The FAT16 File System (with SD cards)

I have been working on a  project where the client requires their device to have it’s firmware updated from a Secure Digital (SD) Card.

The standard way to store and access data on SD cards is to use the MS FAT File System.

There are several libraries available dotted around the Internet which allow developers to access SD using FAT. However, if you are on a tight program space budget, such as in the Boot-Loader of an embedded Microcontroller, then you may need to program the FAT File System access manually.

This process is reasonably difficult to accomplish in raw code as there are quite a few steps to perform before you gain access to the actual file you are after.

I’ll outline here the basic features of FAT16, which for the purposes of this example, will be the simplest to begin with.

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The FAT16 Layout

The layout of the MS FAT16 File System can be seen below;

The Master Boot Record (MBR):

Offset	0x0000
Size	512 Bytes

 

 

The Master Boot Record is the first location that the Host File System parses in order to navigate the File Structure of the SD Card. It is always located at Sector zero and not only sets out the addressing parameters of the File System but also contains operating code run by the host at startup.

Address (Offset)	Item	Length	Notes
0x0000	Executable Code	446 Bytes	Executed by the Host system at startup
0x01BE (00h)	Partition 1: State (Partition 1 Entry Begin)	1 Byte	Beginning of the Partition 1 Entry. 16 Bytes Total. The State determines if the Partition is Active (80h) or Inactive (00h)
0x01BF (01h)	Partition 1: Start Head	1 Byte
0x01C0 (02h)	Partition 1: Start Sector / Cylinder	2 Bytes	Defines the Start Sector and Cylinder of Partition 1
0x01C2 (04h)	Partition 1: Partition Type	1 Byte	Defines the Type of Partition; 01h = 12bit FAT 04h = 16bit FAT (<32Mb) 05h = Ex MSDOS 06h = 16bit FAT (>32Mb) 0Bh = 32bit FAT (<2Gb)
0x01C3 (05h)	Partition 1: End Head	1 Byte
0x01C4 (06h)	Partition 1: End Sector / Cylinder	2 Bytes	Defines the End Sector and Cylinder of Partition 1
0x01C6 (08h)	Partition 1: Start Sector	4 Bytes	Contains the Start Sector Address of Partition 1. Defined as the number of Sectors between the MBR and the first Sector of the Partition.
0x01CA (0Ch)	Partition 1: Partition Length (Partition 1 Entry End)	4 Bytes	End of the Partition 1 Entry. 16 Bytes Total. Defines the number of Sectors in the Partition
0x01CE	Partition 2 Entry	16 Bytes	Defined as above
0x01DE	Partition 3 Entry	16 Bytes	Defined as above
0x01EE	Partition 4 Entry	16 Bytes	Defined as above
0x01FE	Executable Marker	2 Bytes	Set as 0x55AA

 

The Boot Sector Entry (BSE):

Offset	1st sector of Partition
Size	512 Bytes
Address Location Calculation	Partition Start Sector (From MBR) * Bytes / Sector

The Boot Sector Entry contains all the pertinent information regarding the Partitions’ structure. The basic layout is shown below;

Offset	Item	Length	Notes
00h	Jump Code	3 Bytes	Contains executable Machine code to allow the Host System to jump past the BSE
03h	OEM ID	8 Bytes	The system name which formatted the device
0Bh	Bytes Per Sector	2 Bytes	Normally 0x2000 (512 )
0Dh	Sectors Per Cluster	1 Byte
0Eh	No of Reserved Sectors	2 Bytes
10h	No of FATs	1 Byte	The Number of FAT Entries
11h	Max Root Directory Entries	2 Bytes	For SD Cards this is usually 0x2000 (512)
13h	Total Sectors in Partition	2 Bytes	Only set if the device is < 32Mb. Will likely be unset 0x0000
15h	Media Type	1 Byte	0xF8 = Hard Drive
16h	Sectors Per FAT	2 Bytes	Used for reaching the Root Directory
18h	Sectors Per Track	2 Bytes
1Ah	No of Heads	2 Bytes
1Ch	No of Hidden Sectors	4 Bytes	Number of hidden sectors between the start of the disk and the BSE
20h	No of Sectors in a Partition	4 Bytes	Total number of Sectors in the Partition
24h	Logical Drive No	1 Byte	0x80 for Hard Drives
25h	Head No	1 Byte	Usually 0x00
26h	Extended Boot Signature	1 Byte	Always 0x29 denoting that Serial No, Label and Type fields are valid
27h	Serial No	4 Bytes	Serial No of the Device
2Bh	Partition Name	11 Bytes
36h	FAT Type	8 Bytes
3Eh	Executable Boot Code	448 Bytes	Allows the host to find the first file which boots the system
0x01FE	Executable Signature	2 Bytes	End of BSE

FAT Entries (FAT):

Offset	FAT1 = 1st sector after Reserved Sectors
Size	Sectors Per FAT (From BSE)
Address Location Calculation	FAT1 = (Partition Start Sector(From MBR) + Reserved Sectors(From BSE)) * Bytes / Sector

The FAT Entries lay out how the Partitions’ identically sized clusters are laid out. There are normally two FAT Entries in order to help prevent FAT corruption problems.

The FAT is located in the first sector after the Reserved Sectors. All sectors between Partition Start Sector and the FAT, which includes the Boot Sector Entry, are contained within the Reserved Sectors Allocation.

So, to find the first FAT we navigate from the beginning of the Device, adding together the Partition Start Sector and the Number of bytes contained in the Reserved Sectors section.

Root Directory Entry (RDE):

Offset	After last FAT
Size	No of Root Entries(From BSE) * 32
Address Location Calculation	RDE = FAT1 Start Address + ((No of FATS(From BSE) * Sectors Per FAT(From BSE)) * Bytes Per Sector)

The Root Directory contains the Main Directory and File Structure for the entire Partition

The Root directory is almost identical to a standard File System Directory with only a few differences, namely; The first byte is the first character of the Partition Name. The Create Date and Time are set to 0x00.

The Root directory appears after the last FAT, and so can be found by adding the Start Address of the first FAT to the number of Bytes occupied by all FAT’s, which is invariably 2 for SD Cards. Thus the formula can be simplifed to;

RDE = FAT1 Start Address +
      ((2 * Sectors Per FAT) * 512)

Each entry in the Root Directory consumes a total of 32 Bytes as shown below;

Offset	Item	Length	Notes
00h	DOS Filename	8 Bytes	For all other Directories the first byte has special values.
08h	DOS File Extension	3 Bytes
0Bh	File Attributes	1 Byte	8 bits to denote various attributes
0Ch	NT Case Info	1 Byte	Used for Windows NT Casing Info
0Dh	Create Time (ms)	1 Byte	10ms Units
0Eh	Create Time (Hrs/Mins/Secs)	2 Bytes
10h	Create Date	2 Bytes
12h	Last Access Date	2 Bytes
14h	File / Folder Start Cluster (High)	2 Bytes	Only used in FAT32 Systems
16h	Last Modified Time	2 Bytes
18h	Last Modified Date	2 Bytes
1Ah	File / Folder Start Cluster (Low)	2 Bytes
1Ch	File Size (Bytes)	4 Bytes	Folders will have a File Size of 0x0000

Sub Directory / Files:

Offset	After Root Directory
Size	N/a
Address Location Calculation	Root Folder Address + (Max Root Directory Entries(From BSE) * 32) + ((( File / Folder Start Cluster(From RDE) – 2) * Sectors Per Cluster(From BSE) ) * Bytes Per Sector(From BSE) ) Normally For SD Cards using FAT16; Max Root Directory Entries = 512 Bytes Per Sector = 512

A Sub Directory takes on almost exactly the same format as the root Directory above.

All data stored after the Root Directory is located in the third Data Cluster, thus in order to find the location of all data we must subtract these two clusters from the Start Cluster given in the File / Folder Start Cluster Field.

The first data is found using the formula above, however for SD Cards using FAT16, the maximum Root Directory Entries and Bytes Per Sector are invariably 512. Thus the formula can be simplified to the following;

First Data = Root Folder Address + 16384 +
             (((File/Folder St Cluster – 2) *
             Sectors Per Cluster) * 512)

A Worked Example:

Card Type	Sandisk 2Gb SD
MBR Length	512
Partition 1 Start Sector	129
Bytes Per Sector	512
Sectors Per Cluster	64
Reserved Sectors	2
Number of FAT’s	2
Sectors Per FAT	239
Hidden Sectors	129
BSE Length	512
Number of Root Entries	512

The table above gives all the information taken from the MBR and BSE of the Card.

Calculating the BSE Address:

BSE = Partition Start Sector *
         Bytes Per Sector

BSE = 129 * 512

BSE = 66048 = 0x10200

Calculating the FAT1 Address:

FAT1 = (Partition Start Sector +
       Reserved Sectors) * Bytes Per Sector)

FAT1 = (129 + 2) * 512

FAT1 = 131 * 512

FAT1 = 67072 = 0x10600

Calculating the Root Directory Address:

RDE = FAT1 Start Address +
      ((No of FATS * Sectors Per FAT) *
      Bytes Per Sector)

RDE = 67072 +
      ((2 * 239) *
      512)

RDE = 67072 + (478 * 512)

RDE = 67072 + 244736

RDE = 311808 = 0x4C200

Calculating the First File Address:

For a file or folder which has a starting cluster value of 0x04, then the following will locate the start address;

File = Root Folder Address +
      (Max Root Entries * 32) +
      (((File/Folder St Cluster – 2) *
      Sectors Per Cluster) *
      Bytes Per sector)

File = 311808 +
       (512 * 32) +
       (((4 – 2) *
       64) *
       512)

File = 311808 +
       16384 +
       ((2 * 64) * 512)

File = 311808 + 16384 + (128 * 512)

File = 311808 + 16384 + 65536

File = 393728 = 0x60200