FreeBSD Boot0 代码阅读 ！

  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 a
104  * 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 printing
110  * out enabled partition or disks, and waits for user input.
111  *
112  * When a partition is selected, or a timeout expires, the currently
113  * selected partition is used to load the next stage boot code,
114  * %dl and %si are set appropriately as when we were called, and
115  * control is transferred to the newly loaded code at 0:0x7c00.
116  */
117 
118 /*
119  *    CONSTANTS
120  *
121  * NHRDRV is the address in segment 0 where the BIOS writes the
122  *    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 need
125  *     to change the value passed to the linker in the Makefile
126  * 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 so
128  *    it cannot be changed. Computed as a negative offset from 0x200
129  * MAGIC is the signature of a boot block.
130  */
131 
132         .set NHRDRV,0x475        # Number of hard drives
133         .set ORIGIN,0x600        # Execution address
134         .set LOAD,0x7c00        # Load address
135 
136         .set PRT_OFF,0x1be        # Partition table
137         .set B0_OFF,(B0_BASE-0x200)    # Offset of boot0 data
138 
139         .set MAGIC,0xaa55        # Magic: bootable
140 
141         .set KEY_ENTER,0x1c        # Enter key scan code
142         .set KEY_F1,0x3b        # F1 key scan code
143         .set KEY_1,0x02            # #1 key scan code
144 
145         .set ASCII_BEL,‘#‘        # ASCII code for <BEL>
146         .set ASCII_CR,0x0D        # ASCII code for <CR>
147 
148 /*
149  * Offsets of variables in the block at B0_OFF, and in the volatile
150  * data area, computed as displacement from %bp.
151  * We need to define them as constant as the assembler cannot
152  * compute them in its single pass.
153  */
154         .set _NXTDRV,    B0_OFF+6    # Next drive
155         .set _OPT,    B0_OFF+7    # Default option
156         .set _SETDRV,    B0_OFF+8    # Drive to force
157         .set _FLAGS,    B0_OFF+9    # Flags
158         .set SETDRV,    0x20        # the ‘setdrv‘ flag
159         .set NOUPDATE,    0x40        # the ‘noupdate‘ flag
160         .set USEPACKET,    0x80        # the ‘packet‘ flag
161 
162     /* ticks is at a fixed position */
163         .set _TICKS,    (PRT_OFF - 0x200 - 2)    # Timeout ticks
164         .set _MNUOPT, 0x10        # Saved menu entries
165 
166         .set TLEN, (desc_ofs - bootable_ids)    # size of bootable ids
167         .globl start            # Entry point
168         .code16                # This runs in real mode
169 
170 /*
171  *     MAIN ENTRY POINT
172  * 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 and
177  * the 0x55, 0xaa at the end, as an identifier for version 1.0
178  * of the boot code. Do not change it.
179  * In version 1.0 the parameter table (_NEXTDRV etc) is at 0x1b9
180  */
181 start:        cld                # String ops inc
182         xorw %ax,%ax            # Zero
183         movw %ax,%es            # Address
184         movw %ax,%ds            #  data
185         movw %ax,%ss            # Set up
186         movw $LOAD,%sp            #  stack
187 
188     /*
189      * Copy this code to the address it was linked for, 0x600 by default.
190      */
191         movw %sp,%si            # Source
192         movw $start,%di            # Destination
193         movw $0x100,%cx            # Word count
194         rep                # Relocate
195         movsw                #  code
196     /*
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 variables
203         movb $0x8,%cl            # Words to clear
204         rep                # Zero
205         stosw                #  them
206         incb -0xe(%di)            # Set the S field to 1
207 
208         jmp main-LOAD+ORIGIN        # Jump to relocated code
209 
210 main:
211 #if defined(SIO) && COMSPEED != 0
212     /*
213      * Init the serial port. bioscom preserves the driver number in DX.
214      */
215         movw $COMSPEED,%ax        # defined by Makefile
216         callw bioscom
217 #endif
218 
219     /*
220      * If the ‘setdrv‘ flag is set in the boot sector, use the drive
221      * number from the boot sector at ‘setdrv_num‘.
222      * Optionally, do the same if the BIOS gives us an invalid number
223      * (note though that the override prevents booting from a floppy
224      * 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 default
230 #else
231         jnz disable_update        # Yes
232         testb %dl,%dl            # Drive number valid?
233         js save_curdrive        # Possibly (0x80 set)
234 #endif
235     /*
236      * Disable updates if the drive number is forced.
237      */
238 disable_update:    orb $NOUPDATE,_FLAGS(%bp)    # Disable updates
239         movb _SETDRV(%bp),%dl        # Use stored drive number
240 
241     /*
242      * Whatever drive we decided to use, store it at (%bp). The byte
243      * 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 number
248         pushw %dx            # Also in the stack
249 #ifdef    TEST    /* test code, print internal bios drive */
250         rolb $1, %dl
251         movw $drive, %si
252         call putkey
253 #endif
254         callw putn            # Print a newline
255     /*
256      * Start out with a pointer to the 4th byte of the first table entry
257      * so that after 4 iterations it‘s beyond the end of the sector
258      * and beyond a 256 byte boundary. We use the latter trick to check for
259      * 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 number
263 
264     /*
265      * Loop around on the partition table, printing values until we
266      * 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            # No
271         movb (%bx),%al            # Load type
272         test %al, %al            # skip empty partition
273         jz next_entry
274     /*
275      * Scan the table of bootable ids, which starts at %di and has
276      * length TLEN. On a match, %di points to the element following the
277      * 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 match
279      * within the first TLEN entries, we hit the ‘unknown‘ entry.
280      */
281         movw $bootable_ids,%di        # Lookup tables
282         movb $(TLEN+1),%cl        # Number of entries
283         repne                # Locate
284         scasb                #  type
285     /*
286      * Get the matching element in the next array.
287      * The byte at $(TLEN-1)(%di) contains the offset of the description
288      * string from %di, so we add the number and print the string.
289      */
290         addw $(TLEN-1), %di        # Adjust
291         movb (%di),%cl            # Partition
292         addw %cx,%di            #  description
293         callw putx            # Display it
294 
295 next_entry:    incw %dx            # Next item
296         addb $0x10,%bl            # Next entry
297         jnc read_entry            # Till done
298     /*
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 still
302      * get an entry for the next drive, which is the first Hard Disk.
303      */
304         popw %ax            # Drive number
305         subb $0x80-0x1,%al        # Does next
306         cmpb NHRDRV,%al            #  drive exist? (from BIOS?)
307         jb print_drive            # Yes
308     /*
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            # Yes
313     /*
314      * If it was illegal or we cycled through them, go back to drive 0.
315      */
316         xorb %al,%al            # Drive 0
317     /*
318      * Whatever drive we selected, make it an ascii digit and save it
319      * 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 0x80
321      * to indicate end of string.
322      */
323 print_drive:    addb $‘0‘|0x80,%al        # Save next
324         movb %al,_NXTDRV(%bp)        #  drive number
325         movw $drive,%di            # Display
326         callw putx            #  item
327     /*
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        # Display
333         callw putstr            #  prompt
334         movb _OPT(%bp),%dl        # Display
335         decw %si            #  default
336         callw putkey            #  key
337         jmp start_input            # Skip beep
338 
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 beep
343         callw putchr
344 
345 start_input:
346     /*
347      * Actual Start of input loop.  Take note of time
348      */
349         xorb %ah,%ah            # BIOS: Get
350         int $0x1a            #  system time
351         movw %dx,%di            # Ticks when
352         addw _TICKS(%bp),%di        #  timeout
353 read_key:
354     /*
355      * Busy loop, looking for keystrokes but keeping one eye on the time.
356      */
357 #ifndef SIO
358         movb $0x1,%ah            # BIOS: Check
359         int $0x16            #  for keypress
360 #else /* SIO */
361         movb $0x03,%ah            # BIOS: Read COM
362         call bioscom
363         testb $0x01,%ah            # Check line status
364                         # (bit 1 indicates input)
365 #endif /* SIO */
366         jnz got_key             # Have input
367         xorb %ah,%ah            # BIOS: int 0x1a, 00
368         int $0x1a            #  get system time
369         cmpw %di,%dx            # Timeout?
370         jb read_key            # No
371 
372     /*
373      * Timed out or default selection
374      */
375 use_default:    movb _OPT(%bp),%al        # Load default
376         orb $NOUPDATE,_FLAGS(%bp)     # Disable updates
377         jmp check_selection        # Join common code
378 
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 the
383      * selection is valid.
384      * The SIO code uses ascii chars, the console code uses scancodes.
385      */
386 got_key:
387 #ifndef SIO
388         xorb %ah,%ah            # BIOS: int 0x16, 00
389         int $0x16            #  get keypress
390         movb %ah,%al            # move scan code to %al
391         cmpb $KEY_ENTER,%al
392 #else
393         movb $0x02,%ah            # BIOS: Receive
394         call bioscom
395         cmpb $ASCII_CR,%al
396 #endif
397         je use_default            # enter -> default
398     /*
399      * Check if the key is acceptable, and loop back if not.
400      * The console (non-SIO) code looks at scancodes and accepts
401      * 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..6
403      * The SIO code only takes 1..6
404      */
405 #ifdef SIO /* SIO mode, use ascii values */
406         subb $‘1‘,%al            # Subtract ‘1‘ ascii code
407 #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..F6
411         jna 3f                # Yes
412         subb $(KEY_1 - KEY_F1),%al    # Less #1 scan code
413     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 INT18
421     1:
422 #endif /* PXE */
423         jae beep            # Not in F1..F5, beep
424 
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            # No
433     /*
434      * Save the info in the original tables
435      * for rewriting to the disk.
436      */
437         movb %al,_OPT(%bp)        # Save option
438 
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 value
444      * to present to the next stage boot code.
445      */
446         movw %bp,%si            # Partition for write
447         movb (%si),%dl            # Drive number, saved above
448         movw %si,%bx            # Partition for read
449         cmpb $0x4,%al            # F5/#5 pressed?
450         pushf                # Save results for later
451         je 1f                # Yes, F5
452 
453     /*
454      * F1..F4 was pressed, so make %bx point to the currently
455      * selected partition, and leave the drive number unchanged.
456      */
457         shlb $0x4,%al            # Point to
458         addw $partbl,%ax        #  selected
459         xchgw %bx,%ax             #  partition
460         movb $0x80,(%bx)        # Flag active
461     /*
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 and
464      * restore to %si which is where the next stage expects it.
465      */
466     1:    pushw %bx            # Save
467         testb $NOUPDATE,_FLAGS(%bp)    # No updates?
468         jnz 2f                # skip update
469         movw $start,%bx            # Data to write
470         movb $0x3,%ah            # Write sector
471         callw intx13            #  to disk
472     2:    popw %si            # Restore
473 
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 results
479         jne 3f                # If not F5/#5
480         movb _NXTDRV(%bp),%dl        # Next drive
481         subb $‘0‘,%dl            #  number
482     /*
483      * Load selected bootsector to the LOAD location in RAM. If read
484      * fails or there is no 0x55aa marker, treat it as a bad selection.
485      */
486     3:    movw $LOAD,%bx            # Address for read
487         movb $0x2,%ah            # Read sector
488         callw intx13            #  from disk
489         jc beep                # If error
490         cmpw $MAGIC,0x1fe(%bx)        # Bootable?
491         jne beep            # No
492         pushw %si            # Save ptr to selected part.
493         callw putn            # Leave some space
494         popw %si            # Restore, next stage uses it
495         jmp *%bx            # Invoke bootstrap
496 
497 /*
498  * Display routines
499  * putkey    prints the option selected in %dl (F1..F5 or 1..5) followed by
500  *        the string at %si
501  * putx:    print the option in %dl followed by the string at %di
502  *        also record the drive as valid.
503  * putn:    print a crlf
504  * putstr:    print the string at %si
505  * putchr:    print the char in al
506  */
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 does
511  * a test and set. We don‘t care for the test part.
512  */
513 putx:        btsw %dx,_MNUOPT(%bp)        # Enable menu option
514         movw $item,%si            # Display
515         callw putkey            #  key
516         movw %di,%si            # Display the rest
517         callw putstr            # Display string
518 
519 putn:        movw $crlf,%si            # To next line
520         jmp putstr
521 
522 putkey:
523 #ifndef SIO
524         movb $‘F‘,%al            # Display
525         callw putchr            #  ‘F‘
526 #endif
527         movb $‘1‘,%al            # Prepare
528         addb %dl,%al            #  digit
529 
530 putstr.1:    callw putchr            # Display char
531 putstr:        lodsb                # Get byte
532         testb $0x80,%al         # End of string?
533         jz putstr.1            # No
534         andb $~0x80,%al         # Clear MSB then print last 
535 
536 putchr:
537 #ifndef SIO
538         pushw %bx            # Save
539         movw $0x7,%bx             # Page:attribute
540         movb $0xe,%ah            # BIOS: Display
541         int $0x10            #  character
542         popw %bx            # Restore
543 #else /* SIO */
544         movb $0x01,%ah            # BIOS: Send
545 bioscom:
546         pushw %dx            # Save
547         xorw %dx,%dx             # Use COM1
548         int $0x14            #  Character
549         popw %dx            # Restore
550 #endif /* SIO */
551         retw                # To caller
552 
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 block
558  * 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 parameters
562         movb 0x1(%si),%dh        # Load head
563         movw 0x2(%si),%cx        # Load cylinder:sector
564         movb $0x1,%al            # Sector count
565         pushw %si            # Save
566         movw %sp,%di            # Save
567 #ifndef CHECK_DRIVE                /* floppy support */
568         testb %dl, %dl            # is this a floppy ?
