mbr.S revision 1.8
1 /* $NetBSD: mbr.S,v 1.8 2004/07/04 00:38:51 mycroft 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 * If we haven't output any menu entries, then the system sits waiting 219 * for a keypress with the user looking at a blank screen wondering 220 * why nothing is happening. 221 * To stop this we generate <ENTER> - to boot the active partition 222 * if we haven't output any menu texts. 223 * The non-bootsel code also traverses this code path. 224 */ 225 226 #ifndef BOOTSEL 227 movb $SCAN_ENTER - SCAN_F1, %al 228 #else 229 movb $SCAN_ENTER, %al /* default to active partition */ 230 cmpb $'0', prefix /* did we output a menu ? */ 231 je check_key /* Nope - process SCAN_ENTER */ 232 /* 233 * Get the initial time value for the timeout comparison. It is returned 234 * by int 1a in cx:dx. We do sums modulo 2^16 so it doesn't matter if 235 * the counter wraps (which it does every hour) - so we can safely 236 * ignore 'cx'. 237 * 238 * Loop around checking for a keypress until we have one, or timeout is 239 * reached. 240 */ 241 wait_key: 242 xorb %ah, %ah 243 int $0x1a 244 mov %dx, %di /* start time to di */ 245 3: 246 movb $1, %ah /* looks to see if a */ 247 int $0x16 /* key has been pressed */ 248 jnz get_key 249 xorb %ah, %ah 250 int $0x1a /* current time to cx:dx */ 251 sub %di, %dx 252 movw timeout, %ax 253 cmp %ax, %dx /* always wait for 1 tick... */ 254 jbe 3b /* 0xffff means never timeout */ 255 def_key: 256 movb defkey, %al /* timedout - pick default key */ 257 jmp check_key 258 get_key: 259 xorb %ah, %ah 260 int $0x16 /* 'read key', code ah, ascii al */ 261 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 push %ax 329 movw $errtxt, %si 330 call message 331 pop %si 332 call message 333 movw $crlf, %si 334 call message 335 #endif 336 pop %dx /* drive we errored on */ 337 xor %ax,%ax /* only need %ah = 0 */ 338 int $0x13 /* reset disk subsystem */ 339 #ifdef BOOTSEL 340 pop %dx /* original drive number */ 341 push %dx 342 push %dx 343 jmp get_key 344 #else 345 int $0x18 /* BIOS might ask for a key */ 346 /* press and retry boot seq. */ 347 1: sti 348 hlt 349 jmp 1b 350 #endif 351 352 #ifndef NO_CHS 353 /* 354 * Active partition pointed to by si. 355 * Read the first sector. 356 * 357 * We can either do a CHS (Cylinder Head Sector) or an LBA (Logical 358 * Block Address) read. Always doing the LBA one 359 * would be nice - unfortunately not all systems support it. 360 * Also some may contain a separate (eg SCSI) bios that doesn't 361 * support it even when the main bios does. 362 * 363 * There is also the additional problem that the CHS values may be wrong 364 * (eg if fdisk was run on a different system that used different BIOS 365 * geometry). We convert the CHS value to a LBA sector number using 366 * the geometry from the BIOS, if the number matches we do a CHS read. 367 */ 368 boot_si: 369 movl 8(%si), %ebp /* get sector # */ 370 371 testb $MBR_BS_READ_LBA, flags 372 jnz boot_lba /* fdisk forced LBA read */ 373 374 pop %dx /* collect saved drive... */ 375 push %dx /* ...number to dl */ 376 movb $8, %ah 377 int $0x13 /* chs info */ 378 379 /* 380 * Validate geometry, if the CHS sector number doesn't match the LBA one 381 * we'll do an LBA read. 382 * calc: (cylinder * number_of_heads + head) * number_of_sectors + sector 383 * and compare against LBA sector number. 384 * Take a slight 'flier' and assume we can just check 16bits (very likely 385 * to be true because the number of sectors per track is 63). 386 */ 387 movw 2(%si), %ax /* cylinder + sector */ 388 push %ax /* save for sector */ 389 shr $6, %al 390 xchgb %al, %ah /* 10 bit cylinder number */ 391 shr $8, %dx /* last head */ 392 inc %dx /* number of heads */ 393 mul %dx 394 mov 1(%si), %dl /* head we want */ 395 add %dx, %ax 396 and $0x3f, %cx /* number of sectors */ 397 mul %cx 398 pop %dx /* recover sector we want */ 399 and $0x3f, %dx 400 add %dx, %ax 401 dec %ax 402 403 cmp %bp, %ax 404 je read_chs 405 406 #ifndef NO_LBA_CHECK 407 /* 408 * Determine whether we have int13-extensions, by calling int 13, function 41. 409 * Check for the magic number returned, and the disk packet capability. 410 */ 411 movw $0x55aa, %bx 412 movb $0x41, %ah 413 pop %dx 414 push %dx 415 int $0x13 416 jc 1f /* no int13 extensions */ 417 cmpw $0xaa55, %bx 418 jnz 1f 419 testb $1, %cl 420 jnz boot_lba 421 1: set_err(ERR_NO_LBA) 422 jmp err_msg 423 #endif /* NO_LBA_CHECK */ 424 #endif /* NO_CHS */ 425 426 /* 427 * Save sector number (passed in %ebp) into lba parameter block, 428 * read the sector and leap into it. 429 */ 430 boot_lba: 431 movl %ebp, lba_sector /* save sector number */ 432 movw $lba_info, %si 433 movb $0x42, %ah 434 pop %dx /* recover drive # */ 435 do_read: 436 push %dx /* save drive */ 437 int $0x13 438 439 set_err(ERR_READ) 440 jc err_msg 441 442 /* 443 * Check signature for valid bootcode 444 */ 445 movb BOOTADDR, %al /* first byte non-zero */ 446 test %al, %al 447 jz 1f 448 movw BOOTADDR + MBR_MAGIC_OFFSET, %ax 449 1: cmp $MBR_MAGIC, %ax 450 set_err(ERR_NOOS) 451 jnz err_msg 452 453 /* We pass the sector number through to the next stage boot. 454 * It doesn't have to use it (indeed no other mbr code will generate) it, 455 * but it does let us have a NetBSD pbr that can identify where it was 456 * read from! This lets us use this code to select between two 457 * NetBSD system on the same physical driver. 458 * (If we've read the mbr of a different disk, it gets a random number 459 * - but it wasn't expecting anything...) 460 */ 461 movl %ebp, %esi 462 pop %dx /* recover drive # */ 463 jmp start - LOADADDR + BOOTADDR 464 465 466 #ifndef NO_CHS 467 /* 468 * Sector below CHS limit 469 * Do a cylinder-head-sector read instead. 470 */ 471 read_chs: 472 pop %dx /* recover drive # */ 473 movb 1(%si), %dh /* head */ 474 movw 2(%si), %cx /* ch=cyl, cl=sect */ 475 movw $BOOTADDR, %bx /* es:bx is buffer */ 476 movw $0x201, %ax /* command 2, 1 sector */ 477 jmp do_read 478 #endif 479 480 /* 481 * Control block for int-13 LBA read. 482 * We need a xx, 00, 01, 00 somewhere to load chs for sector zero, 483 * by a complete fluke there is one here! 484 */ 485 chs_zero: 486 lba_info: 487 .word 0x10 /* control block length */ 488 .word 1 /* sector count */ 489 .word BOOTADDR /* offset in segment */ 490 .word 0 /* segment */ 491 lba_sector: 492 .long 0x0000 /* sector # goes here... */ 493 .long 0x0000 494 495 errtxt: .ascii "Error " /* runs into crlf if errcod set */ 496 errcod: .byte 0 497 crlf: .asciz "\r\n" 498 499 #ifdef BOOTSEL 500 prefix: .asciz "0: " 501 #endif 502 503 #ifndef TERSE_ERROR 504 ERR_INVPART: .asciz "No active partition" 505 ERR_READ: .asciz "Disk read error" 506 ERR_NOOS: .asciz "No operating system" 507 #ifndef NO_LBA_CHECK 508 ERR_NO_LBA: .asciz "Invalid CHS read" 509 #endif 510 #ifdef BOOTSEL 511 ERR_KEY: .asciz "bad key" 512 #endif 513 #endif 514 515 /* 516 * I hate #including source files, but the stuff below has to be at 517 * the correct absolute address. 518 * Clearly this could be done with a linker script. 519 */ 520 521 #include <message.S> 522 #if 0 523 #include <dump_eax.S> 524 #endif 525 526 /* 527 * Stuff from here on is overwritten by fdisk - the offset must not change... 528 * 529 * Get amount of space to makefile can report it. 530 * (Unfortunately I can't seem to get the value reported when it is -ve) 531 */ 532 mbr_space = defkey - . 533 . = start + MBR_BS_OFFSET 534 /* 535 * Default action, as a keyvalue we'd normally read from the BIOS. 536 */ 537 defkey: 538 .byte SCAN_ENTER /* ps/2 code */ 539 #ifndef BOOTSEL_FLAGS 540 #define BOOTSEL_FLAGS 0 541 #endif 542 flags: .byte MBR_BS_NEWMBR | BOOTSEL_FLAGS 543 /* 544 * Timeout value. ~65536 ticks per hour, which is ~18.2 times per second. 545 * 0xffff means never timeout. 546 */ 547 timeout: 548 .word 182 /* default to 10 seconds */ 549 /* 550 * mbr_bootsel 551 */ 552 nametab: 553 .fill MBR_PART_COUNT * (MBR_BS_PARTNAMESIZE + 1), 0x01, 0x00 554 555 /* space for mbr_dsn */ 556 . = start + MBR_DSN_OFFSET 557 .long 0 558 559 /* mbr_bootsel_magic */ 560 . = start + MBR_BS_MAGIC_OFFSET 561 .word MBR_BS_MAGIC 562 563 /* 564 * MBR partition table 565 */ 566 . = start + MBR_PART_OFFSET 567 parttab: 568 .fill 0x40, 0x01, 0x00 569 570 . = start + MBR_MAGIC_OFFSET 571 .word MBR_MAGIC 572 573 /* zeroed data space */ 574 bss_off = 0 575 bss_start = . 576 #define BSS(name, size) name = bss_start + bss_off; bss_off = bss_off + size 577 BSS(ptn_list, 256 * 4) /* long[]: boot sector numbers */ 578 BSS(bss_end, 0) 579
Indexes created Mon Oct 13 08:09:50 GMT 2025