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+<H2><A NAME="s2">2. Properties of partition tables.</A></H2>
+
+<P>
+<H2><A NAME="ss2.1">2.1 Why partitions?</A>
+</H2>
+
+<P>
+<P>The partition table of a disk cuts it into 'logical disks'.
+There are several reasons for wanting to do this.
+DOS does not support filesystems larger than 2 GB, so
+partitioning is required to break this '2 GB barrier'.
+Different partitions may carry different operating systems
+or different filesystems (FAT, HPFS, NTFS, ext2, ...) to be used by one
+operating system. Sometimes small partitions are used for special purposes
+(OS/2 Boot Manager uses a small partition for itself, various laptops
+have a 'hibernation' partition where the state of the system is stored
+when it goes asleep). Some 'reliable' systems have backup partitions.
+For backup purposes, say to tape, it is often convenient to have
+partitions of a size such that the entire partition can be written
+to a single tape.
+<P>It is a good idea to keep your own things (say under /home)
+and privately installed packages (say under /usr/local)
+separate from the software installed from a distribution.
+In case these are on a different partition, it is easier
+to do a complete reinstall (or switch to a different distribution)
+without losing your own stuff.
+<P>For well-designed systems it is often possible to have all basic
+system software on a read-only partition, thus diminishing the probability
+of corruption and saving backup time.
+There is also a security aspect; for example on a Unix system one
+might mount all filesystems other than the root filesystem 'nosuid,nodev',
+and have /tmp, /home, /var not on the root filesystem, to minimize the
+possibility that some suid program is tricked into overwriting a
+vital system file via a hard link to it.
+<P>Finally there is the old BIOS problem that can make it impossible to boot
+a system that lives past cylinder 1024. This may mean that one has to
+have a partition that ends before the 1024 cylinder limit where the
+stuff needed at boot time is stored.
+<P>Some reasons why you want to avoid Disk Managers are given on
+<A HREF="http://www.morgan-cybersys.com/lcd.html">The Invircible Anti-virus Manual, Appendix C</A>.
+<P>
+<H2><A NAME="ss2.2">2.2 What does a partition table look like?</A>
+</H2>
+
+<P>
+<P>One may have an arbitrary number of partitions on a disk.
+However, the Master Boot Record (MBR, sector 0 of the disk)
+only holds descriptors for 4 partitions, called the <I>primary</I>
+partitions. Usually the BIOS can boot only from a primary partition.
+(Of course it can boot a boot loader that itself is able to access
+nonprimary partitions or other disks.)
+The descriptors for the remaining partitions, called <I>logical</I> partitions,
+are scattered along the disk in a linked list of partition table sectors,
+starting with the MBR.
+<P>Each partition table sector contains 4 partition descriptors.
+A partition descriptor may be of type <B>05</B> (DOS extended partition),
+<B>0f</B> (W95 extended partition), <B>85</B> (Linux extended partition),
+or <B>c5</B> (DRDOS/secured extended partition),
+in which case it points to another partition table sector.
+In this way, we obtain a quaternary tree of partitions.
+Linux accepts <B>85</B> as a synonym for <B>05</B> - this is useful if one
+wants to have extended partitions past the 1024 cylinder limit
+(to prevent DOS fdisk from crashing or hanging).
+Windows 95 uses <B>0f</B> for LBA mapped extended partitions.
+Thus, an extended partition is not a partition containing data,
+but is a box containing other partitions.
+Nevertheless, the partition table sector that starts an extended partition
+has enough room left to contain a boot loader like LILO, so that it is
+possible to boot an extended partition.
+<P>Most operating systems severely restrict the accepted trees.
+Usually branching is not allowed, and one gets a linear chain of
+partition table sectors.
+Linux will accept several extended primary partitions.
+<P>
+<P>
+<H2><A NAME="ss2.3">2.3 Partition descriptors</A>
+</H2>
+
+<P>
+<P>A partition table entry is 16 bytes long and contains 6 items
+(not listed in order).
+1. A byte that is 0x80 or 0 denoting 'bootable' or not.
+The standard DOS MBR will not boot a partition unless it is the unique
+bootable primary partition. For nonprimary partitions this byte is
+unused.
+2. A byte that gives the type.
+3. A 4-byte starting sector number.
+4. A 4-byte length (in sectors).
+5. A 3-byte starting sector given in C/H/S (cylinder/head/sector) format.
+6. A 3-byte final sector given in C/H/S format.