569         jz 1f                # Yes, use CHS mode
570 #endif
571         testb $USEPACKET,_FLAGS(%bp)    # Use packet interface?
572         jz 1f                # No
573         pushl $0x0            # Set the
574         pushl 0x8(%si)            # LBA address
575         pushw %es            # Set the transfer
576         pushw %bx            #  buffer address
577         push  $0x1            # Block count
578         push  $0x10            # Packet size
579         movw %sp,%si            # Packet pointer
580         decw %ax            # Verify off
581         orb $0x40,%ah            # Use disk packet
582     1:    int $0x13            # BIOS: Disk I/O
583         movw %di,%sp            # Restore
584         popw %si            # Restore
585         retw                # To caller
586 
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 PXE
593         .ascii "\nF6 PXE\r"
594 #endif
595         .ascii "\nBoot:"
596 item:        .ascii " ";         .byte ‘ ‘|0x80
597 crlf:        .ascii "\r";         .byte ‘\n‘|0x80
598 
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, 0x0b
608 #ifndef SAVE_MORE_MEMORY
609         .byte 0x05    # extended partition
610 #endif
611 #ifndef SAVE_MEMORY    /* other DOS partitions */
612         .byte 0x01    # FAT12
613         .byte 0x04    # FAT16 < 32M
614 #endif
615 
616 desc_ofs:
617     /*
618      * Offsets that match the known types above, used to point to the
619      * 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, Linux
623         .byte os_freebsd-.        # 165, FreeBSD
624         .byte os_bsd-.            # 166, OpenBSD
625         .byte os_bsd-.            # 169, NetBSD
626         .byte os_dos-.            #   6, FAT16 >= 32M
627         .byte os_win-.            #   7, NTFS
628         .byte os_win-.            #  11, FAT32
629 
630 #ifndef SAVE_MORE_MEMORY
631         .byte os_ext-.            #   5, DOS Ext
632 #endif
633 #ifndef SAVE_MEMORY
634         .byte os_dos-.            #   1, FAT12 DOS
635         .byte os_dos-.            #   4, FAT16 <32M
636 #endif
637         .byte os_misc-.         # Unknown
638 
639     /*
640      * And here are the strings themselves. The last byte of
641      * the string has bit 7 set.
642      */
643 os_misc:    .byte ‘?‘|0x80
644 os_dos:
645 #ifndef SAVE_MORE_MEMORY    /* ‘DOS‘ remapped to ‘WIN‘ if no room */
646         .ascii "DO";   .byte ‘S‘|0x80
647 #endif
648 os_win:        .ascii "Wi";   .byte ‘n‘|0x80
649 os_linux:    .ascii "Linu"; .byte ‘x‘|0x80
650 os_freebsd:    .ascii "Free"
651 os_bsd:        .ascii "BS";   .byte ‘D‘|0x80
652 #ifndef SAVE_MORE_MEMORY
653 os_ext:        .ascii "EX";   .byte ‘T‘|0x80
654 #endif
655 
656         .org (0x200 + B0_OFF),0x90
657 /*
658  * The boot0 version 1.0 parameter table.
659  * Do not move it nor change the "Drive " string, boot0cfg
660  * uses its offset and content to identify the boot sector.
661  * The other fields are sometimes changed before writing back to the drive
662  * Be especially careful that nxtdrv: must come after drive:, as it
663  * is part of the same string.
664  */
665 drive:        .ascii "Drive "
666 nxtdrv:        .byte 0x0            # Next drive number
667 opt:        .byte 0x0            # Option
668 setdrv_num:    .byte 0x80            # Drive to force
669 flags:        .byte FLAGS            # Flags
670 #ifdef VOLUME_SERIAL
671         .byte 0xa8,0xa8,0xa8,0xa8       # Volume Serial Number
672 #endif
673 ticks:        .word TICKS            # Delay
674 
675         .org PRT_OFF
676 /*
677  * Here is the 64 byte partition table that fdisk would fiddle with.
678  */
679 partbl:        .fill 0x40,0x1,0x0        # Partition table
680         .word MAGIC            # Magic number
681         .org 0x200            # again, safety check
682 endblock: