+
+
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. +
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. +
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. +
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. +
Some reasons why you want to avoid Disk Managers are given on +The Invircible Anti-virus Manual, Appendix C. +
+
+
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 primary +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 logical partitions, +are scattered along the disk in a linked list of partition table sectors, +starting with the MBR. +
Each partition table sector contains 4 partition descriptors. +A partition descriptor may be of type 05 (DOS extended partition), +0f (W95 extended partition), 85 (Linux extended partition), +or c5 (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 85 as a synonym for 05 - 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 0f 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. +
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. +
+
+
+
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 +large disk HOWTO. +
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 <dburton@burtonsys.com> 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.
+
+
+
+
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 01, 04, 06, 07 +to 11, 14, 16, 17. +
Also other programs or systems use this 'partition hiding'. +For example, +System Commander will OR the type with 0x10, +changing the Linux 83 into the Amoeba 93. +
+
Some partition IDs imply a particular method of disk access. +In particular, IDs 0c, 0e, 0f +(the LBA versions of 0b, 06, 05) +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. +
+
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. +
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). +
(Matthias Paul) +
+
+
+
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. +
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. +
+
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. +
Just before the partition table some operating systems save some +interesting stuff. For example, DRDOS stores a password starting +at offset 0x1b6. +
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. +
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. +
Some operating systems are reported to have 8 instead of 4 +partition descriptors in the MBR. Cf. AST DOS under 14 +and NEC DOS under 24 above. +
+
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. +
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." +
+
+
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.) +
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. +Drive letters in Windows NT, +Drive letters in Windows 2000 for unsupported partition types. +
+
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.) +
+
+
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). +
+
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. +
+
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). +
+
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 +over2gb.htm. +
+
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 +over3gb.htm. +
+
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 +over4gb.htm. +
+
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. +
+
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).) +
+
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. +
+
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. +
For another discussion of this topic, see +Breaking the Barriers, and, with more details, +IDE Hard Drive Capacity Barriers. +
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. +
+
+
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). +
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. +
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). +
Many systems are willing to accept more than two nonempty parts +in an extended partition, but will not create such themselves. +
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. +
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. +
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. +
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. +Cannot set an installable partition with FDISK. +
The Windows NT Disk Administrator will corrupt your disk +when it writes a signature on a disk with two or more +logical partitions. See +Disk Administrator Corrupts Partitions. +
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 +Cannot View NTFS Logical Drive After Using FDISK. +
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 +Windows NT 4.0 Supports Maximum of 7.8-GB System Partition +and +Windows NT Partitioning Rules During Setup +and +Boot Partition Created During Setup Limited to 4 Gigabytes +and +Windows NT Does Not Boot to a Partition That Starts More Than 4 GB into Disk. +
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 +How Windows NT Handles Drive Translation. +
Windows 2000 seems to require that the partition order agrees +with the disk order. +
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. +
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. +
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. +
+ +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 +kb/q265003. +Update both \WINNT\SYSTEM\DRIVERS\FASTFAT.SYS and +\WINNT\SYSTEM\DLLCACHE\FASTFAT.SYS . +
+ +Then there is the problem of what to write in +(c,h,s) if the numbers +do not fit. The main strategies seem to be +
1. Mark (c,h,s) as invalid by writing +some fixed value. +
1a. Write (1023,255,63) for any nonrepresentable CHS. +
1b. Write (1022,254,63) for any nonrepresentable CHS. +
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. +
2. Leave h, s but do something to c. +Of course, these fail if h or s does not fit. +
2a. Truncate c to 1023, writing +(1023, #heads-1, #sectors). +
2b. Truncate c to 1022, writing +(1022, #heads-1, #sectors). +
2c. Reduce c mod 1024, writing only its last 10 bits. +
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: 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!).
+Some versions of Linux fdisk used 2a or 2c, and this confuses OS/2 fdisk - cf.
+Linux, OS/2 and >1024 Cylinder HDDs.
+David A. Burton <dburton@burtonsys.com> reports
+that System Commander Deluxe (from
+V Communications) uses
+
1c. Mark (c,h,s) as invalid by writing +c=1022. +(Maybe this is really 2b?) +
+
+
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. +
(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.) +
+
+
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.) +
+
The LBA Number of sectors value in the partition table is 0. +
+
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.) +
+
The Sector value of CHS begin is not 1. (Same comment.) +
+
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 /boot partition containing LILO stuff
+should be accessible.)
+
+
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.) +
+
The Cylinder value of CHS end is larger than the number +of cylinders that the BIOS reports. +
+
+
+
(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.) +
+
+
+
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.) +
+
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. +
+
(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.) +
+
+
+
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.) +
+
+
A lot of useful information was supplied by various people:
+Thomas Wolfram (thomas@aeon.in-berlin.de) - the author of os-bs,
+Peter Gutmann (pgut01@cs.auckland.ac.nz) - the author of SFS,
+Cody Batt (codyb@powerquest.com),
+Christian Carey (ccarey@CapAccess.ORG),
+Dan Fandrich (dan@fch.wimsey.bc.ca),
+David Faulks (david@santana.ca),
+Kai Henningsen (kai@khms.westfalen.de),
+Dan Hildebrand (danh@qnx.com),
+Todd Larason (jtl@molehill.org),
+Mark Morgan Lloyd (markMLl.in@telemetry.co.uk).
+Marek Michalkiewicz (marekm@i17linuxb.ists.pwr.wroc.pl),
+David C. Niemi (niemidc@clark.net),
+Matthias Paul (Matthias.Paul@post.rwth-aachen.de),
+Loek Weerd (loekw@worldonline.nl),
+S. Widlake (s.widlake@rl.ac.uk).
+