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  * $FreeBSD: release/10.0.0/sys/boot/i386/boot0/boot0.S 228738 2011-12-20 15:19:29Z jhb $ 18  */ 19  20 /* build options: */ 21 #ifdef SIO        /* use serial console on COM1.    */ 22 #endif 23  24 #ifdef PXE        /* enable PXE/INT18 booting with F6 */ 25 #define SAVE_MORE_MEMORY 26 #endif 27  28 #ifdef CHECK_DRIVE    /* make sure we boot from a HD. */ 29 #endif 30  31 #ifdef ONLY_F_KEYS    /* Only F1..F6, no digits on console */ 32 #endif 33  34 #ifdef VOLUME_SERIAL    /* support Volume serial number */ 35 #define B0_BASE    0x1ae    /* move the internal data area */ 36 #define SAVE_MEMORY 37 #else 38 #define B0_BASE    0x1b2 39 #endif 40  41 #ifdef TEST        /* enable some test code */ 42 #define SAVE_MEMORY 43 #define SAVE_MORE_MEMORY 44 #endif 45  46 /* 47  * Note - this code uses many tricks to save space and fit in one sector. 48  * This includes using side effects of certain instructions, reusing 49  * register values from previous operations, etc. 50  * Be extremely careful when changing the code, even for simple things. 51  */ 52  53 /* 54  *        BOOT BLOCK STRUCTURE 55  * 56  * This code implements a Master Boot Record (MBR) for an Intel/PC disk. 57  * It is 512 bytes long and it is normally loaded by the BIOS (or another 58  * bootloader) at 0:0x7c00. This code depends on %cs:%ip being 0:0x7c00 59  * 60  * The initial chunk of instructions is used as a signature by external 61  * tools (e.g. boot0cfg) which can manipulate the block itself. 62  * 63  * The area at offset 0x1b2 contains a magic string ('Drive '), also 64  * used as a signature to detect the block, and some variables that can 65  * be updated by boot0cfg (and optionally written back to the disk). 66  * These variables control the operation of the bootloader itself, 67  * e.g. which partitions to enable, the timeout, the use of LBA 68  * (called 'packet') or CHS mode, whether to force a drive number, 69  * and whether to write back the user's selection back to disk. 70  * 71  * As in every Master Boot Record, the partition table is at 0x1be, 72  * made of four 16-byte entries each containing: 73  * 74  *   OFF SIZE    DESCRIPTION 75  *    0      1    status (0x80: bootable, 0: non bootable) 76  *    1      3    start sector CHS 77  *           8:head, 6:sector, 2:cyl bit 9..8, 8:cyl bit 7..0 78  *    4   1    partition type 79  *    5   3    end sector CHS 80  *    8   4    LBA of first sector 81  *   12   4    partition size in sectors 82  * 83  * and followed by the two bytes 0x55, 0xAA (MBR signature). 84  */ 85  86  87 /* 88  *        BOOT BLOCK OPERATION 89  * 90  * On entry, the registers contain the following values: 91  * 92  *    %cs:%ip    0:0x7c00 93  *    %dl    drive number (0x80, 0x81, ... ) 94  *    %si    pointer to the partition table from which we were loaded. 95  *        Some boot code (e.g. syslinux) use this info to relocate 96  *        themselves, so we want to pass a valid one to the next stage. 97  *        NOTE: the use of %si is not a standard. 98  * 99  * This boot block first relocates itself at a different address (0:0x600),100  * to free the space at 0:0x7c00 for the next stage boot block.101  *102  * It then initializes some memory at 0:0x800 and above (pointed by %bp)103  * to store the original drive number (%dl) passed to us, and to construct a104  * fake partition entry. The latter is used by the disk I/O routine and,105  * in some cases, passed in %si to the next stage boot code.106  *107  * The variables at 0x1b2 are accessed as negative offsets from %bp.108  *109  * After the relocation, the code scans the partition table printing110  * out enabled partition or disks, and waits for user input.111  *112  * When a partition is selected, or a timeout expires, the currently113  * selected partition is used to load the next stage boot code,114  * %dl and %si are set appropriately as when we were called, and115  * control is transferred to the newly loaded code at 0:0x7c00.116  */117 118 /*119  *    CONSTANTS120  *121  * NHRDRV is the address in segment 0 where the BIOS writes the122  *    total number of hard disks in the system.123  * LOAD is the original load address and cannot be changed.124  * ORIGIN is the relocation address. If you change it, you also need125  *     to change the value passed to the linker in the Makefile126  * PRT_OFF is the location of the partition table (from the MBR standard).127  * B0_OFF is the location