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What does it mean to mark a partition as "active"?

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#1 HexiumVII



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Posted 30 May 2003 - 11:33 PM

I'm partitioning some new drives in XP and noticed in the disk manager that you can "mark partition as active" what does this do excatly?

#2 Guest_fafot_*

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Posted 30 May 2003 - 11:47 PM

That means that this will be the BOOT partition. All the others will be just usual.

#3 Big Buck Hunter

Big Buck Hunter


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Posted 31 May 2003 - 05:06 AM

A good look at the change the ACTIVE flag makes to a partition table can be found here:


      - A 100
       xxxx:xxxx MOV AX,201  (ignore xxxx:xxxx segment:offset values on left)
       xxxx:xxxx MOV BX,200
       xxxx:xxxx MOV CX,1
       xxxx:xxxx MOV DX,80   (or use 81 for 2nd drive, 82 for 3rd, 83 for
       xxxx:xxxx INT 13
       xxxx:xxxx INT 3
                    (press ENTER an extra time here to return to dash prompt)
       - G=100
                    (ignore register dump)
       - D 3BE 3FF  (following output is a hex dump of the partition table)

                                                           x1 x2 
       xxxx:xxxx x3 x4 x5 x6 x7 x8 x9 10-11 12 13 14 15 16 xx xx 
       xxxx:xxxx xx xx xx xx xx xx xx xx-xx xx xx xx xx xx xx xx 
       xxxx:xxxx xx xx xx xx xx xx xx xx-xx xx xx xx xx xx xx xx 
       xxxx:xxxx xx xx xx xx xx xx xx xx-xx xx xx xx xx xx 55 AA 

       - Q        (quits back to DOS prompt)

The hex dump resulting from above procedure shows the partition table for the first hard drive. The above values 1 through 16 are the 16 bytes (each represented by 2 digits of hexadecimal) that hold the information for one partition. There are four 16-byte slots - so that four paritions can be defined on the drive - for instance, a Primary DOS parition and an Extended DOS partition may be defined, as well as partitions created by other operating systems. These four 16-byte slots are followed by the the values 55 AA which identifies this as a valid partition table. The 55 AA also indicates the end of the boot sector. The fifth byte in each slot (5 above) indicates the partition type. Here are the partition types used by MS-DOS as indicated in Q69912:

    Type:         Size:     FAT type:    First used in:
    01 - PRI DOS  0-15MB    12-bit FAT   MS-DOS 2.0
    04 - PRI DOS  16-32MB   16-bit FAT   MS-DOS 3.0
    05 - EXT DOS  0-2GB     n/a          MS-DOS 3.3
    06 - PRI DOS  32MB-2GB  16-bit FAT   MS-DOS 4.0
    0E - PRI DOS  32MB      16-bit FAT   Windows 95
    0F - EXT DOS  0-2GB     n/a          Windows 95

Here are what some of the other values indicate:

   1 - Active Status 80 = Active
                      00 = Not Active

   2 - Starting Head of Partition

   3/4 - Starting Cylinder and Sector (combined)

   5 - Partition Type

   6 - Ending Head of Partition (1 less than total number of heads)

   7/8  -  Ending Cylinder and Sector (combined)

   9/10/11/12 - Relative Partition Sector Start
             (no. of sectors between partition table and start of partition)

   13/14/15/16 - Size of Partition (in sectors)

Thank you for your time,
Frank Russo

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