Partitions and the Partition Table

CS 321 2007 Lecture, Dr. Lawlor

So you've got a disk. The disk stores bytes. You can in theory access the raw disk (and some database systems do this for speed!), but it's way more common to build several layers of storage on top of the raw disk bytes:

Disks are divided into "partitions", which are just pieces of the disk used for different things. You can make one disk appear as your C, D, and E drives by splitting it into three partitions. You can install Windows and Linux on the same disk using two partitions.
Partitions are formatted with a "filesystem", like FAT, NTFS, or Linux ext3.
Filesystems contain many files, like "README.txt".

Example: Windows Disk Manager

To get to the Windows Disk Manager, first right-click on "My Computer" and hit "Manage", then select "Disk Manager" near the bottom of the left-hand side list. This shows you all your disks, and importantly all your partitions:

Window XP Partitoin Manager, showing partitions

Window XP Partitoin Manager, showing partitions

The original design of the IBM PC's "boot sector" (the first 512 bytes of the disk) left room for only four "primary" partitions. Since people running multiple operating systems often want more than this, you can also make an "extended" partition (in green above) that counts as a primary partition, but contains other smaller "logical drive" partitions inside of it. You can also leave unpartitioned space on your disk, which can later be used to create new partitions.

If your partition table gets horribly screwed up, you may still be able to recover your partitions and all their data using a tool like TestDisk.

Example: Linux fdisk

The same disk viewed in Windows above can be examined in Linux like so:

root@dellawlor:/home/olawlor/class/cs321/lecture/fs_fat # fdisk /dev/hda
Command (m for help): p

Disk /dev/hda: 80.0 GB, 80026361856 bytes
255 heads, 63 sectors/track, 9729 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/hda1               1           6       48163+  de  Dell Utility
/dev/hda2   *           7        4262    34186320    7  HPFS/NTFS
/dev/hda3   *        4263        7910    29302560   83  Linux
/dev/hda4            7911        9729    14611117+   5  Extended
/dev/hda5            7911        8519     4891761   83  Linux
/dev/hda6            8520        9128     4891761   83  Linux
/dev/hda7            9129        9372     1959898+  83  Linux
/dev/hda8            9373        9495      987966   82  Linux swap
/dev/hda9            9496        9520      200781    b  W95 FAT32
/dev/hda10           9521        9545      200781    b  W95 FAT32
/dev/hda11           9546        9570      200781   83  Linux
/dev/hda12           9571        9729     1277136   83  Linux

Command (m for help): q

Again, I've got a bunch of partitions. /dev/hda2 is my Windows partition. /dev/hda3 is my main Linux partition. /dev/hda4 is my extended partition, which contains hda5 through hda12.

Boot Process

Power flows into the CPU
The CPU begins executing the BIOS, which is stored in flash ROM on your motherboard.
The BIOS looks for a disk whose first sector ends with the two bytes "0xaa 0x55". This is a "boot sector", or "master boot record".
The BIOS executes the boot sector, which has to contain raw 16-bit mode x86 machine code.
The boot sector code looks at the partition table (stored inside itself), and finds the boot loader on your boot partition. This is usually NTLDR for Windows systems, or Grub or LILO for Linux.
The boot loader finds the OS kernel on the disk.
The OS kernel starts running, loads up the disks, starts processes, and the machine can then be used for real work.