FAT File Systems - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 190

Table 8-7: A FAT entry tells whether a cluster is available, bad, or in use and if
in use, the number of the next cluster in a file or directory, if any.
FAT16 Range (hex) FAT32 Range (hex)

(ignore bits 28--31)

Description

0000 0000000 available cluster

0001 0000001 reserved cluster

0002--FFEF 0000002--FFFFFEF the number of the next cluster in

the file

FFF0--FFF6 FFFFFF0--FFFFFF6 reserved (not currently defined)

FFF7 FFFFFF7 bad cluster

FFF8--FFFF FFFFFF8--FFFFFFF the file's last cluster (EOC marker)

Accessing the FAT
The PIC18F4550 FAT16 firmware that follows shows how to convert a
data-cluster number to a logical block address, how to read and write to the
FAT, how to find a file's next cluster, and how to find an empty cluster.

Volume Information

A DISK structure can hold in information about a volume and its FAT:
#define FAT16 2
#define FAT32 3

typedef struct
{
byte* buffer; // pointer to a buffer equal to one sector

dword firsts; // LBA of the volume's first sector

dword fat; // LBA of the volume's FAT

dword root; // LBA of the volume's root directory

dword data; // LBA of the volume's data area

word maxroot; // maximum number of entries in the root directory

dword maxcls; // maximum number of data clusters in the volume

word fatsize; // number of sectors in the FAT

byte fatcopy; // number of copies of the FAT

byte SecPerClus; // number of sectors per cluster

byte type; // type of FAT (FAT16, FAT32)

byte mount; // TRUE if the media is mounted, FALSE if not mounted)

} DISK;