Microsoft Extensible Firmware Initiative fat32 File System Specification

IsEOF = FALSE;

If(FATType == FAT12) {

If(FATContent >= 0x0FF8)

IsEOF = TRUE;

} else if(FATType == FAT16) {

If(FATContent >= 0xFFF8)

IsEOF = TRUE;

} else if (FATType == FAT32) {

If(FATContent >= 0x0FFFFFF8)

IsEOF = TRUE;

}

Note that the cluster number whose cluster entry in the FAT contains the EOC mark is allocated to the file and is also the last cluster allocated to the file. Microsoft operating system FAT drivers use the EOC value 0x0FFF for FAT12, 0xFFFF for FAT16, and 0x0FFFFFFF for FAT32 when they set the contents of a cluster to the EOC mark. There are various disk utilities for Microsoft operating systems that use a different value, however.

There is also a special “BAD CLUSTER” mark. Any cluster that contains the “BAD CLUSTER” value in its FAT entry is a cluster that should not be placed on the free list because it is prone to disk errors. The “BAD CLUSTER” value is 0x0FF7 for FAT12, 0xFFF7 for FAT16, and 0x0FFFFFF7 for FAT32. The other relevant note here is that these bad clusters are also lost clusters—clusters that appear to be allocated because they contain a non-zero value but which are not part of any files allocation chain. Disk repair utilities must recognize lost clusters that contain this special value as bad clusters and not change the content of the cluster entry.
NOTE: It is not possible for the bad cluster mark to be an allocatable cluster number on FAT12 and FAT16 volumes, but it is feasible for 0x0FFFFFF7 to be an allocatable cluster number on FAT32 volumes. To avoid possible confusion by disk utilities, no FAT32 volume should ever be configured such that 0x0FFFFFF7 is an allocatable cluster number.
The list of free clusters in the FAT is nothing more than the list of all clusters that contain the value 0 in their FAT cluster entry. Note that this value must be fetched as described earlier as for any other FAT entry that is not free. This list of free clusters is not stored anywhere on the volume; it must be computed when the volume is mounted by scanning the FAT for entries that contain the value 0. On FAT32 volumes, the BPB_FSInfo sector may contain a valid count of free clusters on the volume. See the documentation of the FAT32 FSInfo sector.
What are the two reserved clusters at the start of the FAT for? The first reserved cluster, FAT[0], contains the BPB_Media byte value in its low 8 bits, and all other bits are set to 1. For example, if the BPB_Media value is 0xF8, for FAT12 FAT[0] = 0x0FF8, for FAT16 FAT[0] = 0xFFF8, and for FAT32 FAT[0] = 0x0FFFFFF8. The second reserved cluster, FAT[1], is set by FORMAT to the EOC mark. On FAT12 volumes, it is not used and is simply always contains an EOC mark. For FAT16 and FAT32, the file system driver may use the high two bits of the FAT[1] entry for dirty volume flags (all other bits, are always left set to 1). Note that the bit location is different for FAT16 and FAT32, because they are the high 2 bits of the entry.
For FAT16:

ClnShutBitMask = 0x8000;

HrdErrBitMask = 0x4000;

For FAT32:

ClnShutBitMask = 0x08000000;

HrdErrBitMask = 0x04000000;

Bit ClnShutBitMask – If bit is 1, volume is “clean”.

If bit is 0, volume is “dirty”. This indicates that the file system driver did not Dismount the volume properly the last time it had the volume mounted. It would be a good idea to run a Chkdsk/Scandisk disk repair utility on it, because it may be damaged.

Bit HrdErrBitMask – If this bit is 1, no disk read/write errors were encountered.

If this bit is 0, the file system driver encountered a disk I/O error on the Volume the last time it was mounted, which is an indicator that some sectors may have gone bad on the volume. It would be a good idea to run a Chkdsk/Scandisk disk repair utility that does surface analysis on it to look for new bad sectors.
Here are two more important notes about the FAT region of a FAT volume:

1. 
   The last sector of the FAT is not necessarily all part of the FAT. The FAT stops at the cluster number in the last FAT sector that corresponds to the entry for cluster number CountofClusters + 1 (see the CountofClusters computation earlier), and this entry is not necessarily at the end of the last FAT sector. FAT code should not make any assumptions about what the contents of the last FAT sector are after the CountofClusters + 1 entry. FAT format code should zero the bytes after this entry though.
   
2. 
   The BPB_FATSz16 (BPB_FATSz32 for FAT32 volumes) value may be bigger than it needs to be. In other words, there may be totally unused FAT sectors at the end of each FAT in the FAT region of the volume. For this reason, the last sector of the FAT is always computed using the CountofClusters + 1 value, never from the BPB_FATSz16/32 value. FAT code should not make any assumptions about what the contents of these “extra” FAT sectors are. FAT format code should zero the contents of these extra FAT sectors though.
   

FAT Volume Initialization

At this point, the careful reader should have one very interesting question. Given that the FAT type (FAT12, FAT16, or FAT32) is dependant on the number of clusters—and that the sectors available in the data area of a FAT volume is dependant on the size of the FAT—when handed an unformatted volume that does not yet have a BPB, how do you determine all this and compute the proper values to put in BPB_SecPerClus and either BPB_FATSz16 or BPB_FATSz32? The way Microsoft operating systems do this is with a fixed value, several tables, and a clever piece of arithmetic.
Microsoft operating systems only do FAT12 on floppy disks. Because there is a limited number of floppy formats that all have a fixed size, this is done with a simple table:

“If it is a floppy of this type, then the BPB looks like this.”

There is no dynamic computation for FAT12. For the FAT12 formats, all the computation for BPB_SecPerClus and BPB_FATSz16 was worked out by hand on a piece of paper and recorded in the table (being careful of course that the resultant cluster count was always less than 4085). If your media is larger than 4 MB, do not bother with FAT12. Use smaller BPB_SecPerClus values so that the volume will be FAT16.

The rest of this section is totally specific to drives that have 512 bytes per sector. You cannot use these tables, or the clever arithmetic, with drives that have a different sector size. The “fixed value” is simply a volume size that is the “FAT16 to FAT32 cutover value”. Any volume size smaller than this is FAT16 and any volume of this size or larger is FAT32. For Windows, this value is 512 MB. Any FAT volume smaller than 512 MB is FAT16, and any FAT volume of 512 MB or larger is FAT32.
Please don’t draw an incorrect conclusion here.
There are many FAT16 volumes out there that are larger than 512 MB. There are various ways to force the format to be FAT16 rather than the default of FAT32, and there is a great deal of code that implements different limits. All we are talking about here is the default cutover value for MS-DOS and Windows on volumes that have not yet been formatted. There are two tables—one is for FAT16 and the other is for FAT32. An entry in these tables is selected based on the size of the volume in 512 byte sectors (the value that will go in BPB_TotSec16 or BPB_TotSec32), and the value that this table sets is the BPB_SecPerClus value.