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•  Real Partition Table Examples
•  20 GB Hard Drive (with NTFS, FAT32 and Linux Partitions)


•  Footnotes

Real Partition Table Examples

 Offset     Size                Description
 ------  ---------   ----------------------------------
  000h   446 bytes    Master bootstrap loader code area
                     (includes the NT Drive S/N if any;
    see the Standard, FAT32 and NTFS MBRs for example.)

  1BEh    16 bytes    Partition entry for partition 1
  1CEh    16 bytes    Partition entry for partition 2
  1DEh    16 bytes    Partition entry for partition 3
  1EEh    16 bytes    Partition entry for partition 4

  1FEh     2 bytes    Signature ID, the WORD: AA55h;
                       indicates a valid MBR sector.

A 20 GB Hard Drive with Three OSs

Figure 5.

This is a Hex view of just the last 80 bytes of a hard disk's MBR sector as they might appear in a disk editor. The Partition Table is enclosed within the thin YELLOW line in the diagram above. Two other items also appear in this diagram: 1) The thin violet/purple line surrounding the Hex bytes "A8 E1 A8 E1" shows an example of an NT Disk Signature (also used by Win 2000/XP) and:  2) The sector's Word-sized Signature ID of AA55h; often called its Magic number in older tech documentation.
[ Note: on Intel x86 CPU systems, Hex Words are stored with the Low-byte first and the High-byte last so AA55h appears as:  55 AA  in a disk editor ].

In the upper-right corner of the table, you'll see a byte with the value of  80 . This is the first byte in the first Partition entry, and it's followed by the bytes  01 01 00 . Most HDDs have these bytes, since the first partition is usually the boot partition on most hard disks [ the 80h byte tells the Bootstrap Loader code (the MBR code) which partition it is supposed to try loading a Boot sector from into Memory; only one partition entry (at boot-up) can have the 80h byte here] and because the next three bytes are the CHS values for the Starting Sector of the first partition (CHS 0,1,1 in this case; which corresponds to Absolute Sector 63 of this disk, where most Boot Records begin).

Until HDDs became larger than 8.4 GB, the Starting and Ending CHS values and the Total Sector value could be checked against each other; which might help to correct an error for a manual entry in the Table. Example: The Last Sector CHS values (877,254,63) can be used to check the Total number of Sectors by first computing:  878 cylinders x 255 heads x 63 sectors = 14,105,070 sectors (don't forget that both the Cylinder and Head counts begin with a zero), and then subtracting the 63 sectors which preceded it to arrive at: 14,105,007 sectors for its Total size or length. Thus, you can see that they must agree with each other!

Before launching into all the calculations for the rest of this drive's partition table, we need to point out that this isn't the typical layout for a drive created by a Microsoft OS. Instead of a single Primary partition followed by an Extended partition, this drive has three Primary partitions and then an Extended partition!

In the table above, you'll find that computing CHS values isn't always very easy to do! Though the whole first byte for each Starting and Ending CHS entry is always for the Head*, the Cylinder and Sector bytes must be regrouped before you can compute them! To compute the Sector value, you must remove the leading two bits of the second (or middle) byte; thus the largest value you can have for a Sector is: 3Fh (__11 1111) = 63. To compute the Cylinder value, the two bits from the middle byte become the first two bits of a ten-bit hex number that ends with the last byte in each CHS entry. So, the largest value you can have for a Cylinder is: 3FFh (11 1111 1111) = 1023 (in the table, which gives you a count of 1024 cylinders). Since these are the largest values that can be used in any Standard Partition Table, all CHS values are limited to: 1024 x 255* x 63 = 16,450,560 sectors (or 8,422,686,720 bytes); thus the 8.4 GB limit for any BIOS without the newer INT 13 Extensions (such as Functions 42, 43 and 48 for reading, writing and getting drive parameters)!
________________
*For those who missed the note about why we do not use 256 heads, see this: Note about a 'bug' in MS-DOS.

Therefore, when any part of a partition reaches further than 8.4 GB into a drive, you lose some redundancy as can be seen in the Last Sector CHS portion of this entry:

If the Cylinder value wasn't limited to 1023, then the Last Sector CHS triple's cylinder value could be computed as follows: 26,394,795 (14,105,070 + 12,289,725; the same as the Relative Sector Offset in the next entry!) divided by 16,065 (255 x 63) = 1643 cylinders. So, the CHS would be: 1642,254,63.

And for any full partition that's beyond 8.4 GB, such as the last two entries in our example Partition Table above, only the values inside of the thin GREEN lines can be used to compute both a partition's actual size and location!  Note both the Starting Sector and Last Sector items in our example Table's third entry:

Finally, the fourth entry in our example Table is that of an LBA Extended Partition (type = 0Fh). In the case of any Extended Partition, its Total Sectors (or Length) includes all of the Logical partitions that may exist inside of it, whether there's only one or many. Extended Partitions often run all the way to the end of a drive's useable sectors, but they don't have to. You should also note that even if all four entries of a Partition Table are being used, that does not necessarily mean there's no empty space left on a drive.

Last Update: June 9, 2007. (2007.06.09)

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