sectors + sector 439 * and compare against LBA sector number. 440 * Take a slight 'flier' and assume we can just check 16bits (very likely 441 * to be true because the number of sectors per track is 63). 442 */ 443 movw 2(%si), %ax /* cylinder + sector */ 444 push %ax /* save for sector */ 445 shr $6, %al 446 xchgb %al, %ah /* 10 bit cylinder number */ 447 shr $8, %dx /* last head */ 448 inc %dx /* number of heads */ 449 mul %dx 450 mov 1(%si), %dl /* head we want */ 451 add %dx, %ax 452 and $0x3f, %cx /* number of sectors */ 453 mul %cx 454 pop %dx /* recover sector we want */ 455 and $0x3f, %dx 456 add %dx, %ax 457 dec %ax 458 459 cmp %bp, %ax 460 je read_chs 461 462 #ifndef NO_LBA_CHECK 463 /* 464 * Determine whether we have int13-extensions, by calling int 13, function 41. 465 * Check for the magic number returned, and the disk packet capability. 466 */ 467 movw $0x55aa, %bx 468 movb $0x41, %ah 469 pop %dx 470 push %dx 471 int $0x13 472 set_err(ERR_NO_LBA) 473 jc err_msg /* no int13 extensions */ 474 cmpw $0xaa55, %bx 475 jnz err_msg 476 testb $1, %cl 477 jz err_msg 478 #endif /* NO_LBA_CHECK */ 479 #endif /* NO_CHS */ 480 481 /* 482 * Save sector number (passed in %ebp) into lba parameter block, 483 * read the sector and leap into it. 484 */ 485 boot_lba: 486 movl %ebp, lba_sector /* save sector number */ 487 movw $lba_info, %si 488 movb $0x42, %ah 489 pop %dx /* recover drive # */ 490 do_read: 491 push %dx /* save drive */ 492 int $0x13 493 494 set_err(ERR_READ) 495 jc err_msg 496 497 /* 498 * Check signature for valid bootcode 499 */ 500 movb BOOTADDR, %al /* first byte non-zero */ 501 test %al, %al 502 jz 1f 503 movw BOOTADDR + MBR_MAGIC_OFFSET, %ax 504 1: cmp $MBR_MAGIC, %ax 505 set_err(ERR_NOOS) 506 jnz err_msg 507 508 /* We pass the sector number through to the next stage boot. 509 * It doesn't have to use it (indeed no other mbr code will generate) it, 510 * but it does let us have a NetBSD pbr that can identify where it was 511 * read from! This lets us use this code to select between two 512 * NetBSD system on the same physical driver. 513 * (If we've read the mbr of a different disk, it gets a random number 514 * - but it wasn't expecting anything...) 515 */ 516 movl %ebp, %esi 517 pop %dx /* recover drive # */ 518 jmp BOOTADDR 519 520 521 #ifndef NO_CHS 522 /* 523 * Sector below CHS limit 524 * Do a cylinder-head-sector read instead. 525 */ 526 read_chs: 527 pop %dx /* recover drive # */ 528 movb 1(%si), %dh /* head */ 529 movw 2(%si), %cx /* ch=cyl, cl=sect */ 530 movw $BOOTADDR, %bx /* es:bx is buffer */ 531 movw $0x201, %ax /* command 2, 1 sector */ 532 jmp do_read 533 #endif 534 535 /* 536 * Control block for int-13 LBA read. 537 * We need a xx, 00, 01, 00 somewhere to load chs for sector zero, 538 * by a complete fluke there is one here! 539 */ 540 chs_zero: 541 lba_info: 542 .word 0x10 /* control block length */ 543 .word 1 /* sector count */ 544 .word BOOTADDR /* offset in segment */ 545 .word 0 /* segment */ 546 lba_sector: 547 .long 0x0000 /* sector # goes here... */ 548 .long 0x0000 549 550 errtxt: .ascii "Error " /* runs into crlf if errcod set */ 551 errcod: .byte 0 552 crlf: .asciz "\r\n" 553 554 #ifndef NO_BANNER 555 #ifdef BOOTSEL 556 #ifdef COM_PORT_VAL 557 banner: .asciz "a: disk" 558 #else 559 banner: .asciz "Fn: diskn" 560 #endif 561 #else 562 banner: .asciz "NetBSD MBR boot" 563 #endif 564 #endif 565 566 #ifdef BOOTSEL 567 prefix: .asciz "0: " 568 #endif 569 570 #ifndef TERSE_ERROR 571 ERR_INVPART: .asciz "No active partition" 572 ERR_READ: .asciz "Disk read error" 573 ERR_NOOS: .asciz "No operating system" 574 #ifndef NO_LBA_CHECK 575 ERR_NO_LBA: .asciz "Invalid CHS read" 576 #endif 577 #ifdef BOOTSEL 578 ERR_KEY: .asciz "bad key" 579 #endif 580 #endif 581 582 #if defined(COM_BAUD) 583 #define COM_DIVISOR (((COM_FREQ / COM_BAUD) + 8) / 16) 584 com_args: 585 .byte 0x80 /* divisor latch enable */ 586 .byte +3 /* io_port + 3 */ 587 .byte COM_DIVISOR & 0xff 588 .byte -3 /* io_port */ 589 .byte COM_DIVISOR >> 8 /* high baud */ 590 .byte +1 /* io_port + 1 */ 591 .byte 0x03 /* 8 bit no parity */ 592 .byte +2 /* io_port + 3 */ 593 num_com_args = (. - com_args)/2 594 #endif 595 596 /* 597 * I hate #including source files, but the stuff below has to be at 598 * the correct absolute address. 599 * Clearly this could be done with a linker script. 600 */ 601 602 message_crlf: 603 call message 604 movw $crlf, %si 605 #include <message.S> 606 #if 0 607 #include <dump_eax.S> 608 #endif 609 610 /* 611 * Stuff from here on is overwritten by fdisk - the offset must not change... 612 * 613 * Get amount of space to makefile can report it. 614 * (Unfortunately I can't seem to get the value reported when it is -ve) 615 */ 616 mbr_space = defkey - . 617 . = start + MBR_BS_OFFSET 618 /* 619 * Default action, as a keyvalue we'd normally read from the BIOS. 620 */ 621 defkey: 622 .byte KEY_ACTIVE /* ps/2 code */ 623 #ifndef BOOTSEL_FLAGS 624 #define BOOTSEL_FLAGS 0 625 #endif 626 flags: .byte MBR_BS_NEWMBR | BOOTSEL_FLAGS 627 /* 628 * Timeout value. ~65536 ticks per hour, which is ~18.2 times per second. 629 * 0xffff means never timeout. 630 */ 631 timeout: 632 .word 182 /* default to 10 seconds */ 633 /* 634 * mbr_bootsel 635 */ 636 nametab: 637 .fill MBR_PART_COUNT * (MBR_BS_PARTNAMESIZE + 1), 0x01, 0x00 638 639 /* space for mbr_dsn */ 640 . = start + MBR_DSN_OFFSET 641 .long 0 642 643 /* mbr_bootsel_magic */ 644 . = start + MBR_BS_MAGIC_OFFSET 645 .word MBR_BS_MAGIC 646 647 /* 648 * MBR partition table 649 */ 650 . = start + MBR_PART_OFFSET 651 parttab: 652 .fill 0x40, 0x01, 0x00 653 654 . = start + MBR_MAGIC_OFFSET 655 .word MBR_MAGIC 656 657 /* zeroed data space */ 658 bss_off = 0 659 bss_start = . 660 #define BSS(name, size) name = bss_start + bss_off; bss_off = bss_off + size 661 BSS(ptn_list, 256 * 4) /* long[]: boot sector numbers */ 662 BSS(dump_eax_buff, 16) 663 BSS(bss_end, 0) 664
Indexes created Sun Oct 12 09:09:55 GMT 2025