The
MultiMediaCard
FAT16 File System Specification
Version 1.0
MMCA Technical Committee
MultiMediaCard System
Revision History
Version
Date
0.1
1.0
2
03/03
Section/ Page
Changes compared to previous issue
all
Initial file system spec proposal to MMCA
Originator:MMCA FS workgroup
Author:Juha Liinamaa
all
Conversion to MMCA look and feel formats
MultiMediaCard FAT16 File System Specification Version 1.0 Official Release (c) March 2003 MMCA
MultiMediaCard System
You acknowledge that the attached standard (the “Standard”) is provided to you on an “AS IS” basis.
MULTIMEDIACARD ASSOCIATION (“MMCA”) MAKES NO EXPRESS, IMPLIED OR
STATUTORY WARRANTIES AND EXPRESSLY DISCLAIMS THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OF THIRD PARTY RIGHTS. MMCA SHALL NOT BE LIABLE FOR (I)
TECHNICAL OR EDITORIAL ERRORS OR OMISSIONS CONTAINED WITHIN THE
STANDARD, OR (II) ANY INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL
DAMAGES (INCLUDING WITHOUT LIMITATION LOST PROFITS OR LOSS OF USE)
RESULTING FROM THE FURNISHING, PERFORMANCE OR USE OF THE STANDARD,
EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
Copyright (c) March 2003 MultiMediaCard Association, 19672 Stevens Creek Blvd., #404,
Cupertino, CA 95014-2465. World rights reserved.
No part of this publication may be transmitted, reproduced or distributed in any way, including but
not limited to photocopying, electronic copying, magnetic or other recording, without the prior
written consent of MMCA.
MultiMediaCard FAT16 File System Specification Version 1.0 Official Release (c) March 2003 MMCA
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Table Of Contents
1
Introduction ............................................................................................................................. 7
2
Physical CHS geometry ........................................................................................................... 7
3
3.1
3.2
Master Boot Record ................................................................................................................. 9
Partition entry format ......................................................................................................... 10
Address translations ............................................................................................................ 11
4
4.1
4.2
4.3
File system............................................................................................................................... 12
Partition Boot Record ......................................................................................................... 12
Variable parameters in PBR ............................................................................................... 13
Miscellaneous notes on allocation tables and PBR ............................................................ 15
5
5.1
5.2
5.3
5.4
5.5
Directory entries .................................................................................................................... 17
Long filename support ........................................................................................................ 17
Filename character sets ....................................................................................................... 18
FAT/VFAT16 caveats ........................................................................................................ 19
Example of long filename entries ....................................................................................... 19
Example of file system memory layout .............................................................................. 20
Appendix A: Abbreviations ........................................................................................................... 23
Appendix B: References ................................................................................................................. 24
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Introduction
Introduction
Purpose of this document is to define the details of FAT16 file system for MultiMediaCards’ user data area.
First of all, various FAT systems have their roots in magnetic media (floppy disks and hard drives). With that
background, FAT file system introduces some need to relate the byte-unit linearly addressed MultiMediaCard
and conventional magnetic media. In order to maintain consistency at all levels, this document deals with the
following specifics:
- CHS geometry: how to translate the card characteristics into the conventional CHS geometry of
magnetic media
- Master Boot Record (partitioning): how and why to introduce the MultiMediaCard as logical
hard drive(s), conversions between physical and logical addresses and addressing modes
- Partition Boot Record(s), aka partition boot sector: how to define the actual FAT16 file system
within a partition
- Directory entries: how to create files and directories (with conventional 8+3 filenames)
- Optional long filename support (VFAT): how to avoid breaking inter operability between systems that do support VFAT and those that don’t
CHS geometry is used as a basis for all calculations to maintain consistency throughout all addressing modes.
2
Physical CHS geometry
The goal of this chapter is to define a way to calculate a physical (media-based) CHS geometry for an arbitrary card with the following guidelines:
- Calculated CHS should reflect card properties and potential as they are expressed in the card
CSD register and MultiMediaCard System Specification
- Singular conversions between addressing modes (CHS, linear sector and linear card address)
- Simple and efficient conversion methods
- Minimal waste of card memory capacity in address conversions
In order to simplify the expressions, the following notations are defined. Practical implementation of these
notations should be straightforward on any platform.
- floor(x): closest integer ≤ x
- ceil(x): closest integer ≥ x
- ceil2(x): closest even integer ≥ x
- rem(x, y): remainder of integer division (x/y). The remainder, x and y are ≥ 0
- min(r): smallest value found in a finite-length range of numbers
- max(r): largest value found in a finite-length range of numbers
The following table summarizes the all the factors that need to be considered at this point
Table 1: Physical CHS
Acronym
calculation parameters
Explanation
Range
232
Unit
M
Card memory capacity as expressed in the card CSD register
98304 to
W
Sector Size
512
byte
S
Number of Sectors per Head
32
sector
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Table 1: Physical CHS
Acronym
calculation parameters
Explanation
Range
Unit
P
Number of Sectors on media (aka Number of Physical sectors, aka floor(min(M)/W) to
LBA’s)
floor(max(M)/W)
sector
H
Number of Heads per Cylinder
[2, 4, 6,..., 256]
head
C
Number of Cylinders on media (aka Number of Physical cylinders)
[3, 4, 5,..., 1024]
cylinder
Min(M) is the smallest card capacity for which the FAT16 system described in this document can be implemented; reasons for this limitation become more obvious later on.
Max(M) is ultimately limited by the MultiMediaCard bus protocol itself with its byte-based 32-bit addressing.
Thus, maximal capacity is 4GB.
Sector size (W) was defined to be 512 bytes as it is the ‘de facto’ standard of current disks, file system implementations and MultiMediaCards. However, if the card does not support data transfer operations at 512-byte
granularity, host SW must provide support for the 512-byte sector size.
Limitations on parameters C, H and S are introduced to maintain compatibility with the traditional BIOS
int13h hard drive interface of PC computers. Parameter H is forced to even values in order to maintain consistency with disk drives; it makes sense that both sides of “disk platters” are used in conventional disk drives.
Parameter S is fixed to 32 sectors for a number of reasons. It simplifies calculations, neatly divides the card
capacity to fixed 16kB chunks and is sufficient to address the maximal card capacity:
max ( C ) ⋅ max ( H ) ⋅ S ⋅ ( W ⁄ 1024 )KB = 1024 ⋅ 256 ⋅ 32 ⋅ 0.5KB = 4194304KB = 4GB
Furthermore, it is the maximum number of card write blocks in the smallest card erase unit (erase sector).
Assuming 512-byte write block and powers-of-two logic in the card memory hierarchy, it exactly covers one
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Introduction
or more card erase units. The variable parameters are calculated as follows:
Equation 1: Number of physical sectors
M
P = floor  -----
 W
Equation 2: Number of physical heads
P
H = ceil2  ----------------------------
S ⋅ max ( C )
Equation 3: Number of physical cylinders
P
C = floor  ------------
S⋅H
Thus, the CHS geometry may waste some capacity because all existing platforms do not allow partial cylinders. Furthermore, address translations remain straightforward if CHS addressing is allowed to be the limiting factor. Amount of wasted capacity is minimized by minimizing the number of heads. Consequently,
number of cylinders is maximal.
3
Master Boot Record
Partition table resides at the very beginning of the media. It's total size is 16kB (32 sectors = 1 head). The
partition table contains Master Boot Record (MBR) which defines the layout of file system(s) on the media.
Master boot record is the very first sector on the media. The other sectors in the partition table may contain
Master Boot Code or some proprietary information which is “don't care”, from standardization point of view.
The MBR itself is arranged as follows:
Table 2: Master Boot Record
structure
Byte
position
Length
(bytes)
0x000
432
Optional MBR code (“don't care”)
0x1B0
14
Media ID entry
(Defined in System Spec 3.0, chapter 11)
0x1BE
16
Partition Table Entry
(see below)
0x1CE
16
Partition Table Entry
(see below)
0x1DE 16
Partition Table Entry
(see below)
0x1EE
16
Partition Table Entry
(see below)
0x1FE
1
Signature
0x55
Entry Description
Value / Range
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Table 2: Master Boot Record
Byte
position
Length
(bytes)
0x1FF
1
3.1
structure
Entry Description
Signature
Value / Range
0xAA
Partition entry format
Every partition entry has the following fields (cylinder example values refer to 1st partition):
Table 3: Partition
Byte
position
Length
(bytes)
entry
Description
Value / Range
Acronym
0x0
1
boot descriptor
0x00 (non-bootable),
0x80 (bootable)
BF
0x1
1
Partition start head
0
0x2, bits[5:0]
6/8
Partition start sector
1
0x2, bits[7:6]
2/8
Partition start cylinder MSB:
bits[9:8]
1 to (C-2)
0x3
1
Partition start cylinder, bits[7:0]
0x4
1
File system descriptor
0 = empty
4 = DOS 16-bit FAT < 32MB
6 = DOS 16-bit FAT > 32MB
Other values indicate nonstandard 3rd party file system
0x5
1
Partition end head
H-1
0x6, bits[5:0]
6/8
Partition end sector
S
0x6, bits[7:6]
2/8
Partition end cylinder MSB:
bits[9:8]
1 to (C-1)
EN
0x7
1
Partition end cylinder,
bits[7:0]
0x8
4
First sector position relative to the
beginning of media (zero-based)
B0*H*S + 0*S +1 –1 =
B0*H*S
ON
0xC
4
Number of sectors in partition
(E0-B0+1)*H*S
PN
BN
All formatted partitions are based on 16-bit FAT entries and may employ VFAT for long filename support. All
sector numbers are stored in Little-Endian format (least significant byte first). All memory capacity related
parameters depend on the CHS geometry defined in the previous chapter. As implied by the table, all partitions must be aligned to full cylinders. Partition allocation does not have to be contiguous, but partitions must
not overlap and their starting/ending cylinder values must not exceed the value (C-1) defined in previous
chapter. Those values with an acronym are inherited to Partition Boot Record one way or another.
Values in unused partition entries are all zeroes.
“Extended” partitions are beyond the scope of this document. Such partitions would require definition of
nested Secondary Master Boot Record before it can be divided to logical partitions. Since supporting this fea-
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Introduction
ture adds to the complication and is not crucial in terms of inter operability, its use is forbidden.
3.2
Address translations
While all partitions reside on the same physical media, purpose of partitioning is to introduce them as separate
pseudo devices which have individual file systems and logical addressing. File system addressing is partitionbased. To summarize, partitions introduce the following division between addresses:
- Logical address: partition-based, either linear or CHS. It is only able to deal with sectors within
a single partition
- Physical address: media-based. It is capable of addressing the whole media capacity across partitions, either linearly or via CHS
Per-partition logical CHS limits are defined in the following table. It is the “apparent” CHS geometry which
will be applied when actual file system is created to a partition.
Table 4: Logical
Acronym
CHS parameters
Description
Range / Value
Unit
Cn
Number of cylinders in partition
EN-BN+1
Cylinder
HN
Number of heads per cylinder in partition
H
Head
SN
Number of sectors per track in partition
S
Sector
Since partitions were previously forced to cylinder boundaries, conversion between physical and logical CHS
addresses remains simple: only cylinder offset needs correction. When converting linear addresses between
physical and logical, offset is corrected by parameter ON.
Conversions between linear address and CHS address are computed as follows (p = arbitrary linear, c = arbitrary cylinder, h = arbitrary head, s = arbitrary sector):
Equation 4: Arbitrary CHS-to-linear address conversion
p = c⋅H⋅S+h⋅S+s–1
Equation 5: Arbitrary linear-to-CHS address conversion

 P -
 c = floor  ----------S ⋅ H


– c ⋅ H ⋅ S-
 h = floor  p--------------------------


S

s = p – c ⋅ H ⋅ S – h ⋅ S + 1
Card address is calculated from arbitrary physical address by multiplying with W.
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MultiMediaCard System
File system
Each partition will have its own FAT16 file system. Therefore, all the parameters described are logical and
only apply to the partition under consideration.
With FAT16, the partition is roughly divided into two areas:
- System area: reserved sectors (including boot sector, aka Partition Boot Record), file allocation
tables (2pcs) and root directory entries (512pcs)
- User data area: divided into allocation units (clusters)
Since there are several ways to arrange the system and data area usage, the goal of this chapter is to define a
FAT system according to following guidelines:
- Define a singular way to arrange system and data area usage
- Adhere to CHS and it’s 16kB boundaries for efficiency and consistency
- Minimize wasted capacity (cluster size)
4.1
Partition Boot Record
The partition Boot Record is the first sector in the partition in (logical) linear address 0. Fields with a defined
acronym are computed or otherwise used later on.
Table 5: PBR structure
Byte
position
Length
(bytes)
0x0
3
Jump command. NOTE: x86 machine language spe- 0xEB 0xXX 0x90
cific, must not be changed
0x3
8
OEM name. NOTE: MS Windows apparently
trashes this field on removable media
Don’t care
0xB
2
Bytes Per Sector
W
0xD
1
Sectors per Cluster
[20, 21,..., 27]
sc
0xE
2
Reserved Sectors (including this PBR)
[2, 4, 6,..., S]
r
0x10
1
Number of FATs
2
0x11
2
Number of root directory entries (32 bytes each)
512
0x13
2
Number of sectors on partition
0 (if partition type 6)
PN (if partition type 1)
0x15
1
Media descriptor
0xF8 (hard disk)
0x16
2
Sectors Per Fat
1 to 256
0x18
2
Sectors Per Track
S
0x1A
2
Number of Heads
H
0x1C
4
Number of Hidden sectors (part. Offset)
ON
0x20
4
Extended number of sectors on partition
PN (if partition type 6)
0 (if partition type 1)
0x24
1
Drive number
BF (partition entry)
0x25
1
Reserved
0
12
Description
Range/Value
Acronym
E
sf
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Table 5: PBR structure
Byte
position
Length
(bytes)
0x26
1
Extended boot signature
0x29
0x27
4
Volume serial number
PSN from card CID +partition index (0 to 3)
0x2B
11
Volume label (example mmcform11)
ASCII string w/o NULL,
SP padding
(“mmcform11 “)
0x36
8
File system type
ASCII string:
(“FAT16”)
0x3E
448
Optional partition boot code
Don’t care
0x1FE 1
Signature
0x55
0x1FF 1
Signature
0xAA
Description
Range/Value
Acronym
As can be seen, lots of information in this record is inherited from physical CHS geometry and partition table
entry. Note: number of logical cylinders can be calculated either from these values or from respective partition entry values; PBR has inherited the needed parameters from partition entry. Using MBR, the logical
number of cylinders CN is:
Equation 6: Number of logical cylinders in partition
CN = EN – BN + 1
4.2
Variable parameters in PBR
At this point, we have three degrees of freedom in our FAT arrangement, namely reserved sectors (r), sectors
per FAT (sf) and sectors per cluster (sc). However, these parameters are not entirely independent of each other
and each has certain limitations.
In the following, reserved sectors and the two FAT tables are combined to parameter hF which shall fulfill the
equation
S ⋅ hF = r + 2 ⋅ sF
Root directory entries (512pcs) take exactly one head; each RDE is 32bytes, 512*32/W = 512*32/512 = 32
sectors = 1 head.
Having these requirements in place aligns all our major file system components (reserved sectors + FATs, root
directory entries and user data area) to 16kB head boundaries, hopefully increasing performance a little.
Because there are two identical FATs, r is implicitly an even integer (since S is even) to satisfy the condition
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rem(r +2sF, S) = 0. Thus, it is always guaranteed that there is at least 1 “free” sector between PBR and FATs
which can optionally contain proprietary information and non-x86 boot code. However, from standardization
point-of-view, data in reserved sectors other than the PBR is “don’t care”.
Each FAT entry is 16bits = 2bytes, yielding W/2 = 256 cluster entries per sector. Keeping in mind that the first
two clusters are reserved, the combined partition capacity must satisfy the following condition
Equation 7: Partition capacity equation
S ⋅ hF – r 
⋅ E- ⋅ S –  W
 C ⋅ H – h – 32
- – 2 ⋅ sc < s c ⇒
------- ⋅ -------------------------------N
F
 2