+Linux only uses items 2-4, and hence is not interested in the 'geometry'
+of the disk, and can use disks with up to 2^32 sectors (4 TB).
+DOS uses 5-6 instead of 3-4, and this leads to the well-known problems
+with geometry, with the 1024 cylinder limit, the 500 MB limit, the 8 GB
+limit. For some details, see the
+<A HREF="http://www.win.tue.nl/~aeb/linux/largedisk.html">large disk HOWTO</A>.
+<P>For an extended partition, only the first sector is important -
+it contains the descriptors for its logical partitions.
+There are various conventions about how the descriptor of an
+extended partition (different from the outer one) should look like.
+There is the paradigm of 'nested boxes', where each extended partition
+covers a disk area containing all the logical partitions inside.
+There is also the paradigm of 'chained boxes', where each extended
+partition (except possibly the outer one) just contains the next
+logical partition.
+I don't know which systems follow which paradigms.
+(David A. Burton <CODE><dburton@burtonsys.com></CODE> reports
+that System Commander uses the nested style.)
+However, for the outer (primary) extended partition it is common
+to contain all logical partitions inside (i.e., have a start and length
+field that describes a piece of the disk that contains all logical
+partitions).
+Of course the 'chained boxes' paradigm is more flexible since it
+allows logical partitions with a primary partition in between.
+<P>
+<P>
+<H2><A NAME="ss2.4">2.4 Partition hiding</A>
+</H2>
+
+<P>
+<P>The OS/2 Boot Manager does not want you to have more than one
+primary DOS partition (MS-DOS itself does not mind), and will
+change the type from <B>01</B>, <B>04</B>, <B>06</B>, <B>07</B>
+to <B>11</B>, <B>14</B>, <B>16</B>, <B>17</B>.
+<P>Also other programs or systems use this 'partition hiding'.
+For example,
+<A HREF="http://www.v-com.com/syscomm.html">System Commander</A> will OR the type with 0x10,
+changing the Linux <B>83</B> into the Amoeba <B>93</B>.
+<P>
+<H2><A NAME="ss2.5">2.5 CHS vs LBA</A>
+</H2>
+
+<P>Some partition IDs imply a particular method of disk access.
+In particular, IDs <B>0c</B>, <B>0e</B>, <B>0f</B>
+(the LBA versions of <B>0b</B>, <B>06</B>, <B>05</B>)
+go with partition table entries that have C/H/S = 1023/255/63
+and expect access via the extended INT-13 functions (AH=4x)
+of the BIOS.
+<P>
+<H2><A NAME="logsectfat"></A> <A NAME="ss2.6">2.6 Logically sectored FAT</A>
+</H2>
+
+<P>Some systems use a filesystem that is fully compatible with
+a standard FAT12 or FAT16 partition, except for using a
+sector size larger than the usual 512 bytes, up to 8192 bytes.
+This is what is meant by "logically sectored FAT" in the above.
+<P>Logically sectored FATs have been a way to circumvent the dreaded 32 MB
+partition size limit before the introduction of DOS 3.31. Since the
+count of sectors was restricted to 16-bit on FAT16 (type 04h) the only
+way to grow the partition above the 32 MB limit in a reasonably compatible
+fashion was to increase the sector size instead. Physical sectors at
+ROM BIOS INT 13h level are always 512 bytes in size, but other devices
+may require support for other sector sizes in the operating system.
+Hence, when DOS logs in drives during bootstrap it will record the
+sector size values indicated in each partition it finds and if it
+is larger than the previously recorded value, it will slide up the
+maximum supported sector size to the found value. Very old DOS versions
+seem to have started with an initial value of 128 (showing some
+CP/M heritance here), but recent DOS versions use an initial value
+of 512 bytes. Once DOS has logged in all drives (including those
+not represented on INT 13h level, for example, SCSI disk, RAM disk
+or such), it will set up its internal buffering logic to use the
+maximum sector size found. This mechanism is present in all
+DOS versions (although it was partially broken in DOS 5.0 - 6.22).
+<P>(Matthias Paul)
+<P>
+<P>
+<H2><A NAME="ss2.7">2.7 What does FDISK /MBR do?</A>
+</H2>
+
+<P>
+<P>People often recommend the undocumented DOS command FDISK /MBR
+to solve problems with the MBR. This command however does not
+rewrite the entire MBR - it just rewrites the boot code, the first
+446 bytes of the MBR, but leaves the 64-byte partition information
+alone. Thus, it won't help when the partition table has problems.
