Reading the Boot Sector of a Drive

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BOOTSEC demonstrates how to use Interrupt 25h (absolute disk read) to read the boot sector (the first sector on head 0, cylinder 0) off of a drive (either a floppy disk drive or hard disk).

BOOTSEC checks to see whether the drive is one of the following:

Drive Detection Method
CD-ROM Interrupt 2F calls to MSCDEX.
Net drive Windows API WNetGetConnection().
RAM drive Checks the boot sector to see if there is one FAT.
Hard disk Checks the media BYTE of the boot sector. If it is
equal to 0xF8h then it is a hard disk.
Floppy disk Checks the media BYTE of the boot sector. If it is
not equal to 0xF8h and it is not a RAM drive, net
drive, or CD-ROM drive, then it is a floppy disk drive.
BOOTSEC also shows how to implement a dialog box as a main window using a private dialog class.

More Information

The following files are available for download from the Microsoft Download Center:


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119591 How to Obtain Microsoft Support Files from Online Services
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The following information is contained in the boot sector:

  • The jump instruction to the boot strap routine
  • The name of the OEM and the version of MS-DOS
  • Bytes per sector
  • Sectors per cluster
  • Reserved sectors
  • Number of file allocation tables
  • Number of root directory entries
  • Number of sectors
  • Media descriptor
  • Number of sectors occupied by each FAT
  • Number of sectors on a single track
  • Number of read/write heads on the drive
  • Number of hidden sectors
  • Number of huge sectors
  • Whether the disk is the first hard disk drive
  • The boot signature
  • The volume serial number
  • The volume label
  • The file system type
NOTE: The information contained in the boot sector was changed in MS-DOS 5.0. If this program is run on a disk that was formatted with a previous version of MS-DOS, then some of the fields in the structure will not be filled out, and the program may display garbage. The elements of the structure that were not changed will be displayed correctly.

Article ID: 102870 - Last Review: Aug 30, 2004 - Revision: 1