2
S ⋅ W
( C N ⋅ H – h F – 1 ) ⋅ 32 – ( 128 ⋅ 32 ⋅ h F – 128 ⋅ r – 2 ) ⋅ s c < s c ⇒
32 ⋅ C N ⋅ H – 32 ⋅ h F – 32 – 4096 ⋅ s c ⋅ h F + 128 ⋅ r ⋅ s c + 2 ⋅ s c < s c ⇒
( 4096 ⋅ s c ) ⋅ h F > 32 ⋅ C N ⋅ H + ( 128 ⋅ r + 1 ) ⋅ s c – 32 ⇒
32 ⋅ C N ⋅ H + ( 128 ⋅ r + 1 ) ⋅ s c – 32
h F > ----------------------------------------------------------------------------------4096 ⋅ s c + 32
Thus, condition for hF is clearer, but we still have three degrees of freedom (hF, r and sc) at this point. However, our earlier definitions and the FAT16 system itself imposes some restrictions:
Equation 8: Partition capacity equation restrictions
 h F ≤ 17

 r ∈ [ 2, 4, 6, …, S ], S = 32

 s c ∈ [ 2 0, 2 1, …, 2 7 ]
Keeping in mind the definition of hF, condition r = 2 represents the ideal case of FAT size. Thus, equation 4-2
simplifies to equation 4-4 (dN is actual number of data clusters in the partition):
Equation 9: Restricted partition capacity equations
⋅ C N ⋅ H + 257 ⋅ s c – 32
 h = ceil  32
------------------------------------------------------------- F


4096 ⋅ s c + 32

 s ∈ [ 2 0, 2 1, …, 2 7 ]
 c
 h < 17
 F

⋅ C N ⋅ H + 32 ⋅ h F – 32
 d = floor  32
------------------------------------------------------------ ≤ 65518
N



sc
Additional limitation to number of actual data clusters dN is imposed by the fact that in addition to cluster
indices 0 and 1, some other indices are restricted as well: 0xFFF0 to 0xFFF6 are reserved, 0xFFF7 denotes
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unusable cluster and 0xFFF8 to 0xFFFF signify end of file allocation chain. Thus, maximum number of
actual data clusters is (65536 – 18 = 65518). This has to be taken into account at h F = 17 boundary.
At this point, the sensible thing to do is to experiment with the various cluster sizes (sC). Cluster size should
be as small as possible to avoid capacity waste. So, there will be a maximum of eight iterations which
repeated until calculated hF and dN meet their respective criteria.
After the iterations, PBR parameter sC is already known. Parameters sF and r are computed from dN as follows:
Equation 10: Computation of sF and r
d N + 2
 s = ceil  --------------F

