mbr.S revision 1.10
1 /* $NetBSD: mbr.S,v 1.10 2004/09/01 20:31:20 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 */ 51 52 #ifdef BOOT_EXTENDED 53 #define NO_CHS 1 54 #define BOOTSEL 1 55 #endif 56 57 #include <machine/asm.h> 58 #include <sys/bootblock.h> 59 60 #define BOOTADDR 0x7c00 61 #define LOADADDR 0x0600 /* address were are linked to */ 62 63 #define TABENTRYSIZE (MBR_BS_PARTNAMESIZE + 1) 64 #define NAMETABSIZE (MBR_PART_COUNT * TABENTRYSIZE) 65 66 /* Scan values for the various keys we use, as returned by the BIOS */ 67 #define SCAN_ENTER 0x1c 68 #define SCAN_F1 0x3b 69 #define SCAN_1 0x2 70 71 /* 72 * Minimum and maximum drive number that is considered to be valid. 73 */ 74 #define MINDRV 0x80 75 #define MAXDRV 0x8f 76 77 #ifdef TERSE_ERROR 78 /* 79 * Error codes. Done this way to save space. 80 */ 81 #define ERR_INVPART '1' /* Invalid partition table */ 82 #define ERR_READ '2' /* Read error */ 83 #define ERR_NOOS '3' /* Magic no. check failed for part. */ 84 #define ERR_KEY '?' /* unknown key press */ 85 #define ERR_NO_LBA 'L' /* sector above chs limit */ 86 87 #define set_err(err) movb $err, %al 88 89 #else 90 #define set_err(err) mov $err, %ax 91 #endif 92 93 .text 94 .code16 95 /* 96 * Move ourselves out of the way first. 97 * (to the address we are linked at - 0x600) 98 * and zero our bss 99 */ 100 ENTRY(start) 101 xor %ax, %ax 102 mov %ax, %ss 103 movw $BOOTADDR, %sp 104 mov %ax, %es 105 mov %ax, %ds 106 mov %sp, %si 107 movw $start, %di 108 movw $(bss_start - start)/2, %cx 109 rep 110 movsw 111 mov $(bss_end - bss_start + 1)/2, %cx 112 rep 113 stosw 114 ljmp $0, $mbr /* leap into copy of code */ 115 116 /* 117 * Sanity check the drive number passed by the BIOS. Some BIOSs may not 118 * do this and pass garbage. 119 */ 120 mbr: 121 cmpb $MAXDRV, %dl /* relies on MINDRV being 0x80 */ 122 jle 1f 123 movb $MINDRV, %dl /* garbage in, boot disk 0 */ 124 1: 125 push %dx /* save drive number */ 126 push %dx /* twice - for err_msg loop */ 127 128 /* 129 * Walk through the selector (name) table printing used entries. 130 */ 131 bootsel_menu: 132 movw $nametab, %bx 133 #ifdef BOOT_EXTENDED 134 xorl %ecx, %ecx /* base of extended partition */ 135 next_extended: 136 xorl %edx, %edx /* for next extended partition */ 137 #endif 138 lea parttab - nametab(%bx), %bp 139 next_ptn: 140 movb 4(%bp), %al /* partition type */ 141 #ifdef NO_CHS 142 movl 8(%bp), %edi /* partition sector number */ 143 #ifdef BOOT_EXTENDED 144 cmpb $MBR_PTYPE_EXT, %al /* Extended partition */ 145 je 1f 146 cmpb $MBR_PTYPE_EXT_LBA, %al /* Extended LBA partition */ 147 je 1f 148 cmpb $MBR_PTYPE_EXT_LNX, %al /* Linux extended partition */ 149 jne 2f 150 1: movl %edi, %edx /* save next extended ptn */ 151 jmp 4f 152 2: 153 #endif 154 addl lba_sector, %edi /* add in extended ptn base */ 155 #endif 156 test %al, %al /* undefined partition */ 157 je 4f 158 cmpb $0x80, (%bp) /* check for active partition */ 159 jne 3f /* jump if not... */ 160 #define ENTER (4 * ((SCAN_ENTER - SCAN_F1) & 0xff)) 161 #ifdef NO_CHS 162 movl %edi, ptn_list + ENTER /* save location of active ptn */ 163 #else 164 mov %bp, ptn_list + ENTER 165 #endif 166 #undef ENTER 167 3: 168 #ifdef BOOTSEL 169 cmpb $0, (%bx) /* check for prompt */ 170 jz 4f 171 /* output menu item */ 172 movw $prefix, %si 173 incb (%si) 174 call message /* menu number */ 175 mov (%si), %si /* ':' << 8 | '1' + count */ 176 shl $2, %si /* const + count * 4 */ 177 #define CONST (4 * ((':' << 8) + '1' - ((SCAN_1 - SCAN_F1) & 0xff))) 178 #ifdef NO_CHS 179 movl %edi, ptn_list - CONST(%si) /* sector to read */ 180 #else 181 mov %bp, ptn_list - CONST(%si) /* partition info */ 182 #endif 183 #undef CONST 184 mov %bx, %si 185 call message /* prompt */ 186 movw $crlf, %si 187 call message 188 #endif 189 4: 190 add $0x10, %bp 191 add $TABENTRYSIZE, %bx 192 cmpb $(nametab - start - 0x100) + 4 * TABENTRYSIZE, %bl 193 jne next_ptn 194 195 #ifdef BOOT_EXTENDED 196 /* 197 * Now check extended partition chain 198 */ 199 testl %edx, %edx 200 je wait_key 201 testl %ecx, %ecx 202 jne 1f 203 xchg %ecx, %edx /* save base of ext ptn chain */ 204 1: addl %ecx, %edx /* sector to read */ 205 movl %edx, lba_sector 206 movw $lba_info, %si 207 movb $0x42, %ah 208 pop %dx /* recover drive # */ 209 push %dx /* save drive */ 210 int $0x13 211 jc wait_key /* abort menu on read fail */ 212 cmpw $MBR_MAGIC, LOADADDR + MBR_MAGIC_OFFSET 213 movw $nametab - LOADADDR + BOOTADDR, %bx 214 je next_extended 215 #endif 216 217 /* 218 * The non-bootsel code traverses this code path, it needs the 219 * correct keycode to select the active partition. 220 */ 221 222 #ifndef BOOTSEL 223 mov $SCAN_ENTER - SCAN_F1, %ax 224 #else 225 /* 226 * Get the initial time value for the timeout comparison. It is returned 227 * by int 1a in cx:dx. We do sums modulo 2^16 so it doesn't matter if 228 * the counter wraps (which it does every hour) - so we can safely 229 * ignore 'cx'. 230 * 231 * Loop around checking for a keypress until we have one, or timeout is 232 * reached. 233 */ 234 wait_key: 235 xorb %ah, %ah 236 int $0x1a 237 mov %dx, %di /* start time to di */ 238 3: 239 movb $1, %ah /* looks to see if a */ 240 int $0x16 /* key has been pressed */ 241 jnz get_key 242 xorb %ah, %ah 243 int $0x1a /* current time to cx:dx */ 244 sub %di, %dx 245 cmpw timeout, %dx /* always wait for 1 tick... */ 246 jbe 3b /* 0xffff means never timeout */ 247 def_key: 248 mov defkey - 1, %ax /* timedout - get default key to %ah */ 249 jmp 4f 250 get_key: 251 xorb %ah, %ah 252 int $0x16 /* 'read key', code ah, ascii al */ 253 4: shr $8, %ax /* code in %al, %ah zero */ 254 255 /* 256 * We have a keycode, see what it means. 257 * If we don't know we generate error '?' and go ask again 258 */ 259 check_key: 260 /* 261 * F1-F10 -> boot disk 0-9. Check if the requested disk isn't above 262 * the number of disks actually in the system as stored in 0:0475 by 263 * the BIOS. 264 * If we trust loc 475, we needn't check the upper bound on the keystroke 265 * This is always sector 0, so always read using chs. 266 */ 267 subb $SCAN_F1, %al 268 cmpb 0x0475, %al 269 jae boot_ptn 270 addb $0x80, %al 271 pop %dx /* dump saved drive # */ 272 push %ax /* replace with new */ 273 #ifdef NO_CHS 274 xorl %ebp, %ebp /* read sector number 0 */ 275 jmp boot_lba 276 #else 277 movw $chs_zero, %si /* chs read sector zero info */ 278 jmp read_chs 279 #endif 280 #endif /* BOOTSEL */ 281 282 /* 283 * Boot requested partition. 284 * Use keycode to index the table we generated when we scanned the mbr 285 * while generating the menu. 286 * 287 * We very carfully saved the values in the correct part of the table. 288 */ 289 290 boot_ptn: 291 shl $2, %ax 292 movw %ax, %si 293 #ifdef NO_CHS 294 movl ptn_list(%si), %ebp 295 testl %ebp, %ebp 296 jnz boot_lba 297 #else 298 mov ptn_list(%si), %si 299 test %si, %si 300 jnz boot_si 301 #endif 302 #ifdef BOOTSEL 303 set_err(ERR_KEY) 304 #else 305 set_err(ERR_INVPART) 306 #endif 307 /* jmp err_msg */ 308 309 /* Something went wrong... 