diskdev_cmds/fdisk.tproj/fdisk.8

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  21 .\" Copyright (c) 1997 Tobias Weingartner
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  49 .Dd January 3, 2002
  50 .Dt FDISK 8
  51 .Os
  52 .Sh NAME
  53 .Nm fdisk
  54 .Nd DOS partition maintenance program
  55 .Sh SYNOPSIS
  56 .Nm fdisk
  57 .Op Fl ieu
  58 .Op Fl f Ar mbrname
  59 .Op Fl c Ar cylinders
  60 .Op Fl h Ar heads
  61 .Op Fl s Ar sectors
  62 .Op Fl S Ar size
  63 .Op Fl b Ar size
  64 .Ar device
  65 .Sh DESCRIPTION
  66 In order for the BIOS to boot the kernel, certain conventions must be
  67 adhered to.
  68 Sector 0 of a bootable hard disk must contain boot code,
  69 an MBR partition table, and a magic number (0xAA55).
  70 These MBR partitions (also
  71 known as BIOS partitions) can be used to break the disk up into several
  72 pieces.
  73 .Pp
  74 The BIOS loads sector 0 of the boot disk into memory, verifies
  75 the magic number, and begins executing the code at the first byte.
  76 The normal DOS MBR boot code searches the MBR partition table for an
  77 .Dq active
  78 partition (indicated by a
  79 .Ql \&*
  80 in the first column), and if one
  81 is found, the boot block from that partition is loaded and executed in
  82 place of the original (MBR) boot block.
  83 .Pp
  84 The options are as follows:
  85 .Bl -tag -width Ds
  86 .It Fl i
  87 Initialize the MBR sector.
  88 .It Fl a Ar style
  89 Specify an automatic partitioning style.
  90 .It Fl e
  91 Edit existing MBR sectors.
  92 .It Fl f Ar mbrname
  93 Specifies an alternate MBR template file.
  94 .It Fl u
  95 Update MBR code, preserving existing partition table.
  96 .It Fl y
  97 Do not ask for confirmation before writing.
  98 .It Fl d
  99 Dump partition table in a format readable by the -r option.
 100 .It Fl r
 101 Read a partition table from the standard input.
 102 .It Fl t
 103 Test if the disk is partitioned.
 104 .It Xo Fl c Ar cylinders ,
 105 .Fl h Ar heads ,
 106 .Fl s Ar sectors
 107 .Xc
 108 Specifies an alternate BIOS geometry for
 109 .Nm
 110 to use.
 111 .It Fl S Ar size
 112 Specify the disk size in blocks.
 113 .It Fl b Ar size
 114 Specify the number of bytes per disk block.
 115 .El
 116 .Pp
 117 The DOS
 118 .Nm
 119 program can be used to divide space on the disk into partitions and set
 120 one active.
 121 This
 122 .Nm
 123 program serves a similar purpose to the DOS program.
 124 When called with no special flags, it prints the MBR partition
 125 table of the specified device, i.e.,
 126 .Bd -literal
 127     # fdisk fd0
 128     Disk: fd0       geometry: 80/2/18 [2880 sectors]
 129     Offset: 0       Signature: 0xAA55
 130              Starting        Ending
 131      #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
 132     ----------------------------------------------------------------------
 133     *1: A6    0   0   1 -   79   1  18 [         0 -       2880] OpenBSD
 134      2: 00    0   0   0 -    0   0   0 [         0 -          0] unused
 135      3: A7    0   0   2 -   79   1  18 [         1 -       2879] NEXTSTEP
 136      4: 00    0   0   0 -    0   0   0 [         0 -          0] unused
 137 .Ed
 138 .Pp
 139 The geometry displayed is a synthetic geometry unless another geometry
 140 has been selected using the
 141 .Fl c ,
 142 .Fl h ,
 143 .Fl s ,
 144 .Fl S ,
 145 and
 146 .Fl b
 147 options.
 148 In the future,
 149 .Nm
 150 will read the BIOS geometry from the IOKit registry.
