Microsoft Extensible Firmware Initiative fat32 File System Specification

FAT Data Structure

The next data structure that is important is the FAT itself. What this data structure does is define a singly linked list of the “extents” (clusters) of a file. Note at this point that a FAT directory or file container is nothing but a regular file that has a special attribute indicating it is a directory. The only other special thing about a directory is that the data or contents of the “file” is a series of 32=byte FAT directory entries (see discussion below). In all other respects, a directory is just like a file. The FAT maps the data region of the volume by cluster number. The first data cluster is cluster 2.
The first sector of cluster 2 (the data region of the disk) is computed using the BPB fields for the volume as follows. First, we determine the count of sectors occupied by the root directory:

RootDirSectors = ((BPB_RootEntCnt * 32) + (BPB_BytsPerSec – 1)) / BPB_BytsPerSec;

Note that on a FAT32 volume the BPB_RootEntCnt value is always 0, so on a FAT32 volume RootDirSectors is always 0. The 32 in the above is the size of one FAT directory entry in bytes. Note also that this computation rounds up.

The start of the data region, the first sector of cluster 2, is computed as follows:

If(BPB_FATSz16 != 0)

FATSz = BPB_FATSz16;

Else

FATSz = BPB_FATSz32;

FirstDataSector = BPB_ResvdSecCnt + (BPB_NumFATs * FATSz) + RootDirSectors;

NOTE: This sector number is relative to the first sector of the volume that contains the BPB (the sector that contains the BPB is sector number 0). This does not necessarily map directly onto the drive, because sector 0 of the volume is not necessarily sector 0 of the drive due to partitioning.
Given any valid data cluster number N, the sector number of the first sector of that cluster (again relative to sector 0 of the FAT volume) is computed as follows:

FirstSectorofCluster = ((N – 2) * BPB_SecPerClus) + FirstDataSector;

NOTE: Because BPB_SecPerClus is restricted to powers of 2 (1,2,4,8,16,32….), this means that division and multiplication by BPB_SecPerClus can actually be performed via SHIFT operations on 2s complement architectures that are usually faster instructions than MULT and DIV instructions. On current Intel X86 processors, this is largely irrelevant though because the MULT and DIV machine instructions are heavily optimized for multiplication and division by powers of 2.

FAT Type Determination

There is considerable confusion over exactly how this works, which leads to many “off by 1”, “off by 2”, “off by 10”, and “massively off” errors. It is really quite simple how this works. The FAT type—one of FAT12, FAT16, or FAT32—is determined by the count of clusters on the volume and nothing else.
Please read everything in this section carefully, all of the words are important. For example, note that the statement was “count of clusters.” This is not the same thing as “maximum valid cluster number,” because the first data cluster is 2 and not 0 or 1.
To begin, let’s discuss exactly how the “count of clusters” value is determined. This is all done using the BPB fields for the volume. First, we determine the count of sectors occupied by the root directory as noted earlier.

RootDirSectors = ((BPB_RootEntCnt * 32) + (BPB_BytsPerSec – 1)) / BPB_BytsPerSec;

Note that on a FAT32 volume, the BPB_RootEntCnt value is always 0; so on a FAT32 volume, RootDirSectors is always 0.

Next, we determine the count of sectors in the data region of the volume:

If(BPB_FATSz16 != 0)

FATSz = BPB_FATSz16;

Else

FATSz = BPB_FATSz32;

If(BPB_TotSec16 != 0)

TotSec = BPB_TotSec16;

Else

TotSec = BPB_TotSec32;

DataSec = TotSec – (BPB_ResvdSecCnt + (BPB_NumFATs * FATSz) + RootDirSectors);

Now we determine the count of clusters:

CountofClusters = DataSec / BPB_SecPerClus;

Please note that this computation rounds down.

Now we can determine the FAT type. Please note carefully or you will commit an off-by-one error!
In the following example, when it says <, it does not mean <=. Note also that the numbers are correct. The first number for FAT12 is 4085; the second number for FAT16 is 65525. These numbers and the ‘<’ signs are not wrong.

If(CountofClusters < 4085) {

/* Volume is FAT12 */

} else if(CountofClusters < 65525) {

/* Volume is FAT16 */

} else {

/* Volume is FAT32 */

}

This is the one and only way that FAT type is determined. There is no such thing as a FAT12 volume that has more than 4084 clusters. There is no such thing as a FAT16 volume that has less than 4085 clusters or more than 65,524 clusters. There is no such thing as a FAT32 volume that has less than 65,525 clusters. If you try to make a FAT volume that violates this rule, Microsoft operating systems will not handle them correctly because they will think the volume has a different type of FAT than what you think it does.

NOTE: As is noted numerous times earlier, the world is full of FAT code that is wrong. There is a lot of FAT type code that is off by 1 or 2 or 8 or 10 or 16. For this reason, it is highly recommended that if you are formatting a FAT volume which has maximum compatibility with all existing FAT code, then you should you avoid making volumes of any type that have close to 4,085 or 65,525 clusters. Stay at least 16 clusters on each side away from these cut-over cluster counts.
Note also that the CountofClusters value is exactly that—the count of data clusters starting at cluster 2. The maximum valid cluster number for the volume is CountofClusters + 1, and the “count of clusters including the two reserved clusters” is CountofClusters + 2.
There is one more important computation related to the FAT. Given any valid cluster number N, where in the FAT(s) is the entry for that cluster number? The only FAT type for which this is complex is FAT12. For FAT16 and FAT32, the computation is simple: