1/* 2 * Copyright (c) 2008 Luigi Rizzo (mostly documentation) 3 * Copyright (c) 2002 Bruce M. Simpson 4 * Copyright (c) 1998 Robert Nordier 5 * All rights reserved. 6 * 7 * Redistribution and use in source and binary forms are freely 8 * permitted provided that the above copyright notice and this 9 * paragraph and the following disclaimer are duplicated in all 10 * such forms. 11 * 12 * This software is provided "AS IS" and without any express or 13 * implied warranties, including, without limitation, the implied 14 * warranties of merchantability and fitness for a particular 15 * purpose. 16 */ 17 18/* build options: */ 19#ifdef SIO /* use serial console on COM1. */ 20#endif 21 22#ifdef PXE /* enable PXE/INT18 booting with F6 */ 23#define SAVE_MORE_MEMORY 24#endif 25 26#ifdef CHECK_DRIVE /* make sure we boot from a HD. */ 27#endif 28 29#ifdef ONLY_F_KEYS /* Only F1..F6, no digits on console */ 30#endif 31 32#ifdef VOLUME_SERIAL /* support Volume serial number */ 33#define B0_BASE 0x1ae /* move the internal data area */ 34#define SAVE_MEMORY 35#else 36#define B0_BASE 0x1b2 37#endif 38 39#ifdef TEST /* enable some test code */ 40#define SAVE_MEMORY 41#define SAVE_MORE_MEMORY 42#endif 43 44/* 45 * Note - this code uses many tricks to save space and fit in one sector. 46 * This includes using side effects of certain instructions, reusing 47 * register values from previous operations, etc. 48 * Be extremely careful when changing the code, even for simple things. 49 */ 50 51/* 52 * BOOT BLOCK STRUCTURE 53 * 54 * This code implements a Master Boot Record (MBR) for an Intel/PC disk. 55 * It is 512 bytes long and it is normally loaded by the BIOS (or another 56 * bootloader) at 0:0x7c00. This code depends on %cs:%ip being 0:0x7c00 57 * 58 * The initial chunk of instructions is used as a signature by external 59 * tools (e.g. boot0cfg) which can manipulate the block itself. 60 * 61 * The area at offset 0x1b2 contains a magic string ('Drive '), also 62 * used as a signature to detect the block, and some variables that can 63 * be updated by boot0cfg (and optionally written back to the disk). 64 * These variables control the operation of the bootloader itself, 65 * e.g. which partitions to enable, the timeout, the use of LBA 66 * (called 'packet') or CHS mode, whether to force a drive number, 67 * and whether to write back the user's selection back to disk. 68 * 69 * As in every Master Boot Record, the partition table is at 0x1be, 70 * made of four 16-byte entries each containing: 71 * 72 * OFF SIZE DESCRIPTION 73 * 0 1 status (0x80: bootable, 0: non bootable) 74 * 1 3 start sector CHS 75 * 8:head, 6:sector, 2:cyl bit 9..8, 8:cyl bit 7..0 76 * 4 1 partition type 77 * 5 3 end sector CHS 78 * 8 4 LBA of first sector 79 * 12 4 partition size in sectors 80 * 81 * and followed by the two bytes 0x55, 0xAA (MBR signature). 82 */ 83 84 85/* 86 * BOOT BLOCK OPERATION 87 * 88 * On entry, the registers contain the following values: 89 * 90 * %cs:%ip 0:0x7c00 91 * %dl drive number (0x80, 0x81, ... ) 92 * %si pointer to the partition table from which we were loaded. 93 * Some boot code (e.g. syslinux) use this info to relocate 94 * themselves, so we want to pass a valid one to the next stage. 95 * NOTE: the use of %si is not a standard. 96 * 97 * This boot block first relocates itself at a different address (0:0x600), 98 * to free the space at 0:0x7c00 for the next stage boot block. 99 * 100 * It then initializes some memory at 0:0x800 and above (pointed by %bp) 101 * to store the original drive number (%dl) passed to us, and to construct a 102 * fake partition entry. The latter is used by the disk I/O routine and, 103 * in some cases, passed in %si to the next stage boot code. 104 * 105 * The variables at 0x1b2 are accessed as negative offsets from %bp. 106 * 107 * After the relocation, the code scans the partition table printing 108 * out enabled partition or disks, and waits for user input. 109 * 110 * When a partition is selected, or a timeout expires, the currently 111 * selected partition is used to load the next stage boot code, 112 * %dl and %si are set appropriately as when we were called, and 113 * control is transferred to the newly loaded code at 0:0x7c00. 114 */ 115 116/* 117 * CONSTANTS 118 * 119 * NHRDRV is the address in segment 0 where the BIOS writes the 120 * total number of hard disks in the system. 121 * LOAD is the original load address and cannot be changed. 122 * ORIGIN is the relocation address. If you change it, you also need 123 * to change the value passed to the linker in the Makefile 124 * PRT_OFF is the location of the partition table (from the MBR standard). 125 * B0_OFF is the location of the data area, known to boot0cfg so 126 * it cannot be changed. Computed as a negative offset from 0x200 127 * MAGIC is the signature of a boot block. 128 */ 129 130 .set NHRDRV,0x475 # Number of hard drives 131 .set ORIGIN,0x600 # Execution address 132 .set LOAD,0x7c00 # Load address 133 134 .set PRT_OFF,0x1be # Partition table 135 .set B0_OFF,(B0_BASE-0x200) # Offset of boot0 data 136 137 .set MAGIC,0xaa55 # Magic: bootable 138 139 .set KEY_ENTER,0x1c # Enter key scan code 140 .set KEY_F1,0x3b # F1 key scan code 141 .set KEY_1,0x02 # #1 key scan code 142 143 .set ASCII_BEL,'#' # ASCII code for <BEL> 144 .set ASCII_CR,0x0D # ASCII code for <CR> 145 146/* 147 * Offsets of variables in the block at B0_OFF, and in the volatile 148 * data area, computed as displacement from %bp. 149 * We need to define them as constant as the assembler cannot 150 * compute them in its single pass. 151 */ 152 .set _NXTDRV, B0_OFF+6 # Next drive 153 .set _OPT, B0_OFF+7 # Default option 154 .set _SETDRV, B0_OFF+8 # Drive to force 155 .set _FLAGS, B0_OFF+9 # Flags 156 .set SETDRV, 0x20 # the 'setdrv' flag 157 .set NOUPDATE, 0x40 # the 'noupdate' flag 158 .set USEPACKET, 0x80 # the 'packet' flag 159 160 /* ticks is at a fixed position */ 161 .set _TICKS, (PRT_OFF - 0x200 - 2) # Timeout ticks 162 .set _MNUOPT, 0x10 # Saved menu entries 163 164 .set TLEN, (desc_ofs - bootable_ids) # size of bootable ids 165 .globl start # Entry point 166 .code16 # This runs in real mode 167 168/* 169 * MAIN ENTRY POINT 170 * Initialise segments and registers to known values. 171 * segments start at 0. 172 * The stack is immediately below the address we were loaded to. 173 * NOTE: the initial section of the code (up to movw $LOAD,%sp) 174 * is used by boot0cfg, together with the 'Drive ' string and 175 * the 0x55, 0xaa at the end, as an identifier for version 1.0 176 * of the boot code. Do not change it. 177 * In version 1.0 the parameter table (_NEXTDRV etc) is at 0x1b9 178 */ 179start: cld # String ops inc 180 xorw %ax,%ax # Zero 181 movw %ax,%es # Address 182 movw %ax,%ds # data 183 movw %ax,%ss # Set up 184 movw $LOAD,%sp # stack 185 186 /* 187 * Copy this code to the address it was linked for, 0x600 by default. 188 */ 189 movw %sp,%si # Source 190 movw $start,%di # Destination 191 movw $0x100,%cx # Word count 192 rep # Relocate 193 movsw # code 194 /* 195 * After the code, (i.e. at %di+0, 0x800) create a partition entry, 196 * initialized to LBA 0 / CHS 0:0:1. 197 * Set %bp to point to the partition and also, with negative offsets, 198 * to the variables embedded in the bootblock (nextdrv and so on). 