mbr.S revision 1.4
1 /* $NetBSD: mbr.S,v 1.4 2003/10/08 04:25:45 lukem Exp $ */ 2 3 /* 4 * Copyright (c) 1999-2003 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 #include <machine/asm.h> 53 #include <sys/bootblock.h> 54 55 #define BOOTADDR 0x7c00 56 #define LOADADDR 0x0600 /* address were are linked to */ 57 58 #define TABENTRYSIZE (MBR_BS_PARTNAMESIZE + 1) 59 #define NAMETABSIZE (MBR_PART_COUNT * TABENTRYSIZE) 60 61 /* Scan values for the various keys we use, as returned by the BIOS */ 62 #define SCAN_ENTER 0x1c 63 #define SCAN_F1 0x3b 64 #define SCAN_1 0x2 65 66 /* 67 * Minimum and maximum drive number that is considered to be valid. 68 */ 69 #define MINDRV 0x80 70 #define MAXDRV 0x87 71 72 #ifdef TERSE_ERROR 73 /* 74 * Error codes. Done this way to save space. 75 */ 76 #define ERR_INVPART '1' /* Invalid partition table */ 77 #define ERR_READ '2' /* Read error */ 78 #define ERR_NOOS '3' /* Magic no. check failed for part. */ 79 #define ERR_KEY '?' /* unknown key press */ 80 #define ERR_NO_LBA 'L' /* sector above chs limit */ 81 82 #define set_err(err) movb $err, %al 83 84 #else 85 #define set_err(err) mov $err, %ax 86 #endif 87 88 .text 89 .code16 90 /* 91 * Move ourselves out of the way first. 92 * (to the address we are linked at - 0x600) 93 * and zero our bss 94 */ 95 ENTRY(start) 96 xor %ax, %ax 97 mov %ax, %ss 98 movw $BOOTADDR, %sp 99 mov %ax, %es 100 mov %ax, %ds 101 mov %sp, %si 102 movw $start, %di 103 movw $(bss_start - start)/2, %cx 104 rep 105 movsw 106 mov $(bss_end - bss_start + 1)/2, %cx 107 rep 108 stosw 109 ljmp $0, $mbr /* leap into copy of code */ 110 111 /* 112 * Sanity check the drive number passed by the BIOS. Some BIOSs may not 113 * do this and pass garbage. 114 */ 115 mbr: 116 cmpb $MAXDRV, %dl /* relies on MINDRV being 0x80 */ 117 jle 1f 118 movb $MINDRV, %dl /* garbage in, boot disk 0 */ 119 1: 120 push %dx /* save drive number */ 121 push %dx /* twice - for err_msg loop */ 122 123 #ifdef BOOTSEL 124 /* 125 * Walk through the selector (name) table printing used entries. 126 */ 127 bootsel_menu: 128 movw $nametab, %bx 129 #ifdef BOOT_EXTENDED 130 xorl %ecx, %ecx /* base of extended partition */ 131 next_extended: 132 xorl %edx, %edx /* for next extended partition */ 133 #endif 134 lea parttab - nametab(%bx), %bp 135 next_ptn: 136 movb 4(%bp), %al /* partition type */ 137 #ifdef BOOT_EXTENDED 138 movl 8(%bp), %edi /* partition sector number */ 139 cmpb $MBR_PTYPE_EXT, %al /* Extended partition */ 140 je 1f 141 cmpb $MBR_PTYPE_EXT_LBA, %al /* Extended LBA partition */ 142 je 1f 143 cmpb $MBR_PTYPE_EXT_LNX, %al /* Linux extended partition */ 144 jne 2f 145 1: movl %edi, %edx /* save next extended ptn */ 146 jmp 3f 147 2: 148 #endif 149 test %al, %al /* undefined partition */ 150 je 3f 151 cmpb $0, (%bx) /* check for prompt */ 152 jz 3f 153 154 /* output menu item */ 155 movw $prefix, %si 156 incb (%si) 157 call message /* menu number */ 158 mov (%si), %si /* ':' << 8 | '1' + count */ 159 shl $2, %si /* const + count * 4 */ 160 #define CONST (4 * ((':' << 8) + '1' - ((SCAN_1 - SCAN_F1) & 0xff))) 161 #ifdef NO_CHS 162 addl lba_sector, %edi 163 movl %edi, ptn_list - CONST(%si) /* sector to read */ 164 #else 165 mov %bp, ptn_list - CONST(%si) /* partition info */ 166 #endif 167 #undef CONST 168 mov %bx, %si 169 call message /* prompt */ 170 movw $crlf, %si 171 call message 172 3: 173 add $0x10, %bp 174 add $TABENTRYSIZE, %bx 175 cmpb $(nametab - start - 0x100) + 4 * TABENTRYSIZE, %bl 176 jne next_ptn 177 178 #ifdef BOOT_EXTENDED 179 /* 180 * Now check extended partition chain 181 */ 182 testl %edx, %edx 183 je wait_key 184 testl %ecx, %ecx 185 jne 1f 186 xchg %ecx, %edx /* save base of ext ptn chain */ 187 1: addl %ecx, %edx /* sector to read */ 188 movl %edx, lba_sector 189 movw $lba_info, %si 190 movb $0x42, %ah 191 pop %dx /* recover drive # */ 192 push %dx /* save drive */ 193 int $0x13 194 jc wait_key /* abort menu on read fail */ 195 cmpw $MBR_MAGIC, LOADADDR + MBR_MAGIC_OFFSET 196 movw $nametab - LOADADDR + BOOTADDR, %bx 197 je next_extended 198 #endif 199 200 /* 201 * Get the initial time value for the timeout comparison. It is returned 202 * by int 1a in cx:dx. We do sums modulo 2^16 so it doesn't matter if 203 * the counter wraps (which it does every hour) - so we can safely 204 * ignore 'cx'. 205 * 206 * Loop around checking for a keypress until we have one, or timeout is 207 * reached. 208 */ 209 wait_key: 210 xorb %ah, %ah 211 int $0x1a 212 mov %dx, %di /* start time to di */ 213 3: 214 movb $1, %ah /* looks to see if a */ 215 int $0x16 /* key has been pressed */ 216 jnz get_key 217 xorb %ah, %ah 218 int $0x1a /* current time to cx:dx */ 219 sub %di, %dx 220 movw timeout, %ax 221 cmp %ax, %dx /* always wait for 1 tick... */ 222 jbe 3b /* 0xffff means never timeout */ 223 def_key: 224 movb defkey, %al /* timedout - pick default key */ 225 jmp check_key 226 get_key: 227 xorb %ah, %ah 228 int $0x16 /* 'read key', code ah, ascii al */ 229 shr $8, %ax /* code in %al, %ah zero */ 230 231 /* 232 * We have a keycode, see what it means. 233 * If we don't know we generate error '?' and go ask again 234 */ 235 check_key: 236 /* 237 * <enter> -> boot active partition. 238 */ 239 cmpb $SCAN_ENTER, %al 240 jne boot_disk 241 #endif /* BOOTSEL */ 242 243 /* 244 * Scan MBR for first active partition, and boot it. 245 */ 246 mov $MBR_PART_COUNT, %cx 247 mov $parttab, %si 248 1: 249 cmpb $0x80, (%si) 250 #ifdef NO_CHS 251 jne 10f /* not active */ 252 movl 8(%si), %ebp /* sector number of ptn */ 253 jmp boot_lba 254 10: 255 #else 256 je boot_si 257 #endif 258 add $0x10, %si 259 loop 1b 260 set_err(ERR_INVPART) 261 jmp err_msg 262 263 #ifdef BOOTSEL 264 /* 265 * F1-F10 -> boot disk 0-9. Check if the requested disk isn't above 266 * the number of disks actually in the system as stored in 0:0475 by 267 * the BIOS. 268 * If we trust loc 475, we needn't check the upper bound on the keystroke 269 * This is always sector 0, so always read using chs. 270 */ 271 boot_disk: 272 subb $SCAN_F1, %al 273 cmpb 0x0475, %al 274 jae boot_ptn 275 addb $0x80, %al 276 pop %dx /* dump saved drive # */ 277 push %ax /* replace with new */ 278 #ifdef NO_CHS 279 xorl %ebp, %ebp /* read sector number 0 */ 280 jmp boot_lba 281 #else 282 movw $chs_zero, %si /* chs read sector zero info */ 283 jmp read_chs 284 #endif 285 286 /* 287 * Boot requested partition. 288 * Use keycode to index the table we generated when we scanned the mbr 289 * while generating the menu. 290 * 291 * We very carfully saved the values in the correct part of the table. 292 */ 293 294 boot_ptn: 295 shl $2, %ax 296 movw %ax, %si 297 #ifdef NO_CHS 298 movl ptn_list(%si), %ebp 299 testl %ebp, %ebp 300 jnz boot_lba 301 #else 302 mov ptn_list(%si), %si 303 test %si, %si 304 jnz boot_si 305 #endif 306 set_err(ERR_KEY) 307 /* jmp err_msg */ 308 #endif /* BOOTSEL */ 309 310 /* Something went wrong... 311 * Output error code, 312 * reset disk subsystem - needed after read failure, 313 * and wait for user key 314 */ 315 err_msg: 316 #ifdef TERSE_ERROR 317 movb %al, errcod 318 movw $errtxt, %si 319 call message 320 #else 321 push %ax 322 movw $errtxt, %si 323 call message 324 pop %si 325 call message 326 movw $crlf, %si 327 call message 328 #endif 329 pop %dx /* drive we errored on */ 330 xor %ax,%ax /* only need %ah = 0 */ 331 int $0x13 /* reset disk subsystem */ 332 #ifdef BOOTSEL 333 pop %dx /* original drive number */ 334 push %dx 335 push %dx 336 jmp get_key 337 #else 338 int $0x18 /* BIOS might ask for a key */ 339 /* press and retry boot seq. */ 340 1: sti 341 hlt 342 jmp 1b 343 #endif 344 345 #ifndef NO_CHS 346 /* 347 * Active partition pointed to by si. 348 * Read the first sector. 349 * 350 * We can either do a CHS (Cylinder Head Sector) or an LBA (Logical 351 * Block Address) read. Always doing the LBA one 352 * would be nice - unfortunately not all systems support it. 353 * Also some may contain a separate (eg SCSI) bios that doesn't 354 * support it even when the main bios does. 355 * 356 * There is also the additional problem that the CHS values may be wrong 357 * (eg if fdisk was run on a different system that used different BIOS 358 * geometry). We convert the CHS value to a LBA sector number using 359 * the geometry from the BIOS, if the number matches we do a CHS read. 360 */ 361 boot_si: 362 movl 8(%si), %ebp /* get sector # */ 363 364 testb $MBR_BS_READ_LBA, flags 365 jnz boot_lba /* fdisk forced LBA read */ 366 367 pop %dx /* collect saved drive... */ 368 push %dx /* ...number to dl */ 369 movb $8, %ah 370 int $0x13 /* chs info */ 371 372 /* 373 * Validate geometry, if the CHS sector number doesn't match the LBA one 374 * we'll do an LBA read. 375 * calc: (cylinder * number_of_heads + head) * number_of_sectors + sector 376 * and compare against LBA sector number. 377 * Take a slight 'flier' and assume we can just check 16bits (very likely 378 * to be true because the number of sectors per track is 63). 379 */ 380 movw 2(%si), %ax /* cylinder + sector */ 381 push %ax /* save for sector */ 382 shr $6, %al 383 xchgb %al, %ah /* 10 bit cylinder number */ 384 shr $8, %dx /* last head */ 385 inc %dx /* number of heads */ 386 mul %dx 387 mov 1(%si), %dl /* head we want */ 388 add %dx, %ax 389 and $0x3f, %cx /* number of sectors */ 390 mul %cx 391 pop %dx /* recover sector we want */ 392 and $0x3f, %dx 393 add %dx, %ax 394 dec %ax 395 396 cmp %bp, %ax 397 je read_chs 398 399 #ifndef NO_LBA_CHECK 400 /* 401 * Determine whether we have int13-extensions, by calling int 13, function 41. 402 * Check for the magic number returned, and the disk packet capability. 403 */ 404 movw $0x55aa, %bx 405 movb $0x41, %ah 406 pop %dx 407 push %dx 408 int $0x13 409 jc 1f /* no int13 extensions */ 410 cmpw $0xaa55, %bx 411 jnz 1f 412 testb $1, %cl 413 jnz boot_lba 414 1: set_err(ERR_NO_LBA) 415 jmp err_msg 416 #endif /* NO_LBA_CHECK */ 417 #endif /* NO_CHS */ 418 419 /* 420 * Save sector number (passed in %ebp) into lba parameter block, 421 * read the sector and leap into it. 422 */ 423 boot_lba: 424 movl %ebp, lba_sector /* save sector number */ 425 movw $lba_info, %si 426 movb $0x42, %ah 427 pop %dx /* recover drive # */ 428 do_read: 429 push %dx /* save drive */ 430 int $0x13 431 432 set_err(ERR_READ) 433 jc err_msg 434 435 /* 436 * Check signature for valid bootcode 437 */ 438 movb BOOTADDR, %al /* first byte non-zero */ 439 test %al, %al 440 jz 1f 441 movw BOOTADDR + MBR_MAGIC_OFFSET, %ax 442 1: cmp $MBR_MAGIC, %ax 443 set_err(ERR_NOOS) 444 jnz err_msg 445 446 /* We pass the sector number through to the next stage boot. 447 * It doesn't have to use it (indeed no other mbr code will generate) it, 448 * but it does let us have a NetBSD pbr that can identify where it was 449 * read from! This lets us use this code to select between two 450 * NetBSD system on the same physical driver. 451 * (If we've read the mbr of a different disk, it gets a random number 452 * - but it wasn't expecting anything...) 453 */ 454 movl %ebp, %esi 455 pop %dx /* recover drive # */ 456 jmp start - LOADADDR + BOOTADDR 457 458 459 #ifndef NO_CHS 460 /* 461 * Sector below CHS limit 462 * Do a cylinder-head-sector read instead. 463 */ 464 read_chs: 465 pop %dx /* recover drive # */ 466 movb 1(%si), %dh /* head */ 467 movw 2(%si), %cx /* ch=cyl, cl=sect */ 468 movw $BOOTADDR, %bx /* es:bx is buffer */ 469 movw $0x201, %ax /* command 2, 1 sector */ 470 jmp do_read 471 #endif 472 473 /* 474 * Control block for int-13 LBA read. 475 * We need a xx, 00, 01, 00 somewhere to load chs for sector zero, 476 * by a complete fluke there is one here! 477 */ 478 chs_zero: 479 lba_info: 480 .word 0x10 /* control block length */ 481 .word 1 /* sector count */ 482 .word BOOTADDR /* offset in segment */ 483 .word 0 /* segment */ 484 lba_sector: 485 .long 0x0000 /* sector # goes here... */ 486 .long 0x0000 487 488 errtxt: .ascii "Error " /* runs into crlf if errcod set */ 489 errcod: .byte 0 490 crlf: .asciz "\r\n" 491 492 #ifdef BOOTSEL 493 prefix: .asciz "0: " 494 #endif 495 496 #ifndef TERSE_ERROR 497 ERR_INVPART: .asciz "No active partition" 498 ERR_READ: .asciz "Disk read error" 499 ERR_NOOS: .asciz "No operating system" 500 #ifndef NO_LBA_CHECK 501 ERR_NO_LBA: .asciz "Invalid CHS read" 502 #endif 503 #ifdef BOOTSEL 504 ERR_KEY: .asciz "bad key" 505 #endif 506 #endif 507 508 /* 509 * I hate #including source files, but the stuff below has to be at 510 * the correct absolute address. 511 * Clearly this could be done with a linker script. 512 */ 513 514 #include <message.S> 515 #if 0 516 #include <dump_eax.S> 517 #endif 518 519 /* 520 * Stuff from here on is overwritten by fdisk - the offset must not change... 521 * 522 * Get amount of space to makefile can report it. 523 * (Unfortunately I can't seem to get the value reported when it is -ve) 524 */ 525 mbr_space = defkey - . 526 . = start + MBR_BOOTSEL_OFFSET 527 /* 528 * Default action, as a keyvalue we'd normally read from the BIOS. 529 */ 530 defkey: 531 .byte SCAN_ENTER /* ps/2 code */ 532 #ifndef BOOTSEL_FLAGS 533 #define BOOTSEL_FLAGS 0 534 #endif 535 flags: .byte MBR_BS_NEWMBR | BOOTSEL_FLAGS 536 /* 537 * Timeout value. ~65536 ticks per hour, which is ~18.2 times per second. 538 * 0xffff means never timeout. 539 */ 540 timeout: 541 .word 182 /* default to 10 seconds */ 542 /* 543 * Space for name/select table and partition table. 544 */ 545 nametab: 546 .fill MBR_PART_COUNT * (MBR_BS_PARTNAMESIZE + 1), 0x01, 0x00 547 548 . = start + MBR_PART_OFFSET - 2 549 .word MBR_MAGIC 550 551 . = start + MBR_PART_OFFSET 552 parttab: 553 .fill 0x40, 0x01, 0x00 554 555 . = start + MBR_MAGIC_OFFSET 556 .word MBR_MAGIC 557 558 /* zeroed data space */ 559 bss_off = 0 560 bss_start = . 561 #define BSS(name, size) name = bss_start + bss_off; bss_off = bss_off + size 562 BSS(ptn_list, 256 * 4) /* long[]: boot sector numbers */ 563 BSS(bss_end, 0) 564
Indexes created Mon Oct 13 08:09:50 GMT 2025