The Partition Sector (boot process)
Here are some info about the partition sector of an hard drive which is used during the booting process. Rcgreen has already written an excellent article about the pc boot process here . I will try to add some elements of my knowledge to complete his article.
During the boot process, the BIOS (for Basic Input/Output System or Basic Integrated/Operating System) launch a script called the Bootstrap. It will search the partition sector which contains all the infos about the partitions into your hard drive and some others interesting things.
Structure of the Partition sector
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octet............contain............size
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00h...............MBR................446
1BEh.......1st partition...........16
1CEh.......2nd partition .........16
1DEh.......3rd partition...........16
1EEh ......4th partition...........16
1FEh....identification code.......2
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Each sector of an hard drive can contain generaly 512 octets of memory and the rule is the same with the Partition sector. It contains in the first 446 octets a partition code most commonly called the MBR (Master Boot Record),it's a script which will read the Partition table, and run the boot sectors of the activated partitions. For example it's in the MBR that you can install LILO in order to select between different OS's on different partitions.
After the first 446 octets, there is the Partition table. This table contains 4 structures of 16 octets providing infos about partitions. This number of 4 explain why it is not possible to create more of 4 primary partitions on a hdd without using extended partitions (in fact the extended partition contain more than 1 partition but count as only 1 primary partition in the partition table). You need to understand that even if you have not partitioned your hdd, you have 1 partition counted in the partition table.
Structure of a Partition table
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octet.........................contain..............................size
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00h...........................state...................................1
01h.................head of begining...........................1
02h...........sector & cylinder of begining...............2
04h....................partition type.............................1
05h......................head of end.............................1
06h.............sector & cylinder of end....................2
08h.....distance with the fist partition sector.......4
0Ch........number of sectors in the partition.........4
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The state can be "00h" if the partition is not activated or "80h" if it is. The info about the head, sectors and cylinders of begining and end of the partition are needed in order to physically find it. We will notice that sectors and cylinders are coded in order to put the cylinder code in only 5 bits and the sectors in 10 bits (1 octet + 2 bits). As a bit can only be a "1" or a "0", this system allow the capability to recognise 2^10 = 1024 sectors. That's why it is not possible to launch boot sectors after the 1024th sector (a real problem with Linux and Windows on the same hdd). The partition type can have differents values like "00h" for a free partition (the partition don't exist) or "04h" for the primary DOS.
Ok, I think that I have told the more interesting about this subject (in fact, I don't know really more than this ;))
If you have coments or if you find an error, don't hesitate to post or to pm me.
Hoping it will help you...