 256 

 r = S – rem ( 2 ⋅ s F, S ) = 32 – rem ( 2 ⋅ s F, 32 )
4.3
Miscellaneous notes on allocation tables and PBR
-
Cluster entries 0 and 1 must have values 0xFFF8 and 0xFFFF, respectively
Both FAT tables are identical
Free cluster entries will have value 0
Possible extraneous cluster entry values at the end of FAT tables are considered “don’t care”. For
purposes of inter operability, no specific value may be assumed
- Cluster entry-to-logical linear address conversions are calculated according to EN ISO
29293:1989, chapter 6.2.1
- Data space allocation from FAT is performed according to EN ISO 29293:1989, chapter 6.4.2
Table 4-2 summarizes cluster values within valid data cluster indices (2 to dN +1):
Table 1: Cluster values within valid data cluster indices (2 to dN +1)
Value/Range
Description
0x0000
Indicates free cluster
0x0001
Invalid value, must not be used
0x0002 to (dN +1) Indicates allocated cluster. The value stored is the index of next cluster entry in an allocation chain
(dN +2) to 0xFFF0 Invalid values, must not be used. Such values would point beyond the partition's data area
0xFFF0 to 0xFFF6 Reserved values, must not be used
0xFFF7
Indicates a bad (malfunctioning) cluster
0xFFF8 to 0xFFFF Indicates the end of an allocation chain
Table 4-3 summarizes the cluster values (if any) in special allocation table indices:
Table 2: Cluster values in special indices
Indices
Value/Description
0x0000
0xFFF8 (F8 is the media byte from PBR)
0x0001
Always 0xFFFF
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Table 2: Cluster values in special indices
(dN +2) to 0xFFEF Last sector of FAT may contain extraneous cluster entries due to sector-boundary allocation. Values of those extraneous cluster entries should be 0x0000. However, any file system driver should
not rely on these values being 0x0000. Instead, it should always be aware of the total number of
data clusters (dN)
0xFFF0 to 0xFFF6 Reserved indices/values, must not be used for allocation. Refer to previous table
0xFFF7
Reserved index/value, must not be used for allocation. Refer to previous table
0xFFF8 to 0xFFFF Reserved indices/values, must not be used for allocation. Refer to previous table
The following chart provides a simple example how cluster chains are formed within allocation table and how
cluster indices are mapped to data area sectors. The first logical LSN of data area – “Beginning of data area”
in the chart – is calculated from PBR as the sum of (r + number_of_fats*sf + 32). Note that cluster chains
need not be contiguous or in ascending order.
Start of allocation chain (f rom
directory entry)
Cluster index
0
8
16
...
...
..., dN +1
Cluster values
> dN +1
Don't care
0xFFF8
0x0000
0x0000
0xFFFF
0x0000
0x0000
Data Cluster 2
Data Cluster 6
...
Free cluster
End of chain marker
0x0004
0x0000
0x0000
0x0000
0x0000
0x0000
Data Cluster 3
Data Cluster 7
Bad cluster
0x0005
0x0000
0x0000
0xFFF8
0x0000
0x0000
Data Cluster 4
Data Cluster 8
0x0000
0x0000
0x0000
0x0000
0xFFF7
0x0000
Data Cluster 5
Data Cluster 9
Beginning of data area.
1 cluster = s c consecutive sectors
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Directory entries
Directory entries
Directory entry structure shall be used according to EN ISO 29293 with the following clarification: for a
directory, the file size field must be zero. The table below describes the directory entry fields, including some
notes.
Table 6: Structure
of directory entries
Offset
Length
(bytes)
0x0
8
Blank-padded filename. First character is set to E5h for deleted files (05h for pending
delete files under Novell DOS / OpenDOS)
0x8
3
Blank-padded file extension
0xB
1
Attributes
0xC
10
MS-DOS 1.0-6.22: reserved
DR DOS: used to store file password / owner
Novell DOS 7: DELWATCH data
MS-DOS 7/Windows95: additional file times
0x16
2
Time of creation or last update, bits
[15:11]: hours (0-23)
[10:5]: minutes
[4:0]: seconds/2
0x18
2
Date of creation or last update, bits
[15:9] year – 1980
[8:5] month
[4:0] day
Notes
0x1A 2
Starting cluster number
0x1C
File size
4
Table 7: Format of MS-DOS 7/Windows95 additional file times
Offset
Length
(bytes)
0x0
1
Reserved (Windows NT –specific?)
0x1
1
10-millisecond units pas creation time below (Windows NT –specific?)
0x2
2
File creation time
0x4
2
File creation date
0x6
2
Last-access date
0x8
2
FAT32: high word of starting cluster number. 0x0000 for VFAT16
5.1
Long filename support
Notes
VFAT16 long filename support is built on top of regular FAT16 by using extra directory entries which have all
their attribute bits (Volume, Directory, Read-only, Archive). Systems without long filename support shall
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ignore such entries. Details of long filename directory entry are as follows:
Table 8: Format of long filename directory entry
Length
(bytes)
Offset
Notes
0x0
1
LFN record sequence and flags, bits
[5-0]: sequence number
[6:6]: set if last long-filename record for file
[7:7]: set if file deleted
0x1
10
Long filename, 1st part
0xB
1
0x0F, impossible file attribute used as signature
0xC
1
Reserved (?). Set to 0x00
0xD
1
Checksum for long filename
0xE
12
Long filename, 2nd part
0x1A 2
First cluster number (always 0x0000 for long filename records)
0x1C
Long filename, 3rd part
4
Long filename entries shall always be stored right prior to the short filename entry. The short filename checksum byte is computed by adding up the eleven bytes of the short filename, rotating the intermediate sum right
one bit before adding the next character. In pseudocode:
Byte sum=0;
I=0;
While (I < 11)
{
rotate (sum);
sum += shortname [I];
I++;
}
5.2
Filename character sets
Short filenames must be all uppercase. Any combination of characters and numbers is valid, a blank (ASCII
code 0x20h), as well as all ASCII characters above 0x7F, and the following special characters:
$%'-_@~`!()^#&
The following additional characters are valid in long filenames, but are not valid in 8.3 filenames:
+,;=[]
Long filenames are stored in 16-bit Unicode format, in practice meaning that there is 0x00 after the actual
ASCII characters. Long filenames are terminated by an Unicode NULL (0x0000). Short filenames are generated from long filenames by:
•
Deleting any invalid characters, including . " / \ [ ] : ; = ,
•
Removing extra periods (.) from the filename. The string after final period is considered extension and is
truncated to maximum three (3) characters if the characters are valid and do not contain blanks in the mid-
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dle. Otherwise, final period is ignored and the string next to final period is used as extension
•
If necessary, name part is truncated to a maximum six characters followed by tilde (~) and running
sequence number (1, 2, 3, 4…)
•
If long filename contains a blank (space, ASCII code 0x20), it is replaced with an underscore (_)
•
The short filename characters are converted to upper case.
•
If filename has been a valid short name to begin with, it is only converted to upper case without any other
modifications
5.3
FAT/VFAT16 caveats
- Even though blank (ASCII code 0x20) is theoretically allowed in short filename, very few applications support it. Thus, it should not be used
- In Windows9x, maximum short name path length is 80 characters. Creation of too deeply nested
short name directory structures should be avoided for purposes of compatibility and inter operability
- In Windows9x, maximum length of long filename paths is 260 characters (including terminating
NULL). Long filenames can be up to 255 characters (including terminating NULL), but very
long names should be avoided since in the worst case they only leave four characters for actual
path
- When copying/moving a directory with mixed set of long filenames and short ones, data loss
may occur when a file with long name is copied, short name is generated to destination and the
source contains a short name which is the same as the generated short name in destination directory
Same volume label should be written both to PBR and root directory (using Label attribute in one of the root
directory entries), as per MS Windows behaviour. Maximum label length is 11 characters.
5.4
Example of long filename entries
Files “VeryLongFilename.txt”, “VeryVeryLongFilename.txt” and “Truly very, very long filename.txt” were
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created to a floppy, using Windows95. The directory entries were arranged as follows:
Figure 1: Example Of Directory Entry
5.5
Example of file system memory layout
The following chart illustrates the memory layout of file system with a 32MB (62720 sectors) card using single partition with a file system. At first, the CHS geometry is calculated (S = 32):
P = 62720
62720
H = ceil2  ---------------------- = 2
32 ⋅ 1024
62720
C = floor  --------------- = 980
32 ⋅ 2
So, size of one cylinder is 32kB = 64 sectors. Since the partition is required to be on the cylinder boundary
and there's a 16kB partition table in the very beginning of the media, the total number of logical cylinders in
the partition is CN = 980 –1 = 979. Experimenting with clusters sizes and Equation 4-4 yields a cluster size sc
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= 1 (one sector per cluster). Thus, Equations 4-4 and 4-5 produce the following results:
 sc = 1

⋅ 979 ⋅ 2 + 257 ⋅ 1 – 32
 h = ceil  32
------------------------------------------------------------ F

 = 16
4096 ⋅ 1 + 32


32 ⋅ 979 ⋅ 2 + 32 ⋅ 16 – 32
 d N = floor  ------------------------------------------------------------- = 62112


1



 621112 + 2- = 243
 s F = ceil  -------------------------
256

 r = 32 – rem ( 2 ⋅ 243, 32 ) = 32 – 6 = 26
In the following chart, data sectors are set to zero in order to highlight the arrangement of partition and system
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areas:
System area layout
640
0
32
576
512
sectors
448
243
384
Data area
RDE's
2nd FAT
1st FAT
Reserved
Gap1
Part. table
320
256
243
192
128
64
26
32
32
0
Figure 2: System Area Layout
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Appendix A:
Directory entries
Abbreviations
aka
Also known as
CHS
Cylinder-Head-Sector. Traditional addressing mode of floppies and hard drives. Their range is C = 0
– 1023, H = 0 – 255, S = 1 – 63
FAT
File Allocation Table, table of linked-list values in the FAT file system containing allocation information. Also generic name for various file system implementations employing FATs
FAT16
16 refers to number of bits in a single allocation unit in the FAT
RDE
Root Directory Entry. Located immediately after FAT(s) in the file system
MBR
Master Boot Record. Aka Master Boot Sector. Contains partition information and optionally master
boot code
PBR
Partition Boot Record. Aka Partition Boot Sector. Contains FAT parameters of the partition and
optionally boot code
SPT
Sectors Per Track. Aka Sectors Per Head. Parameter S of CHS geometry
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Appendix B:
MultiMediaCard System
References
MultiMediaCard System Specification
EN ISO 29293:1989
Undocumented DOS 2nd edition
Andrew Schulman, Ralf Brown, David Maxey, Raymond J. Michels, Jim Kyle
Addison-Wesley, 1993
Interrupt List Release 6.0
Ralf Brown
3. January 1999
http://www.cs.cmu.edu/afs/cs.cmu.edu/user/ralf/pub/WWW/files.html
Detailed Explanation of FAT Boot Sector
Reviewed: October 25, 2000
Microsoft Knowledge Base, Article ID: Q140418
Default Cluster Size for FAT and NTFS
Reviewed: March 1, 2000
Microsoft Knowledge Base, Article ID: Q140365
How Windows Generates 8.3 File Names from Long File Names
Reviewed: December 14, 2000
Microsoft Knowledge Base, Article ID: Q142982
Copying Files with Mix of LFN and SFN May Lead to Data Loss
Last reviewed: March 1, 2000
Microsoft Knowledge Base, Article ID: Q195144
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