310 * Output error code, 311 * reset disk subsystem - needed after read failure, 312 * and wait for user key 313 */ 314 err_msg: 315 #ifdef TERSE_ERROR 316 movb %al, errcod 317 movw $errtxt, %si 318 call message 319 #else 320 push %ax 321 movw $errtxt, %si 322 call message 323 pop %si 324 call message 325 movw $crlf, %si 326 call message 327 #endif 328 pop %dx /* drive we errored on */ 329 xor %ax,%ax /* only need %ah = 0 */ 330 int $0x13 /* reset disk subsystem */ 331 #ifdef BOOTSEL 332 pop %dx /* original drive number */ 333 push %dx 334 push %dx 335 jmp get_key 336 #else 337 int $0x18 /* BIOS might ask for a key */ 338 /* press and retry boot seq. */ 339 1: sti 340 hlt 341 jmp 1b 342 #endif 343 344 #ifndef NO_CHS 345 /* 346 * Active partition pointed to by si. 347 * Read the first sector. 348 * 349 * We can either do a CHS (Cylinder Head Sector) or an LBA (Logical 350 * Block Address) read. Always doing the LBA one 351 * would be nice - unfortunately not all systems support it. 352 * Also some may contain a separate (eg SCSI) bios that doesn't 353 * support it even when the main bios does. 354 * 355 * There is also the additional problem that the CHS values may be wrong 356 * (eg if fdisk was run on a different system that used different BIOS 357 * geometry). We convert the CHS value to a LBA sector number using 358 * the geometry from the BIOS, if the number matches we do a CHS read. 359 */ 360 boot_si: 361 movl 8(%si), %ebp /* get sector # */ 362 363 testb $MBR_BS_READ_LBA, flags 364 jnz boot_lba /* fdisk forced LBA read */ 365 366 pop %dx /* collect saved drive... */ 367 push %dx /* ...number to dl */ 368 movb $8, %ah 369 int $0x13 /* chs info */ 370 371 /* 372 * Validate geometry, if the CHS sector number doesn't match the LBA one 373 * we'll do an LBA read. 374 * calc: (cylinder * number_of_heads + head) * number_of_sectors + sector 375 * and compare against LBA sector number. 376 * Take a slight 'flier' and assume we can just check 16bits (very likely 377 * to be true because the number of sectors per track is 63). 378 */ 379 movw 2(%si), %ax /* cylinder + sector */ 380 push %ax /* save for sector */ 381 shr $6, %al 382 xchgb %al, %ah /* 10 bit cylinder number */ 383 shr $8, %dx /* last head */ 384 inc %dx /* number of heads */ 385 mul %dx 386 mov 1(%si), %dl /* head we want */ 387 add %dx, %ax 388 and $0x3f, %cx /* number of sectors */ 389 mul %cx 390 pop %dx /* recover sector we want */ 391 and $0x3f, %dx 392 add %dx, %ax 393 dec %ax 394 395 cmp %bp, %ax 396 je read_chs 397 398 #ifndef NO_LBA_CHECK 399 /* 400 * Determine whether we have int13-extensions, by calling int 13, function 41. 401 * Check for the magic number returned, and the disk packet capability. 402 */ 403 movw $0x55aa, %bx 404 movb $0x41, %ah 405 pop %dx 406 push %dx 407 int $0x13 408 jc 1f /* no int13 extensions */ 409 cmpw $0xaa55, %bx 410 jnz 1f 411 testb $1, %cl 412 jnz boot_lba 413 1: set_err(ERR_NO_LBA) 414 jmp err_msg 415 #endif /* NO_LBA_CHECK */ 416 #endif /* NO_CHS */ 417 418 /* 419 * Save sector number (passed in %ebp) into lba parameter block, 420 * read the sector and leap into it. 421 */ 422 boot_lba: 423 movl %ebp, lba_sector /* save sector number */ 424 movw $lba_info, %si 425 movb $0x42, %ah 426 pop %dx /* recover drive # */ 427 do_read: 428 push %dx /* save drive */ 429 int $0x13 430 431 set_err(ERR_READ) 432 jc err_msg 433 434 /* 435 * Check signature for valid bootcode 436 */ 437 movb BOOTADDR, %al /* first byte non-zero */ 438 test %al, %al 439 jz 1f 440 movw BOOTADDR + MBR_MAGIC_OFFSET, %ax 441 1: cmp $MBR_MAGIC, %ax 442 set_err(ERR_NOOS) 443 jnz err_msg 444 445 /* We pass the sector number through to the next stage boot. 446 * It doesn't have to use it (indeed no other mbr code will generate) it, 447 * but it does let us have a NetBSD pbr that can identify where it was 448 * read from! This lets us use this code to select between two 449 * NetBSD system on the same physical driver. 