 151 .Pp
 152 In this example,
 153 the disk is divided into two partitions that happen to fill the disk.
 154 The first partition overlaps the third partition.
 155 (Used for debugging purposes.)
 156 .Bl -tag -width "start/size"
 157 .It Em "#"
 158 Number of partition table entry.
 159 A
 160 .Dq \&*
 161 denotes the bootable partition.
 162 .It Em "id"
 163 System identifier.
 164 .Ox
 165 reserves the
 166 magic number 166 decimal (A6 in hex).
 167 If no 166 partition is found, it will use an older
 168 .Fx
 169 partition (with a magic number of 165 or A5 in hex).
 170 .It Em "cyl/hd/sec"
 171 These fields provide the starting and ending address of the partition
 172 in BIOS geometry
 173 .It Em "start/size"
 174 These fields provide the starting sector and size in sectors of the
 175 partition in linear block addresses.
 176 .El
 177 .Pp
 178 .Em NOTE :
 179 The sectors field is
 180 .Dq 1 based ,
 181 and the start field is
 182 .Dq 0 based .
 183 The CHS values may need to be in the BIOS's geometry
 184 for older systems to be able to boot and use the drive correctly;
 185 most modern systems prefer the starting sector and size
 186 in preference to the CHS values.
 187 .Pp
 188 The
 189 .Fl i
 190 flag is used to indicate that the partition data is to be initialized.
 191 In this mode,
 192 .Nm
 193 will completely overwrite the primary MBR and partition table, either
 194 using the default MBR template, or the one specified by the
 195 .Fl f
 196 flag.
 197 .Pp
 198 In the default template, partition number 1 will be configured as a
 199 Darwin boot
 200 partition spanning from cylinder 0, head 1, sector 1, and extending
 201 for 8 megabytes.
 202 Partition number 2 will be configured as a
 203 Darwin HFS
 204 partition spanning the rest of the disk.
 205 This mode is designed to initialize an MBR the very first time,
 206 or when it has been corrupted beyond repair.
 207 .Pp
 208 You can specify other default partition styles with the
 209 .Fl a
 210 flag.  The available styles are:
 211 .Bl -tag -width "start/size"
 212 .It Em "boothfs"
 213 Creates an 8Mb boot partition (type AB hex)
 214 and makes the rest of the disk
 215 a Darwin HFS partition (type AF hex).
 216 .It Em "hfs"
 217 Makes the entire disk one HFS+ partition (type AF hex).
 218 .It Em "dos"
 219 Makes the entire disk one DOS partition (type 0C hex).
 220 .It Em "raid"
 221 Makes the entire disk one type AC hex partition.
 222 .El
 223 .Pp
 224 The
 225 .Fl u
 226 flag is used to update the MBR code on a given drive.
 227 The MBR code extends from offset 0x000 to the start of the partition table
 228 at offset 0x1BE.
 229 It is similar to the
 230 .Fl i
 231 flag, except the existing partition table is preserved. This
 232 is useful for writing new MBR code onto an existing drive, and is
 233 equivalent to the DOS command
 234 .Dq FDISK /MBR .
 235 Note that this option will overwrite the NT disk signature, if present.
 236 The
 237 .Fl u
 238 and
 239 .Fl i
 240 flags may not be specified together.
 241 .Pp
 242 The flag
 243 .Fl e
 244 is used to modify a partition table using a interactive edit mode of the
 245 .Nm
 246 program.
 247 This mode is designed to allow you to change any partition on the
 248 drive you choose, including extended partitions.
 249 It is a very powerful mode,
 250 but is safe as long as you do not execute the
 251 .Em write
 252 command, or answer in the negative (the default) when
 253 .Nm
 254 asks you about writing out changes.
 255 .Sh COMMAND MODE
 256 When you first enter this mode, you are presented with a prompt, that looks
 257 like so:
 258 .Em "fdisk: 0>" .
 259 This prompt has two important pieces of information for you.
 260 It will tell
 261 you if the in-memory copy of the boot block has been modified or not.
 262 If it has been modified, the prompt will change to look like:
 263 .Em "fdisk:*0>" .
