If you have several operating systems on your disks, then each uses one or more disk partitions. A disagreement on where these partitions are may have catastrophic consequences.
(where CHS stands for Cylinder/Head/Sector).
This information is redundant: the location of a partition
is given both by the 24-bit
Linux only uses the
DOS uses the
Windows 95 has support for the Extended INT13 interface, and
uses special partition types (c, e, f instead of b, 6, 5)
to indicate that a partition should be accessed in this way.
When these partition types are used, the
What is this nonsense you get from
Many old IBM PS/2 systems used disks with a defect map written to the end of the disk. (Bit 0x20 in the control word of the disk parameter table is set.) Therefore, FDISK would not use the last cylinder. Just to be sure, the BIOS often already reports the size of the disk as one cylinder smaller than reality, and that may mean that two cylinders are lost. Newer BIOSes have several disk size reporting functions, where internally one calls the other. When both subtract 1 for this reserved cylinder and also FDISK does so, then one may lose three cylinders. These days all of this is irrelevant, but this may provide an explanation if one observes that different utilities have slightly different opinions about the disk size.
A well-known claim says that partitions should start and end at cylinder boundaries.
Since "disk geometry" is something without objective existence, different operating systems will invent different geometries for the same disk. One often sees a translated geometry like */255/63 used by one and an untranslated geometry like */16/63 used by another OS. (People tell me Windows NT uses */64/32 while Windows 2K uses */255/63.) Thus, it may be impossible to align partitions to cylinder boundaries according to each of the the various ideas about the size of a cylinder that one's systems have. Also different Linux kernels may assign different geometries to the same disk. Also, enabling or disabling the BIOS of a SCSI card may change the fake geometry of the connected SCSI disks.
Fortunately, for Linux there is no alignment requirement at all. (Except that some semi-broken installation software likes to be very sure that all is OK; thus, it may be impossible to install RedHat 7.1 on a disk with unaligned partitions because DiskDruid is unhappy.)
People report that it is easy to create nonaligned partitions in Windows NT, without any noticeable bad effects.
But MSDOS 6.22 has an alignment requirement. Extended partition sectors that are not on a cylinder boundary are ignored by its FDISK. The system itself is happy with any alignment, but interprets relative starting addresses as if relative to an aligned address: The starting address of a logical partition is given relative not to the address of the extended partition sector that describes it, but relative to the start of the cylinder that contains that sector. (So, it is not surprising that also PartitionMagic requires alignment.)
What is the definition of alignment?
MSDOS 6.22 FDISK will do the following:
1. If the first sector of the cylinder is a partition
table sector, then the rest of the track is unused,
and the partition starts with the the next track.
This applies to sector 0 (the MBR) and the partition table sectors
preceding logical partitions.
2. Otherwise, the partition starts at the first sector of the
cylinder. Also the extended partition starts at a cylinder boundary.
Use of partition type 85 for the extended partition makes it invisible to DOS, making sure that only Linux will look inside.
As an aside: on a Sparc, the boot partition must start on a cylinder boundary (but there is no requirement on the end).
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