+Moreover, it can be dangerous to restore the boot code to its
+original state:
+if the cause of the problems was a boot sector virus, then
+vital information may have been stored elsewhere by the virus,
+and killing the virus may mean killing access to this information.
+(For example, the stoned.empire.monkey virus encrypts the original
+MBR to sector 0/0/3.)
+However, people who want to uninstall LILO, and do not know that
+LILO has a -u option, can use FDISK /MBR for this purpose.
+<P>In a Linux environment, one can wipe all of the MBR with a command
+like "dd if=/dev/zero of=/dev/hda count=1 bs=512".
+If only the boot code must be removed, but not the partition table,
+then "dd if=/dev/zero of=/dev/hda count=1 bs=446" will do.
+Be very careful with such commands. Usually one regrets them later.
+<P>
+<H2><A NAME="ss2.8">2.8 Structure of the MBR - OS additions</A>
+</H2>
+
+<P>As we saw, the structure of the MBR (Master Boot Record, sector 0)
+is as follows:
+First 446 bytes boot loader code, then 64 bytes partition table
+(starting at offset 0x1be = 446), finally 2 bytes signature 0xaa55.
+<P>Just before the partition table some operating systems save some
+interesting stuff. For example, DRDOS stores a password starting
+at offset 0x1b6.
+<P>Windows NT stores a 4-byte "disk signature" or "volume ID"
+starting at offset 0x1b8. It is used to map drive letters to disks:
+in the HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices registry item
+the drive letter is coupled with this disk signature.
+It is used as a disk label to map disk info to disks
+in the HKEY_LOCAL_MACHINE\SYSTEM\DISK registry item.
+This signature is generated by the Disk Administrator when it
+initializes the disk, unless there already was a nonzero value there.
+<P>Grub had a 4-byte stage2 start address at 0x1b8, and a 2-byte
+version at 0x1bc, but recent versions preserve 0x1b8-0x1bd.
+Also LILO v20 and later preserves this area.
+<P>Some operating systems are reported to have 8 instead of 4
+partition descriptors in the MBR. Cf. AST DOS under <B>14</B>
+and NEC DOS under <B>24</B> above.
+<P>
+<H2><A NAME="ss2.9">2.9 The Advanced Active Partition of PTS</A>
+</H2>
+
+<P>As mentioned above, the DOS MBR boot code will boot the
+(unique) primary partition that has been marked active.
+Usually, in a multi-boot situation, the boot manager toggles
+the active bit of the partition that is to be booted.
+PTS chose a different solution.
+<P>Matthias Paul writes:
+"So far the only DOS being able to boot out of a logical drive
+in an extended partition is PTS-DOS by use of so called
+"Advanced Active Partition" entries in the MBR. In order to
+remain as compatible as possible with existing DOS standards,
+this works a little bit different and requires a special 5th
+partition entry in front of the other four entries in the MBR
+and corresponding AAP-aware MBR bootstrap code. If the MBR
+contains a special AAP signature and this special entry exists
+and is flagged bootable, the MBR will use this instead of one
+of the other four entries. The entry may either point to the
+bootsector of a logical drive or to a 512 bytes long file
+(with system-attribute, so it won't be moved around during
+disk defragmentation) somewhere inside the filesystem, which
+makes up a boot sector (same "IBM" signature, same load address,
+same register interface). In contrast to the usual MBR code,
+this MBR code interprets the boot flag byte as physical drive
+unit (80h..FEh), instead of using it only as a active flag
+(80h or 00h in older DOS issues or bit 7 set or cleared in
+newer DOS issues). This way, the AAP MBR could even load a
+boot sector from other than the first harddisk."
+<P>
+<H2><A NAME="ss2.10">2.10 Naming</A>
+</H2>
+
+<P>
+<P>DOS uses drive letters A: and B: for floppy disk drives, and
+assigns drive letters C: ... Z: in the order: first all primary
+DOS partitions on the first disk, then all primary DOS partitions
+on the second disk, ..., then all logical DOS partitions on first
+disk, etc. DOS will stop investigating logical partitions in a given
+extended partition as soon as a non-DOS partition is encountered.
+(DOS recognizes partition types 1, 4, 6 and 5 for extended.)
+<P>Systems like Windows 95, Windows 98, Windows NT, Windows 2000 and OS/2
+follow a similar convention, but recognize different partition types.
+Thus, a drive can have a different drive letter for each of these
+operating systems. For details, see the Microsoft KnowledgeBase, e.g.
+<A HREF="http://support.microsoft.com/support/kb/articles/q93/3/73.ASP">Drive letters in Windows NT</A>,
+<A HREF="http://support.microsoft.com/support/kb/articles/Q254/9/66.ASP">Drive letters in Windows 2000 for unsupported partition types</A>.
+<P>
+<H2><A NAME="ss2.11">2.11 Limits</A>
+</H2>
+
+<P>The partition table describes the location of partitions both
+in 1-dimensional ('LBA') and in 3-dimensional (CHS) form.
+The former is easy enough, but for the latter one needs to
+know the disk geometry. Note that these days this geometry
+is entirely fake, and different systems use different faked
+geometries for the same disk, giving lots of problems.
+(For example, a modern disk may have 2 or 4 heads, but will
+probably report 15 or 16 heads to the BIOS, which in turn may
+report 255 heads to DOS or Windows.)
+<P>
+<DL>
+<P>
+<DT><B>ATA Specification (for IDE disks) - the 137 GB limit</B><DD><P>At most 65536 cylinders (numbered 0-65535), 16 heads (numbered 0-15),
+255 sectors/track (numbered 1-255), for a maximum total capacity of
+267386880 sectors (of 512 bytes each), that is, 136902082560 bytes (137 GB).
+<P>
+<DT><B>BIOS Int 13 - the 8.4 GB limit</B><DD><P>At most 1024 cylinders (numbered 0-1023), 256 heads (numbered 0-255),
+63 sectors/track (numbered 1-63) for a maximum total capacity of
+8455716864 bytes (8.4 GB). This is a serious limitation today.
+It means that DOS cannot use present day large disks.
+<P>
+<DT><B>The DOS 528 MB limit</B><DD><P>If the same values for c,h,s are used for the BIOS Int 13 call and
+for the IDE disk I/O, then both limitations combine, and one can
+use at most 1024 cylinders, 16 heads, 63 sectors/track, for a
+maximum total capacity of 528482304 bytes (528MB), the infamous
+504 MB limit (if one takes M=2^20).
+This was already a problem many years ago, and all kinds of software,
+firmware and hardware solutions were invented. On the software side,
+there are Disk Managers, that circumvent the BIOS and go directly to
+the hardware. On the firmware side there are translating BIOSes,
+that use one geometry when talking to the disk, and another one
+when talking to the user program. (At best, this again allows access
+to 8.4 GB.) On the hardware side, there is LBA disk access,
+that no longer uses (c,h,s).
+<P>
+<DT><B>The 2.1 GB limit</B><DD><P>Some older BIOSes only allocate 12 bits for the field in CMOS RAM that
+gives the number of cylinders. Consequently, this number can be at most
+4095, and only 4095*16*63*512=2113413120 bytes are accessible.
+See
+<A HREF="http://www.firmware.com/support/bios/over2gb.htm">over2gb.htm</A>.
+<P>
+<DT><B>The 3.2 GB limit</B><DD><P>There was a bug in the Phoenix 4.03 and 4.04 BIOS firmware that would
+cause the system to lock up in the CMOS setup for drives with a capacity
+over 3277 MB. See
+<A HREF="http://www.firmware.com/support/bios/over3gb.htm">over3gb.htm</A>.
+<P>
+<DT><B>The 4.2 GB limit</B><DD><P>Simple BIOS translation (ECHS=Extended CHS, sometimes called 'Large
+disk support' or just 'Large')
+works by repeatedly doubling the number of heads and halving the number
+of cylinders shown to DOS, until the number of cylinders is at most 1024.
+Now DOS and Windows 95 cannot handle 256 heads or more,
+and in the common case that the disk reports 16 heads, this means that
+this simple mechanism only works up to 8192*16*63*512=4227858432 bytes
+(with a fake geometry with 1024 cylinders, 128 heads, 63 sectors/track).
+Note that ECHS does not change the number of sectors per track, so if
+that is not 63, the limit will be lower.
+See
+<A HREF="http://www.firmware.com/support/bios/over2gb.htm">over4gb.htm</A>.
+<P>
+<DT><B>The 7.9 GB limit</B><DD><P>Slightly smarter BIOSes avoid the previous problem by first adjusting the
+number of heads to 15 ('revised ECHS'), so that a fake geometry with
+240 heads can be obtained, good for 1024*240*63*512=7927234560 bytes.
+<P>
+<DT><B>The 8.4 GB limit</B><DD><P>Finally, if the BIOS does all it can to make this translation a success,
+and uses 255 heads and 63 sectors/track ('assisted LBA' or just 'LBA')
+it may reach 1024*255*63*512=8422686720 bytes, slightly less than the
+earlier 8.4 GB limit because the geometries with 256 heads must be avoided.