of the data area, known to boot0cfg so128  *    it cannot be changed. Computed as a negative offset from 0x200129  * MAGIC is the signature of a boot block.130  */131 132         .set NHRDRV,0x475        # Number of hard drives133         .set ORIGIN,0x600        # Execution address134         .set LOAD,0x7c00        # Load address135 136         .set PRT_OFF,0x1be        # Partition table137         .set B0_OFF,(B0_BASE-0x200)    # Offset of boot0 data138 139         .set MAGIC,0xaa55        # Magic: bootable140 141         .set KEY_ENTER,0x1c        # Enter key scan code142         .set KEY_F1,0x3b        # F1 key scan code143         .set KEY_1,0x02            # #1 key scan code144 145         .set ASCII_BEL,'#'        # ASCII code for 
     
      146         .set ASCII_CR,0x0D        # ASCII code for 
      
       147 148 /*149  * Offsets of variables in the block at B0_OFF, and in the volatile150  * data area, computed as displacement from %bp.151  * We need to define them as constant as the assembler cannot152  * compute them in its single pass.153  */154         .set _NXTDRV,    B0_OFF+6    # Next drive155         .set _OPT,    B0_OFF+7    # Default option156         .set _SETDRV,    B0_OFF+8    # Drive to force157         .set _FLAGS,    B0_OFF+9    # Flags158         .set SETDRV,    0x20        # the 'setdrv' flag159         .set NOUPDATE,    0x40        # the 'noupdate' flag160         .set USEPACKET,    0x80        # the 'packet' flag161 162     /* ticks is at a fixed position */163         .set _TICKS,    (PRT_OFF - 0x200 - 2)    # Timeout ticks164         .set _MNUOPT, 0x10        # Saved menu entries165 166         .set TLEN, (desc_ofs - bootable_ids)    # size of bootable ids167         .globl start            # Entry point168         .code16                # This runs in real mode169 170 /*171  *     MAIN ENTRY POINT172  * Initialise segments and registers to known values.173  * segments start at 0.174  * The stack is immediately below the address we were loaded to.175  * NOTE: the initial section of the code (up to movw $LOAD,%sp)176  * is used by boot0cfg, together with the 'Drive ' string and177  * the 0x55, 0xaa at the end, as an identifier for version 1.0178  * of the boot code. Do not change it.179  * In version 1.0 the parameter table (_NEXTDRV etc) is at 0x1b9180  */181 start:        cld                # String ops inc182         xorw %ax,%ax            # Zero183         movw %ax,%es            # Address184         movw %ax,%ds            #  data185         movw %ax,%ss            # Set up186         movw $LOAD,%sp            #  stack187 188     /*189      * Copy this code to the address it was linked for, 0x600 by default.190      */191         movw %sp,%si            # Source192         movw $start,%di            # Destination193         movw $0x100,%cx            # Word count194         rep                # Relocate195         movsw                #  code196     /*197      * After the code, (i.e. at %di+0, 0x800) create a partition entry,198      * initialized to LBA 0 / CHS 0:0:1.199      * Set %bp to point to the partition and also, with negative offsets,200      * to the variables embedded in the bootblock (nextdrv and so on).201      */202         movw %di,%bp            # Address variables203         movb $0x8,%cl            # Words to clear204         rep                # Zero205         stosw                #  them206         incb -0xe(%di)            # Set the S field to 1207 208         jmp main-LOAD+ORIGIN        # Jump to relocated code209 210 main:211 #if defined(SIO) && COMSPEED != 0212     /*213      * Init the serial port. bioscom preserves the driver number in DX.214      */215         movw $COMSPEED,%ax        # defined by Makefile216         callw bioscom217 #endif218 219     /*220      * If the 'setdrv' flag is set in the boot sector, use the drive221      * number from the boot sector at 'setdrv_num'.222      * Optionally, do the same if the BIOS gives us an invalid number223      * (note though that the override prevents booting from a floppy224      * or a ZIP/flash drive in floppy emulation).225      * The test costs 4 bytes of code so it is disabled by default.226      */227         testb $SETDRV,_FLAGS(%bp)    # Set drive number?228 #ifndef CHECK_DRIVE    /* disable drive checks */229         jz save_curdrive        # no, use the default230 #else231         jnz disable_update        # Yes232         testb %dl,%dl            # Drive number valid?233         js save_curdrive        # Possibly (0x80 set)234 #endif235     /*236      * Disable updates if the drive number is forced.237      */238 disable_update:    orb $NOUPDATE,_FLAGS(%bp)    # Disable updates239         movb _SETDRV(%bp),%dl        # Use stored drive number240 241     /*242      * Whatever drive we decided to use, store it at (%bp). The byte243      * is normally used for the state of the partition (0x80 or 0x00),244      * but we abuse it as it is very convenient to access at offset 0.245      * The value is read back after 'check_selection'246      */247 save_curdrive:    movb %dl, (%bp)            # Save drive number248         pushw %dx            # Also in the stack249 #ifdef    TEST    /* test code, print internal bios drive */250         rolb $1, %dl251         movw $drive, %si252         call putkey253 #endif254         callw putn            # Print a newline255     /*256      * Start out with a pointer to the 4th byte of the first table entry257      * so that after 4 iterations it's beyond the end of the sector258      * and beyond a 256 byte boundary. We use the latter trick to check for259      * end of the loop without using an extra register (see start.5).260      */261         movw $(partbl+0x4),%bx        # Partition table (+4)262         xorw %dx,%dx            # Item number263 264     /*265      * Loop around on the partition table, printing values until we266      * pass a 256 byte boundary.267      */268 read_entry:    movb %ch,-0x4(%bx)        # Zero active flag (ch == 0)269         btw %dx,_FLAGS(%bp)        # Entry enabled?270         jnc next_entry            # No271         movb (%bx),%al            # Load type272         test %al, %al            # skip empty partition273         jz next_entry274     /*275      * Scan the table of bootable ids, which starts at %di and has276      * length TLEN. On a match, %di points to the element following the277      * match; the corresponding offset to the description is $(TLEN-1)278      * bytes ahead. We use a count of TLEN+1 so if we don't find a match279      * within the first TLEN entries, we hit the 'unknown' entry.280      */281         movw $bootable_ids,%di        # Lookup tables282         movb $(TLEN+1),%cl        # Number of entries283         repne                # Locate284         scasb                #  type285     /*286      * Get the matching element in the next array.287      * The byte at $(TLEN-1)(%di) contains the offset of the description288      * string from %di, so we add the number and print the string.289      */290         addw $(TLEN-1), %di        # Adjust291         movb (%di),%cl            # Partition292         addw %cx,%di            #  description293         callw putx            # Display it294 295 next_entry:    incw %dx            # Next item296         addb $0x10,%bl            # Next entry297         jnc read_entry            # Till done298     /*299      * We are past a 256 byte boundary: the partition table is finished.300      * Add one to the drive number and check it is valid.301      * Note that if we started from a floppy, %dl was 0 so we still302      * get an entry for the next drive, which is the first Hard Disk.303      */304         popw %ax            # Drive number305         subb $0x80-0x1,%al        # Does next306         cmpb NHRDRV,%al            #  drive exist? (from BIOS?)307         jb print_drive            # Yes308     /*309      * If this is the only drive, don't display it as an option.310      */311         decw %ax            # Already drive 0?312         jz print_prompt            # Yes313     /*314      * If it was illegal or we cycled through them, go back to drive 0.315      */316         xorb %al,%al            # Drive 0317     /*318      * Whatever drive we selected, make it an ascii digit and save it319      * back to the "nxtdrv" location in case we want to save it to disk.320      * This digit is also part of the printed drive string, so add 0x80321      * to indicate end of string.322      */323 print_drive:    addb $'0'|0x80,%al        # Save next324         movb %al,_NXTDRV(%bp)        #  drive number325         movw $drive,%di            # Display326         callw putx            #  item327     /*328      * Menu is complete, display a prompt followed by current selection.329      * 'decw %si' makes the register point to the space after 'Boot: '330      * so we do not see an extra CRLF on the screen.331      */332 print_prompt:    movw $prompt,%si        # Display333         callw putstr            #  prompt334         movb _OPT(%bp),%dl        # Display335         decw %si            #  default336         callw putkey            #  key337         jmp start_input            # Skip beep338 339 /*340  * Here we have the code waiting for user input or a timeout.341  */342 beep:        movb $ASCII_BEL,%al        # Input error, print or beep343         callw putchr344 345 start_input:346     /*347      * Actual Start of input loop.  