1 .\" $FreeBSD: src/sbin/i386/fdisk/fdisk.8,v 1.17.2.11 2002/04/25 16:25:12 trhodes Exp $
2 .\" $DragonFly: src/sbin/i386/fdisk/fdisk.8,v 1.10 2006/09/10 01:26:28 dillon Exp $
9 .Nd PC slice table maintenance program
22 In order for the BIOS to boot the kernel,
23 certain conventions must be adhered to.
24 Sector 0 of the disk must contain boot code,
27 BIOS slices can be used to break the disk up into several pieces.
28 The BIOS brings in sector 0 and verifies the magic number. The sector
29 0 boot code then searches the slice table to determine which
32 This boot code then brings in the bootstrap from the
34 slice and, if marked bootable, runs it.
36 you can have one or more slices with one
40 program can be used to divide space on the disk into slices and set one
47 serves a similar purpose to the DOS program. The first form is used to
48 display slice information or to interactively edit the slice
49 table. The second is used to write a slice table using a
51 and is designed to be used by other scripts/programs.
54 .Bl -tag -width indent
56 Change the active slice only. Ignored if
60 Get the boot code from the file
65 The disk image is specified as a normal file instead of as a device,
66 which is useful when building emulated disks for vmware, bochs, etc.
68 Reinitialize the boot code contained in sector 0 of the disk. Ignored
73 Normally if the CHS fields for a partition are set to all 1's if they
74 would otherwise wrap. This typically causes BIOSes to properly detect
75 that the disk should be put in Large mode. If this option is specified
76 the CHS fields will be set to wrapped values instead. This option may
77 be needed on very old PCs.
78 .It Fl f Ar configfile
79 Set slice values using the file
83 always modifies existing slices, unless
85 is also given, in which case all existing slices are deleted (marked
86 as "unused") before the
90 can be "-", in which case
93 .Sx CONFIGURATION FILE ,
94 below, for file syntax.
99 is used, you are not asked if you really want to write the slices
100 table (as you are in the interactive mode). Use with caution!
102 Initialize sector 0 of the disk. This implies
108 Initialize the contents of sector 0
111 slice covering the entire disk.
113 Print a summary of all slices on the disk and exit.
114 All other options will be ignored.
116 Test mode; do not write slice values. Generally used with the
118 option to see what would be written to the slice table. Implies
121 Is used for updating (editing) sector 0 of the disk. Ignored if
129 prints out the slice table that is written to the disk.
131 Operate on a single fdisk entry only. Ignored if
136 The final disk name can be provided as a
138 disk name only, e.g.\&
140 or as a fully qualified device node under
142 If omitted, the disks
146 are searched in that order, until one is found to respond.
148 When called without options,
150 prints the sector 0 slice table.
153 ******* Working on device /dev/ad0 *******
154 parameters extracted from in-core disklabel are:
155 cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)
157 parameters to be used for BIOS calculations are:
158 cylinders=769 heads=15 sectors/track=33 (495 blks/cyl)
160 Warning: BIOS sector numbering starts with sector 1
161 Information from DOS bootblock is:
162 The data for partition 1 is:
163 sysid 165,(DragonFly/FreeBSD/NetBSD/386BSD)
164 start 495, size 380160 (185 Meg), flag 0
165 beg: cyl 1/ sector 1/ head 0;
166 end: cyl 768/ sector 33/ head 14
167 The data for partition 2 is:
169 start 378180, size 2475 (1 Meg), flag 0
170 beg: cyl 764/ sector 1/ head 0;
171 end: cyl 768/ sector 33/ head 14
172 The data for partition 3 is:
174 The data for partition 4 is:
175 sysid 99,(ISC UNIX, other System V/386, GNU HURD or Mach)
176 start 380656, size 224234 (109 Meg), flag 80
177 beg: cyl 769/ sector 2/ head 0;
178 end: cyl 197/ sector 33/ head 14
181 The disk is divided into three slices that happen to fill the disk.
182 The second slice overlaps the end of the first.
183 (Used for debugging purposes.)
184 .Bl -tag -width "cyl, sector and head"
186 is used to label the slice.
189 magic number 165 decimal (A5 in hex).
190 .It Em start No and Em size
191 fields provide the start address
192 and size of a slice in sectors.
194 specifies that this is the active slice.
195 .It Em cyl , sector No and Em head
196 fields are used to specify the beginning and end addresses of the slice.
198 these numbers are calculated using BIOS's understanding of the disk geometry
199 and saved in the bootblock.