+(This translation will use for the number of heads the first value H
+in the sequence 16, 32, 64, 128, 255 for which the total disk capacity
+fits in 1024*H*63*512, and then computes the number of cylinders C as
+total capacity divided by (H*63*512).)
+<P>
+<DT><B>The 33.8 GB limit</B><DD><P>Large disks report 16 heads, 63 sectors/track and 16383 cylinders.
+Many BIOSes compute an actual number of cylinders by dividing
+the total capacity by 16*63. For disks larger than 33.8 GB this
+leads to a number of cylinders larger than 65535. Now the BIOS
+crashes or hangs. The solution is to upgrade the BIOS. If that is
+impossible, it sometimes helps to take the disk out of the BIOS,
+but that won't work if one has to boot from the disk, and may also
+fail because the BIOS already hangs during initial probing.
+Usually one can use a jumper to make the disk appear smaller.
+Also many operating systems have problems - only the most recent
+versions work with these disks.
+<P>
+<DT><B>The 137 GB limit</B><DD><P>As already noted, the old ATA specification does not allow access
+to all of a disk that is larger than 137 GB. Indeed, it uses only
+28 bits to specify a sector number. However, ATA-6 defines an extension
+with 48-bit sector number. The first disks needing the extension were
+Maxtor 160 GB disks, that came to market in Fall 2001.
+</DL>
+<P>For another discussion of this topic, see
+<A HREF="http://www.maxtor.com/products/DiamondMax/techsupport/Q&A/30004.html">Breaking the Barriers</A>, and, with more details,
+<A HREF="http://www.maxtor.com/technology/whitepapers/63001.html">IDE Hard Drive Capacity Barriers</A>.
+<P>Hard drives over 8.4 GB are supposed to report their geometry as 16383/16/63.
+This in effect means that the 'geometry' is obsolete, and the total disk
+size can no longer be computed from the geometry.
+<P>
+<H2><A NAME="ss2.12">2.12 Details for various operating systems</A>
+</H2>
+
+<P>
+<P>Early MSDOS filled the partition table starting at the end.
+In particular, in the case of only one partition, the descriptor
+was stored in the fourth primary slot. These days DOS FDISK
+starts at the beginning, but other systems, like Unixware, still
+start at the end.
+Also Iomega writes the single partition of a ZIP disk
+in the last entry (so that it has to be mounted as /dev/sda4 or
+/dev/hdc4 or so).
+<P>MSDOS 6.22 FDISK creates the four entries in the partition table sector
+that starts an extended partition as 1. a data partition (or empty),
+2. the next extended partition, 3. and 4. empty.
+(But old versions of MS-DOS start at the end, and first fill entry 4.)
+If the first logical partition (that is not the last one) is removed,
+only the link in position 2 remains.
+An extended partition table sector can describe only a single
+data partition (the first one encountered). When reading a table,
+FDISK accepts the entries in any order and position, but it will
+write the sector normalized as described.
+<P>DRDOS on the other hand expects zero to four entries in an extended
+partition table sector. Data partitions,
+possibly followed by the link to the next extended partition.
+Thus, this link is always the last significant entry, and will
+be the first entry if there is no data partition (because
+it has been deleted).
+<P>Many systems are willing to accept more than two nonempty parts
+in an extended partition, but will not create such themselves.
+<P>It is rumoured that the outer extended partition should be the 4th
+in the MBR, but I don't know any systems that have this restriction.
+DRDOS FDISK always puts the extended partition in the fourth entry
+no matter how many other entries you may have.
+<P>MSDOS fdisk shows 4 primary partitions, and of the logical partitions
+only those that have a DOS type (1, 4 or 6). It will list the type of
+a logical partition as 'Unknown' if the partition is not formatted.
+<P>It is rumoured that DRDOS ignores the high-order bit of the ID
+(and that is the reason for the additional Linux IDs 41, 42, 43),
+but I don't know whether that is true (and for which versions of DRDOS).
+It is also rumoured that DRDOS will write 1 sector past the end
+of a partition - I have never seen this either. Confirmation?
+It is however true, that DRDOS fdisk only looks at the last 4 bits
+when printing a type, so that types 11, 21, etc are printed as DOS 2.0,
+but such types are not acceptable for DRDOS itself.
+<P>The OS/2 Warp fdisk is very instable, and hangs or crashes with
+general protection fault as soon as the partition table is somewhat
+unusual, cf.
+<A HREF="http://www.teamos2.org/pharmacy/FDISKbug.html">Cannot set an installable partition with FDISK</A>.
+<P>The Windows NT Disk Administrator will corrupt your disk
+when it writes a signature on a disk with two or more
+logical partitions. See
+<A HREF="http://support.microsoft.com/support/kb/articles/Q135/3/08.asp">Disk Administrator Corrupts Partitions</A>.
+<P>The use of Win95/Win98 FDISK in a mixed system is dangerous.
+It will delete a non-FAT logical partition when you had actually
+told it to delete a FAT partition somewhere farther down the chain
+of logical partitions. See
+<A HREF="http://support.microsoft.com/support/kb/articles/q179/1/44.asp">Cannot View NTFS Logical Drive After Using FDISK</A>.
+<P>The system partition in Windows NT 4 must be contained in the first
+7.8 GB of the disk (or less, in case the BIOS geometry does not have
+255 heads and 63 sectors/track; the actual restriction is that all of it
+must be accessible using BIOS Int 13).
+It must not be larger than 4 GB because Windows NT 4 first installs
+into a FAT16 partition and then converts it into NTFS during
+the second phase of the installation.
+It must start before the 4 GB mark (bug fixed in Service Pack 5).
+See
+<A HREF="http://support.microsoft.com/support/kb/articles/Q224/5/26.ASP">Windows NT 4.0 Supports Maximum of 7.8-GB System Partition</A>
+and
+<A HREF="http://support.microsoft.com/support/kb/articles/Q138/3/64.asp">Windows NT Partitioning Rules During Setup</A>
+and
+<A HREF="http://support.microsoft.com/support/kb/articles/Q119/4/97.ASP">Boot Partition Created During Setup Limited to 4 Gigabytes</A>
+and
+<A HREF="http://support.microsoft.com/support/kb/articles/Q197/2/95.ASP">Windows NT Does Not Boot to a Partition That Starts More Than 4 GB into Disk</A>.
+<P>Windows NT and Windows 2000 use for SCSI disks whatever the BIOS says
+(usually C/H/S=C/255/63) for the boot drive, and C/64/32
+for all other SCSI drives. See
+<A HREF="http://support.microsoft.com/support/kb/articles/Q161/5/63.asp">How Windows NT Handles Drive Translation</A>.
+<P>Windows 2000 seems to require that the partition order agrees
+with the disk order.
+<P>The OS/2 fdisk writes some strange length in the descriptor of the
+last extended partition. This is probably a bug.
+OS/2 fdisk fails to update the length of the (outer) extended partition
+when a primary partition is created in the free space (space not used
+by a logical partition) at the end of this extended partition.
+This can lead to overlapping partitions.
+<P>OS/2 FDISK does not know about type f, but accepts DOS Extended Partitions
+extending beyond cylinder 1023. When some other partition handler,
+like Partition Magic 4.0, changes the type of a large extended
+partition from 05 to 0f, OS/2 loses access.
+<P>OS/2 Boot Manager keeps a private copy of the partition table data.
+This leads to problems when changing the partition table
+with 3rd party tools.
+<P>
+<A NAME="w2kandos2bm"></A>
+Windows 2000 tries to destroy OS/2 Boot Manager. Upon boot it ignores
+the 0a partition ID, and sees something resembling a FAT boot sector
+describing 2 FAT copies. When FASTFAT.SYS marks this partition as clean
+in the first reserved FAT entry, the mirror (2nd) FAT sector is also updated.
+However, there is no mirror FAT, and FASTFAT.SYS writes into the middle of
+the OS/2 Boot Manager code. This aggression was built into FASTFAT.SYS
+at a fairly late stage, and prerelease versions work without problems.
+See also
+<A HREF="http://support.microsoft.com/default.aspx?scid=kb;en-us;265003">kb/q265003</A>.
+Update both \WINNT\SYSTEM\DRIVERS\FASTFAT.SYS and
+\WINNT\SYSTEM\DLLCACHE\FASTFAT.SYS .
+<P>
+<A NAME="above1024chs"></A>
+Then there is the problem of what to write in
+(<I>c</I>,<I>h</I>,<I>s</I>) if the numbers
+do not fit. The main strategies seem to be
+<P>1. Mark (<I>c</I>,<I>h</I>,<I>s</I>) as invalid by writing
+some fixed value.
+<P>1a. Write (1023,255,63) for any nonrepresentable CHS.
+<P>1b. Write (1022,254,63) for any nonrepresentable CHS.
+<P>1c. Write (1023,0,1) for the begin CHS of a partition that
+starts at or past cylinder 1024, and write (1023,255,63) for the end.
+<P>2. Leave <I>h</I>, <I>s</I> but do something to <I>c</I>.
+Of course, these fail if <I>h</I> or <I>s</I> does not fit.
+<P>2a. Truncate <I>c</I> to 1023, writing
+(1023, #heads-1, #sectors).
+<P>2b. Truncate <I>c</I> to 1022, writing
+(1022, #heads-1, #sectors).
+<P>2c. Reduce <I>c</I> mod 1024, writing only its last 10 bits.
+<P>Solaris 8 follows 1b or 2b.
+Andreas Jellinghaus reports that Partition Magic follows 1c
+and detects a problem if start CHS is set to (1023,255,63).
+Jeff Merkey reports that Novell Netware follows 2b.
+He writes: <I>If you do not use their methods on NetWare
+partitions, NetWare will not recognize the partition entries correctly,
+and will attempt to reinitialize the entire partition table on a system
+if they are wrong (Ouch!).</I>
+Some versions of Linux fdisk used 2a or 2c, and this confuses OS/2 fdisk - cf.
+<A HREF="http://www.os2forum.or.at/pharmacy/HDDlinux.html">Linux, OS/2 and >1024 Cylinder HDDs</A>.
+David A. Burton <CODE><dburton@burtonsys.com></CODE> reports
+that System Commander Deluxe (from
+<A HREF="http://www.v-com.com/">V Communications</A>) uses
+<P>1c. Mark (<I>c</I>,<I>h</I>,<I>s</I>) as invalid by writing
+<I>c</I>=1022.
+(Maybe this is really 2b?)
+<P>
+<P>
+<H2><A NAME="ss2.13">2.13 Partition Magic</A>
+</H2>
+
+<P>A very convenient tool for manipulating partitions is Partition Magic,
+a commercial program from PowerQuest. Below a description of some of
+its error numbers. (The URL that gave this information no longer exists.)
+This is of interest also for those who do not have this program: it indicates
+what conditions the PowerQuest people think a partition table should satisfy.
+<P>(Not all of these conditions are complied with by DRDOS or OS/2 or Linux
+or Windows NT on Alpha, so a partition manipulator should accept a much
+wider range of partition tables, but such a program might try to follow
+these rules when creating partitions.)
+<P>
+<DL>
+<P>
+<DT><B>100 - A forked extended partition</B><DD><P>The MBR or some EPBR contains two extended partitions.
+(PowerQuest uses the acronym EPBR for a link in the chain of
+extended partition table sectors.)
+(Linux comment: there are three partition types indicating an
+extended partition, namely 0x5, 0xf, 0x85. DOS only recognizes the first.
+Recent Windows only recognizes the first two. Linux will accept
+two or more extended partitions in the MBR, and often it is useful
+to have a 0x5 chain for use by DOS (where this chain
+stays below the 1024 cylinder boundary) and a 0x85 chain for use by Linux.
+Nothing is wrong with having both 0x85 and one of 0x5, 0xf in the MBR.
+However, it is bad to have both 0x5 and 0xf. This is sometimes seen when
+people use some fdisk-type program that does not yet know about 0xf on a
+disk that already contains such an extended partition.)
+<P>
+<DT><B>104 - Partition contains no sectors</B><DD><P>The LBA Number of sectors value in the partition table is 0.
+<P>
+<DT><B>105 - Partition does not start on cylinder boundary</B><DD><P>The Head value of CHS begin is not 0 or 1.
+PartitionMagic expects all FAT, HPFS and NTFS partitions to start
+and end on cylinder boundaries.
+(Comment: Windows NT on Alpha does not comply with this rule, and
+can create partitions starting on arbitrary sectors. There is no
+known operating system that requires this restriction. However,
+there exists software that tries to guess the disk geometry by
+looking at the CHS start and end values in a partition table.
+Note that with large disks CHS values are entirely meaningless.)
+<P>
+<DT><B>106 - Partition does not start with sector 1</B><DD><P>The Sector value of CHS begin is not 1. (Same comment.)
+<P>
+<DT><B>107 - Partition begins beyond the end of the disk</B><DD><P>The Cylinder value of CHS begin is larger than the number
+of cylinders that the BIOS reports.