Take note of time348      */349         xorb %ah,%ah            # BIOS: Get350         int $0x1a            #  system time351         movw %dx,%di            # Ticks when352         addw _TICKS(%bp),%di        #  timeout353 read_key:354     /*355      * Busy loop, looking for keystrokes but keeping one eye on the time.356      */357 #ifndef SIO358         movb $0x1,%ah            # BIOS: Check359         int $0x16            #  for keypress360 #else /* SIO */361         movb $0x03,%ah            # BIOS: Read COM362         call bioscom363         testb $0x01,%ah            # Check line status364                         # (bit 1 indicates input)365 #endif /* SIO */366         jnz got_key             # Have input367         xorb %ah,%ah            # BIOS: int 0x1a, 00368         int $0x1a            #  get system time369         cmpw %di,%dx            # Timeout?370         jb read_key            # No371 372     /*373      * Timed out or default selection374      */375 use_default:    movb _OPT(%bp),%al        # Load default376         orb $NOUPDATE,_FLAGS(%bp)     # Disable updates377         jmp check_selection        # Join common code378 379     /*380      * Get the keystroke.381      * ENTER or CR confirm the current selection (same as a timeout).382      * Otherwise convert F1..F6 (or '1'..'6') to 0..5 and check if the383      * selection is valid.384      * The SIO code uses ascii chars, the console code uses scancodes.385      */386 got_key:387 #ifndef SIO388         xorb %ah,%ah            # BIOS: int 0x16, 00389         int $0x16            #  get keypress390         movb %ah,%al            # move scan code to %al391         cmpb $KEY_ENTER,%al392 #else393         movb $0x02,%ah            # BIOS: Receive394         call bioscom395         cmpb $ASCII_CR,%al396 #endif397         je use_default            # enter -> default398     /*399      * Check if the key is acceptable, and loop back if not.400      * The console (non-SIO) code looks at scancodes and accepts401      * both F1..F6 and 1..6 (the latter costs 6 bytes of code),402      * relying on the fact that F1..F6 have higher scancodes than 1..6403      * The SIO code only takes 1..6404      */405 #ifdef SIO /* SIO mode, use ascii values */406         subb $'1',%al            # Subtract '1' ascii code407 #else /*  console mode -- use scancodes */408         subb $KEY_F1,%al        /* Subtract F1 scan code */409 #if !defined(ONLY_F_KEYS)410         cmpb $0x5,%al            # F1..F6411         jna 3f                # Yes412         subb $(KEY_1 - KEY_F1),%al    # Less #1 scan code413     3:414 #endif /* ONLY_F_KEYS */415 #endif /* SIO */416 check_selection:417         cmpb $0x5,%al            # F1..F6 or 1..6 ?418 #ifdef PXE /* enable PXE/INT18 using F6 */419         jne 1f;420         int $0x18            # found F6, try INT18421     1:422 #endif /* PXE */423         jae beep            # Not in F1..F5, beep424 425     /*426      * We have a selection.  If it's a bad selection go back to complain.427      * The bits in MNUOPT were set when the options were printed.428      * Anything not printed is not an option.429      */430         cbtw                # Extend (%ah=0 used later)431         btw %ax,_MNUOPT(%bp)         # Option enabled?432         jnc beep            # No433     /*434      * Save the info in the original tables435      * for rewriting to the disk.436      */437         movb %al,_OPT(%bp)        # Save option438 439     /*440      * Make %si and %bx point to the fake partition at LBA 0 (CHS 0:0:1).441      * Because the correct address is already in %bp, just use it.442      * Set %dl with the drive number saved in byte 0.443      * If we have pressed F5 or 5, then this is a good, fake value444      * to present to the next stage boot code.445      */446         movw %bp,%si            # Partition for write447         movb (%si),%dl            # Drive number, saved above448         movw %si,%bx            # Partition for read449         cmpb $0x4,%al            # F5/#5 pressed?450         pushf                # Save results for later451         je 1f                # Yes, F5452 453     /*454      * F1..F4 was pressed, so make %bx point to the currently455      * selected partition, and leave the drive number unchanged.456      */457         shlb $0x4,%al            # Point to458         addw $partbl,%ax        #  selected459         xchgw %bx,%ax             #  partition460         movb $0x80,(%bx)        # Flag active461     /*462      * If not asked to do a write-back (flags 0x40) don't do one.463      * Around the call, save the partition pointer to %bx and464      * restore to %si which is where the next stage expects it.465      */466     1:    pushw %bx            # Save467         testb $NOUPDATE,_FLAGS(%bp)    # No updates?468         jnz 2f                # skip update469         movw $start,%bx            # Data to write470         movb $0x3,%ah            # Write sector471         callw intx13            #  to disk472     2:    popw %si            # Restore473 474     /*475      * If going to next drive, replace drive with selected one.476      * Remember to un-ascii it. Hey 0x80 is already set, cool!477      */478         popf                # Restore %al test results479         jne 3f                # If not F5/#5480         movb _NXTDRV(%bp),%dl        # Next drive481         subb $'0',%dl            #  number482     /*483      * Load selected bootsector to the LOAD location in RAM. If read484      * fails or there is no 0x55aa marker, treat it as a bad selection.485      */486     3:    movw $LOAD,%bx            # Address for read487         movb $0x2,%ah            # Read sector488         callw intx13            #  from disk489         jc beep                # If error490         cmpw $MAGIC,0x1fe(%bx)        # Bootable?491         jne beep            # No492         pushw %si            # Save ptr to selected part.493         callw putn            # Leave some space494         popw %si            # Restore, next stage uses it495         jmp *%bx            # Invoke bootstrap496 497 /*498  * Display routines499  * putkey    prints the option selected in %dl (F1..F5 or 1..5) followed by500  *        the string at %si501  * putx:    print the option in %dl followed by the string at %di502  *        also record the drive as valid.503  * putn:    print a crlf504  * putstr:    print the string at %si505  * putchr:    print the char in al506  */507 508 /*509  * Display the option and record the drive as valid in the options.510  * That last point is done using the btsw instruction which does511  * a test and set. We don't care for the test part.512  */513 putx:        btsw %dx,_MNUOPT(%bp)        # Enable menu option514         movw $item,%si            # Display515         callw putkey            #  key516         movw %di,%si            # Display the rest517         callw putstr            # Display string518 519 putn:        movw $crlf,%si            # To next line520         jmp putstr521 522 putkey:523 #ifndef SIO524         movb $'F',%al            # Display525         callw putchr            #  'F'526 #endif527         movb $'1',%al            # Prepare528         addb %dl,%al            #  digit529 530 putstr.1:    callw putchr            # Display char531 putstr:        lodsb                # Get byte532         testb $0x80,%al         # End of string?533         jz putstr.1            # No534         andb $~0x80,%al         # Clear MSB then print last 535 536 putchr:537 #ifndef SIO538         pushw %bx            # Save539         movw $0x7,%bx             # Page:attribute540         movb $0xe,%ah            # BIOS: Display541         int $0x10            #  character542         popw %bx            # Restore543 #else /* SIO */544         movb $0x01,%ah            # BIOS: Send545 bioscom:546         pushw %dx            # Save547         xorw %dx,%dx             # Use COM1548         int $0x14            #  Character549         popw %dx            # Restore550 #endif /* SIO */551         retw                # To caller552 553 /* One-sector disk I/O routine */554 555 /*556  * %dl: drive, %si partition entry, %es:%bx transfer buffer.557  * Load the CHS values and possibly the LBA address from the block558  * at %si, and use the appropriate method to load the sector.559  * Don't use packet mode for a floppy.560  */561 intx13:                        # Prepare CHS parameters562         movb 0x1(%si),%dh        # Load head563         movw 0x2(%si),%cx        # Load cylinder:sector564         movb $0x1,%al            # Sector count565         pushw %si            # Save566         movw %sp,%di            # Save567 #ifndef CHECK_DRIVE                /* floppy support */568         testb %dl, %dl            # is this a floppy ?569         jz 1f                # Yes, use CHS mode570 #endif571         testb $USEPACKET,_FLAGS(%bp)    # Use packet interface?572         jz 1f                # No573         pushl $0x0            # Set the574         pushl 0x8(%si)            # LBA address575         pushw %es            # Set the transfer576         pushw %bx            #  buffer address577         push  $0x1            # Block count578         push  $0x10            # Packet size579         movw %sp,%si            # Packet pointer580         decw %ax            # Verify off581         orb $0x40,%ah            # Use disk packet582     1:    int $0x13            # BIOS: Disk I/O583         movw %di,%sp            # Restore584         popw %si            # Restore585         retw                # To caller586 587 /*588  * Various menu strings. 'item' goes after 'prompt' to save space.589  * Also use shorter versions to make room for the PXE/INT18 code.590  */591 prompt:592 #ifdef PXE593         .ascii "\nF6 PXE\r"594 #endif595         .ascii "\nBoot:"596 item:        .ascii " ";         .byte ' '|0x80597 crlf:        .ascii "\r";         .byte '\n'|0x80598 599 /* Partition type tables */600 601 bootable_ids:602     /*603      * These values indicate bootable types we know about.604      * Corresponding descriptions are at desc_ofs:605      * Entries don't need to be sorted.606      */607         .byte 0x83, 0xa5, 0xa6, 0xa9, 0x06, 0x07, 0x0b608 #ifndef SAVE_MORE_MEMORY609         .byte 0x05    # extended partition610 #endif611 #ifndef SAVE_MEMORY    /* other DOS partitions */612         .byte 0x01    # FAT12613         .byte 0x04    # FAT16 < 32M614 #endif615 616 desc_ofs:617     /*618      * Offsets that match the known types above, used to point to the619      * actual partition name. The last entry must point to os_misc,620      * which is used for non-matching names.621      */622         .byte os_linux-.        # 131, Linux623         .byte os_freebsd-.        # 165, FreeBSD624         .byte os_bsd-.            # 166, OpenBSD625         .byte os_bsd-.            # 169, NetBSD626         .byte os_dos-.            #   6, FAT16 >= 32M627         .byte os_win-.            #   7, NTFS628         .byte os_win-.            #  11, FAT32629 630 #ifndef SAVE_MORE_MEMORY631         .byte os_ext-.            #   5, DOS Ext632 #endif633 #ifndef SAVE_MEMORY634         .byte os_dos-.            #   1, FAT12 DOS635         .byte os_dos-.            #   4, FAT16 <32M636 #endif637         .byte os_misc-.         # Unknown638 639     /*640      * And here are the strings themselves. The last byte of641      * the string has bit 7 set.642      */643 os_misc:    .byte '?'|0x80644 os_dos:645 #ifndef SAVE_MORE_MEMORY    /* 'DOS' remapped to 'WIN' if no room */646         .ascii "DO";   .byte 'S'|0x80647 #endif648 os_win:        .ascii "Wi";   .byte 'n'|0x80649 os_linux:    .ascii "Linu"; .byte 'x'|0x80650 os_freebsd:    .ascii "Free"651 os_bsd:        .ascii "BS";   .byte 'D'|0x80652 #ifndef SAVE_MORE_MEMORY653 os_ext:        .ascii "EX";   .byte 'T'|0x80654 #endif655 656         .org (0x200 + B0_OFF),0x90657 /*658  * The boot0 version 1.0 parameter table.659  * Do not move it nor change the "Drive " string, boot0cfg660  * uses its offset and content to identify the boot sector.661  * The other fields are sometimes changed before writing back to the drive662  * Be especially careful that nxtdrv: must come after drive:, as it663  * is part of the same string.664  */665 drive:        .ascii "Drive "666 nxtdrv:        .byte 0x0            # Next drive number667 opt:        .byte 0x0            # Option668 setdrv_num:    .byte 0x80            # Drive to force669 flags:        .byte FLAGS            # Flags670 #ifdef VOLUME_SERIAL671         .byte 0xa8,0xa8,0xa8,0xa8       # Volume Serial Number672 #endif673 ticks:        .word TICKS            # Delay674 675         .org PRT_OFF676 /*677  * Here is the 64 byte partition table that fdisk would fiddle with.678  */679 partbl:        .fill 0x40,0x1,0x0        # Partition table680         .word MAGIC            # Magic number681         .org 0x200            # again, safety check682 endblock: