Overview of FAT32

Overview

This figure summarized the FAT32 filesystem.

The first area contains the reserved area and FAT area. They are counted in term of sectors.

Immediately after FAT area, it is the starting point of cluster 2. It contains the information on the root directory and all data stored. In these areas they are counted in term of clusters.

Boot sector

It is always at the first sector of the disk, and it is used for starting up your computer. It contains:

• An x86-based CPU jump instruction.

• The original equipment manufacturer identification (OEM ID).

• The BIOS parameter block (BPB), a data structure.

• The extended BPB.

• The executable boot code (or bootstrap code) that starts the operating system.

Among these elements, the important thing is the BIOS parameter block (BPB), which allows us to obtain the fundamental information about the FAT file system.

Here is the summary table on the fields for FAT32.

Byte Offset	Field Length	Sample Value	Field Name and Definition
0x0B	2 bytes	00 02	Bytes Per Sector. The size of a hardware sector. Valid decimal values for this field are 512, 1024, 2048, and 4096.
0x0D	1 byte	10	Sectors Per Cluster.The number of sectors in a cluster. The default cluster size for a volume depends on the volume size. Valid decimal values for this field are 1, 2, 4, 8, 16, 32, 64, and 128. The Windows Server 2003 implementation of FAT32 allows for creation of volumes up to a maximum of 32 GB. However, larger volumes created by other operating systems (Windows 95 OSR2 and later) are accessible in Windows Server 2003.
0x0E	2 bytes	24 00	Reserved Sectors. The number of sectors that precede the start of the first FAT, including the boot sector.
0x10	1 byte	02	Number of FATs. The number of copies of the FAT on the volume. The value of this field is always 2.
0x11	2 bytes	00 00	Root Entries (FAT12/FAT16 only). For FAT32 volumes, this field must be set to zero.
0x13	2 bytes	00 00	Small Sectors (FAT12/FAT16 only). For FAT32 volumes, this field must be set to zero.
0x15	1 byte	F8	Media Descriptor. Provides information about the media being used. A value of 0xF8 indicates a hard disk and 0xF0 indicates a high-density 3.5-inch floppy disk. Media descriptor entries are a legacy of MS-DOS FAT16 disks and are not used in Windows Server 2003.
0x16	2 bytes	00 00	Sectors Per FAT (FAT12/FAT16 only). For FAT32 volumes, this field must be set to zero.
0x18	2 bytes	3F 00	Sectors Per Track. Contains the "sectors per track" geometry value for disks that use INT 13h. The volume is broken down into tracks by multiple heads and cylinders.
0x1A	2 bytes	FF 00	Number of Heads. Contains the "count of heads" geometry value for disks that use INT 13h. For example, on a 1.44-MB, 3.5-inch floppy disk this value is 2.
0x1C	4 bytes	3F 00 00 00	Hidden Sectors. The number of sectors on the volume before the boot sector. This value is used during the boot sequence to calculate the absolute offset to the root directory and data areas. This field is generally only relevant for media that are visible on interrupt 13h. It must always be zero on media that are not partitioned.
0x20	4 bytes	1D 91 11 01	Large Sectors. Contains the total number of sectors in the FAT32 volume.
0x24	4 bytes	2A 22 00 00	Sectors Per FAT (FAT32 only). The number of sectors occupied by each FAT on the volume. The computer uses this number and the number of FATs and reserved sectors (described in this table) to determine where the root directory begins. The computer can also determine where the user data area of the volume begins based on the number of entries in the root directory.
0x28	2 bytes	00 00	Not used by Windows Server 2003.
0x2A	2 bytes	00 00	File System Version (FAT32 only). The high byte is the major revision number; the low byte is the minor revision number. This field supports the ability to extend the FAT32 media type in the future with concern for old FAT32 drivers mounting the volume. Both bytes are zero in Windows Server 2003, Windows 2000, and Windows Me and earlier.
0x2C	4 bytes	02 00 00 00	Root Cluster Number (FAT32 only). The cluster number of the first cluster of the root directory. This value is typically, but not always, 2.
0x30	2 bytes	01 00	File System Information Sector Number (FAT32 only). The sector number of the File System Information (FSINFO) structure in the reserved area of the FAT32 volume. The value is typically 1. A copy of the FSINFO structure is kept in the Backup Boot Sector, but it is not kept up-to-date.
0x32	2 bytes	06 00	Backup Boot Sector (FAT32 only). A value other than zero specifies the sector number in the reserved area of the volume where a copy of the boot sector is stored. The value of this field is typically 6. No other value is recommended.
0x34	12 bytes	00 00 00 00 00 00 00 00 00 00 00 00	Reserved (FAT32 only). Reserved space for future expansion. The value of this field must always be zero.