+(Comment: Usually this means that programs or operating systems
+that use the BIOS cannot use this partition. It may help to
+change the BIOS translation. For Linux it does not matter,
+except that the <CODE>/boot</CODE> partition containing LILO stuff
+should be accessible.)
+<P>
+<DT><B>108 - Partition does not end on cylinder boundary</B><DD><P>The Head value of CHS end is not one less than the number of
+heads that the BIOS reports, or the Sector value of CHS end
+is not equal to the number of sectors per track that the BIOS reports.
+(See above under 105.)
+<P>
+<DT><B>109 - Partition ends after end of disk</B><DD><P>The Cylinder value of CHS end is larger than the number
+of cylinders that the BIOS reports.
+<P>
+<DT><B>110 - Partition has different CHS and LBA lengths</B><DD><P>
+<P>
+<DT><B>111 - Logical partition starts outside extended</B><DD><P>(Comment: the model here is that the extended partition is one
+big box, taking a consecutive piece of disk area, containing
+the logical partitions. Linux allows the logical partitions
+to be anywhere on the disk, also with primary partitions in between.)
+<P>
+<DT><B>112 - Logical partition ends outside extended</B><DD><P>
+<P>
+<DT><B>113 - Partitions overlap</B><DD><P>A partition ends past the start of another. If the filesystems
+don't actually overlap, which they rarely do, then this can be
+fixed by truncating the overlapping partition.
+(Sometimes overlapping partitions are created by OS/2 fdisk:
+if there is still room in an extended partition it allows the creation
+of a primary partition that overlaps the end of the extended partition.
+Now if someone afterwards creates a logical partition inside the
+extended partition, data loss might occur.)
+<P>
+<DT><B>114 - Logical partition does not start one head away from EPBR</B><DD><P>If the EPBR is found at sector N, and there are 63 sectors per track,
+then Partition Magic expects the logical partition to start at sector
+N+63.
+<P>
+<DT><B>115 - Logical partition does not end where Partition Magic expects</B><DD><P>(Comment: Partition Magic expects the extended partition to be
+a big box containing a chain of pairwise disjoint boxes.
+Here each logical partition except for the first one has the
+same ending sector as the surrounding box.
+Another model one finds is a big box containing a smaller
+box, containing a smaller box ... In that model all EPBR extended
+partition entries will show the same end sector.
+In reality the end sector of an EPBR does not play a role anywhere.)
+<P>
+<DT><B>116 - Partition has different CHS and LBA begin</B><DD><P>
+<P>
+<DT><B>120 - Logical partitions not in ascending order</B><DD><P>PowerQuest states: DOS, OS/2, Windows 95 and Windows NT require
+that logical partitions occur in the chain in the on-disk order.
+(Comment: Linux does not require this. However, reordering the
+links in the chain is trivial (for example with sfdisk). Note
+that disk names will be different after reordering.)
+<P>
+</DL>
+<P>
+<H2><A NAME="ss2.14">2.14 Acknowledgements</A>
+</H2>
+
+<P>A lot of useful information was supplied by various people:
+Thomas Wolfram (<CODE>thomas@aeon.in-berlin.de</CODE>) - the author of os-bs,
+Peter Gutmann (<CODE>pgut01@cs.auckland.ac.nz</CODE>) - the author of SFS,
+Cody Batt (<CODE>codyb@powerquest.com</CODE>),
+Christian Carey (<CODE>ccarey@CapAccess.ORG</CODE>),
+Dan Fandrich (<CODE>dan@fch.wimsey.bc.ca</CODE>),
+David Faulks (<CODE>david@santana.ca</CODE>),
+Kai Henningsen (<CODE>kai@khms.westfalen.de</CODE>),
+Dan Hildebrand (<CODE>danh@qnx.com</CODE>),
+Todd Larason (<CODE>jtl@molehill.org</CODE>),
+Mark Morgan Lloyd (<CODE>markMLl.in@telemetry.co.uk</CODE>).
+Marek Michalkiewicz (<CODE>marekm@i17linuxb.ists.pwr.wroc.pl</CODE>),
+David C. Niemi (<CODE>niemidc@clark.net</CODE>),
+Matthias Paul (<CODE>Matthias.Paul@post.rwth-aachen.de</CODE>),
+Loek Weerd (<CODE>loekw@worldonline.nl</CODE>),
+S. Widlake (<CODE>s.widlake@rl.ac.uk</CODE>).
+<HR>
+Next
+<A HREF="partition_types-1.html">Previous</A>
+<A HREF="partition_types.html#toc2">Contents</A>
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