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