You can also create sub-directories in the Data Area with files that appear to be contained inside them. In Fact, Subdirectories are nothing more than a special file which lists all of the files seemingly contained inside this directory and all the relevant data about each file such as, the location of each file's Starting Cluster, date, time and file size etc.
The DBR also contains some important information about the disk geometry. This information is located in the first sector of every partition, such as:
- Jump Code + NOP
- OEM Name and Version
- Bytes Per Sector
- Sectors Per Cluster
- Reserved Sectors
- Number of Copies of FAT
- Maximum Root Directory Entries (but Not Available for FAT32)
- Number of Sectors in Partition Smaller than 32MB (Therefore Not Available for FAT32)
- Media Descriptor (F8h for Hard Disks)
- Sectors Per FAT (In Older FAT Systems and Not Available for FAT32)
- Sectors Per Track
- Number of Heads
- Number of Hidden Sectors in Partition
- Number of Sectors in Partition
- Number of Sectors Per FAT
- FAT Information Descriptor Flags
- Version of FAT32 Drive
- Cluster Number of the Start of the Root Directory
- Sector Number of the File System Information Sector
- Sector Number of the Backup Boot Sector
- Reserved
- Logical Drive Number of Partition
- Extended Signature (29H)
- Serial Number of Partition
- Volume Name of Partition
- FAT Name
- Executable Code
- Executable Marker or Magic Number (AAH 55H)
The first 3 Bytes of DBR contain a JMP instruction to skip the information and make extensions possible because the MBR loads this sector into memory and transfers execution to it. Usually these three bytes are hexadecimal numbers in format something like E9 XX XX (Hex) or EB XX 90 (Hex).
Following the initial JMP instruction OEM ID is an 8–Bit Field that is reserved by Microsoft for OEM Identification. The OEM ID describes the program that created the boot record. This is often "MSWIN4.0" for Windows 95/98/ME, "IBM 20.0" for OS/2 and "MSDOS5.0" for MS-DOS 4.0 and later.
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