450 * (If we've read the mbr of a different disk, it gets a random number 451 * - but it wasn't expecting anything...) 452 */ 453 movl %ebp, %esi 454 pop %dx /* recover drive # */ 455 jmp start - LOADADDR + BOOTADDR 456 457 458 #ifndef NO_CHS 459 /* 460 * Sector below CHS limit 461 * Do a cylinder-head-sector read instead. 462 */ 463 read_chs: 464 pop %dx /* recover drive # */ 465 movb 1(%si), %dh /* head */ 466 movw 2(%si), %cx /* ch=cyl, cl=sect */ 467 movw $BOOTADDR, %bx /* es:bx is buffer */ 468 movw $0x201, %ax /* command 2, 1 sector */ 469 jmp do_read 470 #endif 471 472 /* 473 * Control block for int-13 LBA read. 474 * We need a xx, 00, 01, 00 somewhere to load chs for sector zero, 475 * by a complete fluke there is one here! 476 */ 477 chs_zero: 478 lba_info: 479 .word 0x10 /* control block length */ 480 .word 1 /* sector count */ 481 .word BOOTADDR /* offset in segment */ 482 .word 0 /* segment */ 483 lba_sector: 484 .long 0x0000 /* sector # goes here... */ 485 .long 0x0000 486 487 errtxt: .ascii "Error " /* runs into crlf if errcod set */ 488 errcod: .byte 0 489 crlf: .asciz "\r\n" 490 491 #ifdef BOOTSEL 492 prefix: .asciz "0: " 493 #endif 494 495 #ifndef TERSE_ERROR 496 ERR_INVPART: .asciz "No active partition" 497 ERR_READ: .asciz "Disk read error" 498 ERR_NOOS: .asciz "No operating system" 499 #ifndef NO_LBA_CHECK 500 ERR_NO_LBA: .asciz "Invalid CHS read" 501 #endif 502 #ifdef BOOTSEL 503 ERR_KEY: .asciz "bad key" 504 #endif 505 #endif 506 507 /* 508 * I hate #including source files, but the stuff below has to be at 509 * the correct absolute address. 510 * Clearly this could be done with a linker script. 511 */ 512 513 #include <message.S> 514 #if 0 515 #include <dump_eax.S> 516 #endif 517 518 /* 519 * Stuff from here on is overwritten by fdisk - the offset must not change... 520 * 521 * Get amount of space to makefile can report it. 522 * (Unfortunately I can't seem to get the value reported when it is -ve) 523 */ 524 mbr_space = defkey - . 525 . = start + MBR_BS_OFFSET 526 /* 527 * Default action, as a keyvalue we'd normally read from the BIOS. 528 */ 529 defkey: 530 .byte SCAN_ENTER /* ps/2 code */ 531 #ifndef BOOTSEL_FLAGS 532 #define BOOTSEL_FLAGS 0 533 #endif 534 flags: .byte MBR_BS_NEWMBR | BOOTSEL_FLAGS 535 /* 536 * Timeout value. ~65536 ticks per hour, which is ~18.2 times per second. 537 * 0xffff means never timeout. 538 */ 539 timeout: 540 .word 182 /* default to 10 seconds */ 541 /* 542 * mbr_bootsel 543 */ 544 nametab: 545 .fill MBR_PART_COUNT * (MBR_BS_PARTNAMESIZE + 1), 0x01, 0x00 546 547 /* space for mbr_dsn */ 548 . = start + MBR_DSN_OFFSET 549 .long 0 550 551 /* mbr_bootsel_magic */ 552 . = start + MBR_BS_MAGIC_OFFSET 553 .word MBR_BS_MAGIC 554 555 /* 556 * MBR partition table 557 */ 558 . = start + MBR_PART_OFFSET 559 parttab: 560 .fill 0x40, 0x01, 0x00 561 562 . = start + MBR_MAGIC_OFFSET 563 .word MBR_MAGIC 564 565 /* zeroed data space */ 566 bss_off = 0 567 bss_start = . 568 #define BSS(name, size) name = bss_start + bss_off; bss_off = bss_off + size 569 BSS(ptn_list, 256 * 4) /* long[]: boot sector numbers */ 570 BSS(bss_end, 0) 571
Indexes created Mon Oct 13 00:09:54 GMT 2025