 264 The second piece of information pertains to the number given in the prompt.
 265 This number specifies the disk offset of the currently selected boot block
 266 you are editing.
 267 This number could be something different that zero when
 268 you are editing extended partitions.
 269 The list of commands and their explanations are given below.
 270 .Bl -tag -width "update"
 271 .It Em help
 272 Display a list of commands that
 273 .Nm
 274 understands in the interactive edit mode.
 275 .It Em manual
 276 Display this manual page.
 277 .It Em reinit
 278 Initialize the currently selected, in-memory copy of the
 279 boot block.
 280 .It Em auto
 281 Partition the disk with one of the automatic partition styles.
 282 .It Em disk
 283 Display the current drive geometry that
 284 .Nm
 285 has
 286 probed.
 287 You are given a chance to edit it if you wish.
 288 .It Em edit
 289 Edit a given table entry in the memory copy of
 290 the current boot block.
 291 You may edit either in BIOS geometry mode,
 292 or in sector offsets and sizes.
 293 .It Em setpid
 294 Change the partition
 295 identifier of the given partition table entry.
 296 This command is particularly useful for reassigning
 297 an existing partition to OpenBSD.
 298 .It Em flag
 299 Make the given partition table entry bootable.
 300 Only one entry can be marked bootable.
 301 If you wish to boot from an extended
 302 partition, you will need to mark the partition table entry for the
 303 extended partition as bootable.
 304 .It Em update
 305 Update the machine code in the memory copy of the currently selected
 306 boot block.
 307 Note that this option will overwrite the NT disk
 308 signature, if present.
 309 .It Em select
 310 Select and load into memory the boot block pointed
 311 to by the extended partition table entry in the current boot block.
 312 .It Em print
 313 Print the currently selected in-memory copy of the boot
 314 block and its MBR table to the terminal.
 315 .It Em write
 316 Write the in-memory copy of the boot block to disk.
 317 You will be asked to confirm this operation.
 318 .It Em exit
 319 Exit the current level of
 320 .Nm fdisk ,
 321 either returning to the
 322 previously selected in-memory copy of a boot block, or exiting the
 323 program if there is none.
 324 .It Em quit
 325 Exit the current level of
 326 .Nm fdisk ,
 327 either returning to the
 328 previously selected in-memory copy of a boot block, or exiting the
 329 program if there is none.
 330 Unlike
 331 .Em exit
 332 it does write the modified block out.
 333 .It Em abort
 334 Quit program without saving current changes.
 335 .El
 336 .Sh NOTES
 337 The automatic calculation of starting cylinder etc. uses
 338 a set of figures that represent what the BIOS thinks is the
 339 geometry of the drive.
 340 These figures are by default taken from the in-core disklabel, or
 341 values that
 342 .Em /boot
 343 has passed to the kernel, but
 344 .Nm
 345 gives you an opportunity to change them if there is a need to.
 346 This allows the user to create a bootblock that can work with drives
 347 that use geometry translation under a potentially different BIOS.
 348 .Pp
 349 If you hand craft your disk layout,
 350 please make sure that the
 351 .Ox
 352 partition starts on a cylinder boundary.
 353 (This restriction may be changed in the future.)
 354 .Pp
 355 Editing an existing partition is risky, and may cause you to
 356 lose all the data in that partition.
 357 .Pp
 358 You should run this program interactively once or twice to see how it works.
 359 This is completely safe as long as you answer the
 360 .Dq write
 361 questions in the
 362 negative.
 363 .Sh FILES
 364 .Bl -tag -width /usr/mdec/mbr -compact
 365 .It Pa /usr/mdec/mbr
 366 default MBR template
 367 .El
 368 .Sh SEE ALSO
 369 .Xr gpt 8 ,
 370 .Xr pdisk 8
 371 .Sh BUGS
 372 There are subtleties
 373 .Nm
 374 detects that are not explained in this manual page.
 375 As well, chances are that some of the subtleties it should detect are being
 376 steamrolled.
 377 Caveat Emptor.