206 flags are used to indicate that the slice data is to be updated.
209 option is also given,
211 will enter a conversational mode.
212 In this mode, no changes will be written to disk unless you explicitly tell
217 will display each slice and ask whether you want to edit it.
220 will step through each field, show you the old value,
221 and ask you for a new one.
222 When you are done with the slice,
224 will display it and ask you whether it is correct.
226 will then proceed to the next entry.
232 fields correct is tricky, so by default,
233 they will be calculated for you;
234 you can specify them if you choose to though.
236 After all the slices are processed,
237 you are given the option to change the
240 Finally, when all the new data for sector 0 has been accumulated,
241 you are asked to confirm whether you really want to rewrite it.
243 The difference between the
250 flag just edits (updates) the fields as they appear on the disk,
253 flag is used to "initialize" sector 0;
254 it will set up the last BIOS slice to use the whole disk for
258 The automatic calculation of starting cylinder etc. uses
259 a set of figures that represent what the BIOS thinks the
260 geometry of the drive is.
261 These figures are taken from the in-core disklabel by default,
264 initially gives you an opportunity to change them.
265 This allows you to create a bootblock that can work with drives
266 that use geometry translation under the BIOS.
268 If you hand craft your disk layout,
269 please make sure that the
271 slice starts on a cylinder boundary.
272 A number of decisions made later may assume this.
273 (This might not be necessary later.)
275 Editing an existing slice will most likely result in the loss of
276 all data in that slice.
280 interactively once or twice to see how it works.
281 This is completely safe as long as you answer the last question
282 in the negative. There are subtleties that
284 detects that are not fully explained in this manual page.
285 .Sh CONFIGURATION FILE
288 option is given, a disk's slice table can be written using values
291 The syntax of this file is very simple;
292 each line is either a comment or a specification, as follows:
298 Lines beginning with a "#" are comments and are ignored.
305 Set the BIOS geometry used in slice calculations. There must be
306 three values specified, with a letter preceding each number:
311 Set the number of cylinders to
316 Set the number of heads to
321 Set the number of sectors/track to
325 These specs can occur in any order, as the leading letter determines
326 which value is which; however, all three must be specified.
328 This line must occur before any lines that specify slice
331 It is an error if the following is not true:
332 .Bd -literal -offset indent
333 1 \(<= number of cylinders
334 1 \(<= number of heads \(<= 256
335 1 \(<= number of sectors/track < 64
338 The number of cylinders should be less than or equal to 1024, but this
339 is not enforced, although a warning will be printed. Note that bootable
341 slices (the "/" filesystem) must lie completely within the
342 first 1024 cylinders; if this is not true, booting may fail.
343 Non-bootable slices do not have this restriction.
345 Example (all of these are equivalent), for a disk with 1019 cylinders,
346 39 heads, and 63 sectors:
347 .Bd -literal -offset indent
359 Set the slice given by
369 Only those slices explicitly mentioned by these lines are modified;
370 any slice not referenced by a "p" line will not be modified.
371 However, if an invalid slice table is present, or the
373 option is specified, all existing slice entries will be cleared
374 (marked as unused), and these "p" lines will have to be used to
375 explicitly set slice information. If multiple slices need to be
376 set, multiple "p" lines must be specified; one for each slice.
378 These slice lines must occur after any geometry specification lines,
385 slices. Specifying a slice type of zero is
386 the same as clearing the slice and marking it as unused; however,
387 dummy values (such as "0") must still be specified for
392 Note: the start offset will be rounded upwards to a head boundary if
393 necessary, and the end offset will be rounded downwards to a cylinder
394 boundary if necessary.
396 Example: to clear slice 4 and mark it as unused:
397 .Bd -literal -offset indent
401 Example: to set slice 1 to a
403 slice, starting at sector 1
404 for 2503871 sectors (note: these numbers will be rounded upwards and
405 downwards to correspond to head and cylinder boundaries):
407 .Bd -literal -offset indent
417 the active slice. Can occur anywhere in the config file, but only
421 line is present, all slices of the disk are made inactive.
423 Example: to make slice 1 the active slice:
424 .Bd -literal -offset indent
429 .Bl -tag -width /boot/mbr -compact
431 The default boot code
438 The default boot code will not necessarily handle all slice types
439 correctly, in particular those introduced since MS-DOS 6.x.
441 The entire program should be made more user-friendly.
445 do not understand the difference between
449 causing difficulty to adjust.
451 You cannot use this command to completely dedicate a disk to
455 command must be used for this.
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