199 */ 200 movw %di,%bp # Address variables 201 movb $0x8,%cl # Words to clear 202 rep # Zero 203 stosw # them 204 incb -0xe(%di) # Set the S field to 1 205 206 jmp main-LOAD+ORIGIN # Jump to relocated code 207 208main: 209#if defined(SIO) && COMSPEED != 0 210 /* 211 * Init the serial port. bioscom preserves the driver number in DX. 212 */ 213 movw $COMSPEED,%ax # defined by Makefile 214 callw bioscom 215#endif 216 217 /* 218 * If the 'setdrv' flag is set in the boot sector, use the drive 219 * number from the boot sector at 'setdrv_num'. 220 * Optionally, do the same if the BIOS gives us an invalid number 221 * (note though that the override prevents booting from a floppy 222 * or a ZIP/flash drive in floppy emulation). 223 * The test costs 4 bytes of code so it is disabled by default. 224 */ 225 testb $SETDRV,_FLAGS(%bp) # Set drive number? 226#ifndef CHECK_DRIVE /* disable drive checks */ 227 jz save_curdrive # no, use the default 228#else 229 jnz disable_update # Yes 230 testb %dl,%dl # Drive number valid? 231 js save_curdrive # Possibly (0x80 set) 232#endif 233 /* 234 * Disable updates if the drive number is forced. 235 */ 236disable_update: orb $NOUPDATE,_FLAGS(%bp) # Disable updates 237 movb _SETDRV(%bp),%dl # Use stored drive number 238 239 /* 240 * Whatever drive we decided to use, store it at (%bp). The byte 241 * is normally used for the state of the partition (0x80 or 0x00), 242 * but we abuse it as it is very convenient to access at offset 0. 243 * The value is read back after 'check_selection' 244 */ 245save_curdrive: movb %dl, (%bp) # Save drive number 246 pushw %dx # Also in the stack 247#ifdef TEST /* test code, print internal bios drive */ 248 rolb $1, %dl 249 movw $drive, %si 250 call putkey 251#endif 252 callw putn # Print a newline 253 /* 254 * Start out with a pointer to the 4th byte of the first table entry 255 * so that after 4 iterations it's beyond the end of the sector 256 * and beyond a 256 byte boundary. We use the latter trick to check for 257 * end of the loop without using an extra register (see start.5). 258 */ 259 movw $(partbl+0x4),%bx # Partition table (+4) 260 xorw %dx,%dx # Item number 261 262 /* 263 * Loop around on the partition table, printing values until we 264 * pass a 256 byte boundary. 265 */ 266read_entry: movb %ch,-0x4(%bx) # Zero active flag (ch == 0) 267 btw %dx,_FLAGS(%bp) # Entry enabled? 268 jnc next_entry # No 269 movb (%bx),%al # Load type 270 test %al, %al # skip empty partition 271 jz next_entry 272 /* 273 * Scan the table of bootable ids, which starts at %di and has 274 * length TLEN. On a match, %di points to the element following the 275 * match; the corresponding offset to the description is $(TLEN-1) 276 * bytes ahead. We use a count of TLEN+1 so if we don't find a match 277 * within the first TLEN entries, we hit the 'unknown' entry. 278 */ 279 movw $bootable_ids,%di # Lookup tables 280 movb $(TLEN+1),%cl # Number of entries 281 repne # Locate 282 scasb # type 283 /* 284 * Get the matching element in the next array. 285 * The byte at $(TLEN-1)(%di) contains the offset of the description 286 * string from %di, so we add the number and print the string. 287 */ 288 addw $(TLEN-1), %di # Adjust 289 movb (%di),%cl # Partition 290 addw %cx,%di # description 291 callw putx # Display it 292 293next_entry: incw %dx # Next item 294 addb $0x10,%bl # Next entry 295 jnc read_entry # Till done 296 /* 297 * We are past a 256 byte boundary: the partition table is finished. 298 * Add one to the drive number and check it is valid. 299 * Note that if we started from a floppy, %dl was 0 so we still 300 * get an entry for the next drive, which is the first Hard Disk. 301 */ 302 popw %ax # Drive number 303 subb $0x80-0x1,%al # Does next 304 cmpb NHRDRV,%al # drive exist? (from BIOS?) 305 jb print_drive # Yes 306 /* 307 * If this is the only drive, don't display it as an option. 308 */ 309 decw %ax # Already drive 0? 310 jz print_prompt # Yes 311 /* 312 * If it was illegal or we cycled through them, go back to drive 0. 313 */ 314 xorb %al,%al # Drive 0 315 /* 316 * Whatever drive we selected, make it an ascii digit and save it 317 * back to the "nxtdrv" location in case we want to save it to disk. 318 * This digit is also part of the printed drive string, so add 0x80 319 * to indicate end of string. 320 */ 321print_drive: addb $'0'|0x80,%al # Save next 322 movb %al,_NXTDRV(%bp) # drive number 323 movw $drive,%di # Display 324 callw putx # item 325 /* 326 * Menu is complete, display a prompt followed by current selection. 327 * 'decw %si' makes the register point to the space after 'Boot: ' 328 * so we do not see an extra CRLF on the screen. 329 */ 330print_prompt: movw $prompt,%si # Display 331 callw putstr # prompt 332 movb _OPT(%bp),%dl # Display 333 decw %si # default 334 callw putkey # key 335 jmp start_input # Skip beep 336 337/* 338 * Here we have the code waiting for user input or a timeout. 339 */ 340beep: movb $ASCII_BEL,%al # Input error, print or beep 341 callw putchr 342 343start_input: 344 /* 345 * Actual Start of input loop. Take note of time 346 */ 347 xorb %ah,%ah # BIOS: Get 348 int $0x1a # system time 349 movw %dx,%di # Ticks when 350 addw _TICKS(%bp),%di # timeout 351read_key: 352 /* 353 * Busy loop, looking for keystrokes but keeping one eye on the time. 354 */ 355#ifndef SIO 356 movb $0x1,%ah # BIOS: Check 357 int $0x16 # for keypress 358#else /* SIO */ 359 movb $0x03,%ah # BIOS: Read COM 360 call bioscom 361 testb $0x01,%ah # Check line status 362 # (bit 1 indicates input) 363#endif /* SIO */ 364 jnz got_key # Have input 365 xorb %ah,%ah # BIOS: int 0x1a, 00 366 int $0x1a # get system time 367 cmpw %di,%dx # Timeout? 368 jb read_key # No 369 370 /* 371 * Timed out or default selection 372 */ 373use_default: movb _OPT(%bp),%al # Load default 374 orb $NOUPDATE,_FLAGS(%bp) # Disable updates 375 jmp check_selection # Join common code 376 377 /* 378 * Get the keystroke. 379 * ENTER or CR confirm the current selection (same as a timeout). 380 * Otherwise convert F1..F6 (or '1'..'6') to 0..5 and check if the 381 * selection is valid. 382 * The SIO code uses ascii chars, the console code uses scancodes. 383 */ 384got_key: 385#ifndef SIO 386 xorb %ah,%ah # BIOS: int 0x16, 00 387 int $0x16 # get keypress 388 movb %ah,%al # move scan code to %al 389 cmpb $KEY_ENTER,%al 390#else 391 movb $0x02,%ah # BIOS: Receive 392 call bioscom 393 cmpb $ASCII_CR,%al 394#endif 395 je use_default # enter -> default 396 /* 397 * Check if the key is acceptable, and loop back if not. 398 * The console (non-SIO) code looks at scancodes and accepts 399 * both F1..F6 and 1..6 (the latter costs 6 bytes of code), 400 * relying on the fact that F1..F6 have higher scancodes than 1..6 401 * The SIO code only takes 1..6 402 */ 403#ifdef SIO /* SIO mode, use ascii values */ 404 subb $'1',%al # Subtract '1' ascii code 405#else /* console mode -- use scancodes */ 406 subb $KEY_F1,%al /* Subtract F1 scan code */ 407#if !defined(ONLY_F_KEYS) 408 cmpb $0x5,%al # F1..F6 409 jna 3f # Yes 410 subb $(KEY_1 - KEY_F1),%al # Less #1 scan code 411 3: 412#endif /* ONLY_F_KEYS */ 413#endif /* SIO */ 414check_selection: 415 cmpb $0x5,%al # F1..F6 or 1..6 ? 416#ifdef PXE /* enable PXE/INT18 using F6 */ 417 jne 1f; 418 int $0x18 # found F6, try INT18 419 1: 420#endif /* PXE */ 421 jae beep # Not in F1..F5, beep 422 423 /* 424 * We have a selection. If it's a bad selection go back to complain. 425 * The bits in MNUOPT were set when the options were printed. 426 * Anything not printed is not an option. 427 */ 428 cbtw # Extend (%ah=0 used later) 429 btw %ax,_MNUOPT(%bp) # Option enabled? 430 jnc beep # No 431 /* 432 * Save the info in the original tables 433 * for rewriting to the disk. 434 */ 435 movb %al,_OPT(%bp) # Save option 436 437 /* 438 * Make %si and %bx point to the fake partition at LBA 0 (CHS 0:0:1). 439 * Because the correct address is already in %bp, just use it. 440 * Set %dl with the drive number saved in byte 0. 441 * If we have pressed F5 or 5, then this is a good, fake value 442 * to present to the next stage boot code. 443 */ 444 movw %bp,%si # Partition for write 445 movb (%si),%dl # Drive number, saved above 446 movw %si,%bx # Partition for read 447 cmpb $0x4,%al # F5/#5 pressed? 448 pushf # Save results for later 449 je 1f # Yes, F5 450 451 /* 452 * F1..F4 was pressed, so make %bx point to the currently 453 * selected partition, and leave the drive number unchanged. 454 */ 455 shlb $0x4,%al # Point to 456 addw $partbl,%ax # selected 457 xchgw %bx,%ax # partition 458 movb $0x80,(%bx) # Flag active 459 /* 460 * If not asked to do a write-back (flags 0x40) don't do one. 461 * Around the call, save the partition pointer to %bx and 462 * restore to %si which is where the next stage expects it. 463 */ 464 1: pushw %bx # Save 465 testb $NOUPDATE,_FLAGS(%bp) # No updates? 466 jnz 2f # skip update 467 movw $start,%bx # Data to write 468 movb $0x3,%ah # Write sector 469 callw intx13 # to disk 470 2: popw %si # Restore 471 472 /* 473 * If going to next drive, replace drive with selected one. 474 * Remember to un-ascii it. Hey 0x80 is already set, cool! 475 */ 476 popf # Restore %al test results 477 jne 3f # If not F5/#5 478 movb _NXTDRV(%bp),%dl # Next drive 479 subb $'0',%dl # number 480 /* 481 * Load selected bootsector to the LOAD location in RAM. If read 482 * fails or there is no 0x55aa marker, treat it as a bad selection. 483 */ 484 3: movw $LOAD,%bx # Address for read 485 movb $0x2,%ah # Read sector 486 callw intx13 # from disk 487 jc beep # If error 488 cmpw $MAGIC,0x1fe(%bx) # Bootable? 489 jne beep # No 490 pushw %si # Save ptr to selected part. 491 callw putn # Leave some space 492 popw %si # Restore, next stage uses it 493 jmp *%bx # Invoke bootstrap 494 495/* 496 * Display routines 497 * putkey prints the option selected in %dl (F1..F5 or 1..5) followed by 498 * the string at %si 499 * putx: print the option in %dl followed by the string at %di 500 * also record the drive as valid. 501 * putn: print a crlf 502 * putstr: print the string at %si 503 * putchr: print the char in al 504 */ 505 506/* 507 * Display the option and record the drive as valid in the options. 508 * That last point is done using the btsw instruction which does 509 * a test and set. We don't care for the test part. 510 */ 511putx: btsw %dx,_MNUOPT(%bp) # Enable menu option 512 movw $item,%si # Display 513 callw putkey # key 514 movw %di,%si # Display the rest 515 callw putstr # Display string 516 517putn: movw $crlf,%si # To next line 518 jmp putstr 519 520putkey: 521#ifndef SIO 522 movb $'F',%al # Display 523 callw putchr # 'F' 524#endif 525 movb $'1',%al # Prepare 526 addb %dl,%al # digit 527 528putstr.1: callw putchr # Display char 529putstr: lodsb # Get byte 530 testb $0x80,%al # End of string? 531 jz putstr.1 # No 532 andb $~0x80,%al # Clear MSB then print last 533 534putchr: 535#ifndef SIO 536 pushw %bx # Save 537 movw $0x7,%bx # Page:attribute 538 movb $0xe,%ah # BIOS: Display 539 int $0x10 # character 540 popw %bx # Restore 541#else /* SIO */ 542 movb $0x01,%ah # BIOS: Send character 543bioscom: 544 pushw %dx # Save 545 xorw %dx,%dx # Use COM1 546 int $0x14 # BIOS: Serial I/O 547 popw %dx # Restore 548#endif /* SIO */ 549 retw # To caller 550 551/* One-sector disk I/O routine */ 552 553/* 554 * %dl: drive, %si partition entry, %es:%bx transfer buffer. 555 * Load the CHS values and possibly the LBA address from the block 556 * at %si, and use the appropriate method to load the sector. 557 * Don't use packet mode for a floppy. 558 */ 559intx13: # Prepare CHS parameters 560 movb 0x1(%si),%dh # Load head 561 movw 0x2(%si),%cx # Load cylinder:sector 562 movb $0x1,%al # Sector count 563 pushw %si # Save 564 movw %sp,%di # Save 565#ifndef CHECK_DRIVE /* floppy support */ 566 testb %dl, %dl # is this a floppy ? 567 jz 1f # Yes, use CHS mode 568#endif 569 testb $USEPACKET,_FLAGS(%bp) # Use packet interface? 570 jz 1f # No 571 pushl $0x0 # Set the 572 pushl 0x8(%si) # LBA address 573 pushw %es # Set the transfer 574 pushw %bx # buffer address 575 push $0x1 # Block count 576 push $0x10 # Packet size 577 movw %sp,%si # Packet pointer 578 decw %ax # Verify off 579 orb $0x40,%ah # Use disk packet 580 1: int $0x13 # BIOS: Disk I/O 581 movw %di,%sp # Restore 582 popw %si # Restore 583 retw # To caller 584 585/* 586 * Various menu strings. 'item' goes after 'prompt' to save space. 587 * Also use shorter versions to make room for the PXE/INT18 code. 588 */ 589prompt: 590#ifdef PXE 591 .ascii "\nF6 PXE\r" 592#endif 593 .ascii "\nBoot:" 594item: .ascii " "; .byte ' '|0x80 595crlf: .ascii "\r"; .byte '\n'|0x80 596 597/* Partition type tables */ 598 599bootable_ids: 600 /* 601 * These values indicate bootable types we know about. 602 * Corresponding descriptions are at desc_ofs: 603 * Entries don't need to be sorted. 604 */ 605 .byte 0x83, 0xa5, 0xa6, 0xa9, 0x06, 0x07, 0x0b 606#ifndef SAVE_MORE_MEMORY 607 .byte 0x05 # extended partition 608#endif 609#ifndef SAVE_MEMORY /* other DOS partitions */ 610 .byte 0x01 # FAT12 611 .byte 0x04 # FAT16 < 32M 612#endif 613 614desc_ofs: 615 /* 616 * Offsets that match the known types above, used to point to the 617 * actual partition name. The last entry must point to os_misc, 618 * which is used for non-matching names. 619 */ 620 .byte os_linux-. # 131, Linux 621 .byte os_freebsd-. # 165, FreeBSD 622 .byte os_bsd-. # 166, OpenBSD 623 .byte os_bsd-. # 169, NetBSD 624 .byte os_dos-. # 6, FAT16 >= 32M 625 .byte os_win-. # 7, NTFS 626 .byte os_win-. # 11, FAT32 627 628#ifndef SAVE_MORE_MEMORY 629 .byte os_ext-. # 5, DOS Ext 630#endif 631#ifndef SAVE_MEMORY 632 .byte os_dos-. # 1, FAT12 DOS 633 .byte os_dos-. # 4, FAT16 <32M 634#endif 635 .byte os_misc-. # Unknown 636 637 /* 638 * And here are the strings themselves. The last byte of 639 * the string has bit 7 set. 640 */ 641os_misc: .byte '?'|0x80 642os_dos: 643#ifndef SAVE_MORE_MEMORY /* 'DOS' remapped to 'WIN' if no room */ 644 .ascii "DO"; .byte 'S'|0x80 645#endif 646os_win: .ascii "Wi"; .byte 'n'|0x80 647os_linux: .ascii "Linu"; .byte 'x'|0x80 648os_freebsd: .ascii "Free" 649os_bsd: .ascii "BS"; .byte 'D'|0x80 650#ifndef SAVE_MORE_MEMORY 651os_ext: .ascii "EX"; .byte 'T'|0x80 652#endif 653 654 .org (0x200 + B0_OFF),0x90 655/* 656 * The boot0 version 1.0 parameter table. 657 * Do not move it nor change the "Drive " string, boot0cfg 658 * uses its offset and content to identify the boot sector. 659 * The other fields are sometimes changed before writing back to the drive 660 * Be especially careful that nxtdrv: must come after drive:, as it 661 * is part of the same string. 662 */ 663drive: .ascii "Drive " 664nxtdrv: .byte 0x0 # Next drive number 665opt: .byte 0x0 # Option 666setdrv_num: .byte 0x80 # Drive to force 667flags: .byte FLAGS # Flags 668#ifdef VOLUME_SERIAL 669 .byte 0xa8,0xa8,0xa8,0xa8 # Volume Serial Number 670#endif 671ticks: .word TICKS # Delay 672 673 .org PRT_OFF 674/* 675 * Here is the 64 byte partition table that fdisk would fiddle with. 676 */ 677partbl: .fill 0x40,0x1,0x0 # Partition table 678 .word MAGIC # Magic number 679 .org 0x200 # again, safety check 680endblock: 681