FAT32 Study Notes (1)

1. Basic Concepts

FAT32 spec
The full name of fat is File Allocation Table. Fat was developed by Microsoft in the 1980s, mainly including fat12, fat16, and FAT32, there are 12 bits in fat12 fat entry, 16 bits in fat16, and 32bit in FAT32 fat entry.

 

2, fat format

The size of a single sector (sector) is 512 bytes. The size of a cluster is usually the number of clusters of the integer power of 2. the maximum size is 32 KB. A file occupies at least one cluster, it is usually an integer cluster, which is linked to each other.

FAT32 format (data is calculated from the cluster and does not contain cluster0 or cluster1 .)

 

MBR

Boot

Fsinfo

CLR

Fat1

Fat2

Data

 

 

Fat16 format (data starts with cluster2)

 

MBR

Boot

CLR

Fat1

Fat2

Rotdir

Data

 

 

Boot refers to the boot sector. The boot sector occupies the first logical sector and the size is bytes. For detailed explanations of the boot sector, refer to FAT32 spec. The main useful members are as follows:

 

Name	Offset (byte)	Size (byte)	Description
Bpb_bytspersec	11	2	Description of the number of bytes for each sector, usually 512 (Forward compatible). The value can also be 4096, Note: If the physical sector of a device is N and N is greater than 512, the value is N but cannot exceed 4096.
Bpb_secperclus	13	1	Indicates the number of sector s in each cluster, which is usually an integer power of 2, but a cluster generally does not exceed 32 K.
Bpb_rsvdseccnt	14	2	Retain the number of sector in reserved region. For fat12 fat16, this value is 1, for FAT32, this value is 32.
Bpb_numfats	16	1	The number of fat data structures, usually 2, fat1, and fat2. Security is mutually backed up.
Bpb_rootentcnt	17	2	For FAT32, this value is 0. For fat12 and fat16, this value indicates the number of entries in the 32byte root directory.
Bpb_totsec16	19	2	For fat12 and fat16, this value indicates the number of sector in fatimg. For FAT32, this value is 0, the number of sector in fatimg is expressed in the range of offset 32 length 4.
Bpb_media	21	1	0xf8 indicates non-removable media, and 0xf0 indicates removable media. The value must be the same as the low position of fat [0 ].
Bpb_fatsz16	22	2	For fat12 and fat16, it indicates that a fat occupies the sector size. For FAT32, this value is 0.
Bpb_hiddsec	28	4	For devices without partitions, this value is 0. For other devices, this value indicates the number of hidden sector before partition.
Bpb_totsec32	32	4	FAT32 indicates the total number of devices. Does not contain the MBR

 

 

 

The following is for FAT32.

Name	Offset (byte)	Size (byte)	Description
Bpb_fatsz32	36	4	The number of sector occupied by a fat table. FAT32 has two fat tables that are mutually backed up. fat12 and fat16 are empty.
Bpb_extflags	40	2	When bit7 is 0, fat table 1 and fat Table 2 are mirrored to each other during running. If bit7 is 1, only one fat table is active during running, fat12 and fat16 are empty.
Bpb_rootclus	44	4	The serial number of the cluster where the root directory is located, usually 2, that is, the second cluster of Data region is the root directory, in this way, you can easily find the root directory from cluster2 (I personally understand that cluster starts from data Region)
Bpb_fsinfo	48	2	Indicates the number of sector occupied by fsinfo.
Bpb_bkbootsec	50	2	Number of Boot Sector backups in the reserved area.
Bpb_reserved	52	12	Future Extension
Bs_drvnum	64	1
Bs_filpolicype	82	8	FAT32 is usually set as the string "FAT32"
	510	2	The fixed value is 0x55 AA. The fixed value offset must be at the 510 position and does not change with the sector.

FAT32 Study Notes (1)