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Partition Letter Assignment

Partition letters (C:, D:, etc) are assigned by DOS/Windows9x when the system boots. These are my observations on how it is done.

The first available drive letter is C:

  • The first (by the ordering in the MBR's partition table) primary DOS FAT12/FAT16 partition on drive 0 is assigned the next available drive letter.

  • If this is Win95B and no primary FAT12/FAT16 partitions were found, then the first primary FAT32 on drive 0 is assigned the next available drive letter.

  • Each additional drive is then processed, assigning the next available drive letter to the first primary FAT12/16/32 partition on the drive.

  • The first extended DOS partition on each drive is processed. The next available drive letters are assigned to logical DOS paritions in the extended partition.

  • Additional primary DOS partitions on each drive are processed. The next available drive letters are assigned. It is not possible to created more than one primary DOS partition on a drive without using special tools.

  • Additional extended DOS partitions on each drive are ignored. It is not possible to created more than one extended DOS partition on a drive without using special tools.

Here's a typical example of standard partitioning.
  Drive 0:
    Primary DOS FAT16           C:
    Extended DOS
      Logical DOS FAT12         E:
      Logical DOS FAT16         F:
  Drive 1:
    Primary DOS FAT12           D:
    Extended DOS
      Logical DOS FAT16         G:
      Logical DOS FAT16         H:
Here's an oddball example of standard partitioning. Although this configuration is valid, it is unusable since there is no primary bootable partition on drive 0.
  Drive 0:
    Extended DOS
      Logical DOS FAT12         D:
      Logical DOS FAT16         E:
  Drive 1:
    Primary DOS FAT16           C:
    Extended DOS
      Logical DOS FAT16         F:
      Logical DOS FAT16         G:
Lance Costanzo, [email protected]