Annex A PC File System(Normative)

A.1 PC File System mapping to media formats

Several of the removable media utilize the PC file system. For any media that use the PC file system, the following rules apply, except as overridden in the applicable annex.

A.1.1 File-set ID mapping

The PC File System mapping does not provide a File-set ID.

Note: On systems that permit user access to the media volume label, the volume label can be used to provide a File-set ID. Not all operating systems permit routine user access to this information.

A.1.2 File ID Mapping

The PC File System provides a hierarchical structure for directories and files within directories. Each structure has a root directory that may contain references to both files and subdirectories. Subdirectories may contain references to both files and other subdirectories. The nomenclature for referring to files and directories in the PC File System is:

a) \ – For the root directory

b) \filename – For a file in the root directory

c) \subdir\filename – For a file in the subdirectory subdir

The PC File System name corresponding to a File ID shall be the DICOM File ID prefixed with the character "\", with the "\" character separating File ID components.

Note: Example File ID mappings

File ID PC File system name

The DICOMDIR file shall be in the root directory for media that do not support multiple file-sets on a single medium. DICOMDIR location is described for the multiple file-set situation in the annex for such media.

Note: It is recommended but not required that the File-set Descriptor File ID (0004,1141) be "README" (see PS 3.10).

A.1.3 File management information

The PC File System provides the following information for each file:


Filename 1 to 8 characters
Extension 0 to 3 characters
Time Time of last modification (or creation)
Date Date of last modification (or creation)
Size Size of file (in bytes)

The PC File System Filename shall correspond to a DICOM File ID Component. The PC File System Extension for a DICOM file shall not contain any characters. The PC File System date and time shall be used to provide the DICOM facilities for examining the modification or creation date and time. Unused characters in Filename and Extension (see Table A.1-1) should be filled with null characters.

Notes: 1. The PC File System does not specify or control the time base used for date and time. Coordination of reference time zones is outside the scope of this standard.

2. The typical written form of a filename is filename.extension (e.g. "FILE.EXT"). The period between filename and extension is a convention used in most programs for entering and displaying the filename and extension. The period is not actually recorded on disk and is not permitted as part of a filename. A file with no extension is recorded as a file with zero extension characters (i.e. all null filled) although it is often written and displayed without the period.

The PC File system does not provide ownership or access control facilities. Write protection is addressed in the relevant physical media specific annex. Protection mechanisms are not available for the generic PC File System.

A.2 Logical format

The PC File System requires that the media be organized into sectors. The media specific value for bytes/sector and the mechanism for doing this is in each media annex.

The PC File System shall be organized as an "mtools" unpartitioned file system (see Note), using either 12-bit or 16-bit File Allocation Table (FAT). The layout of the boot sector shall be as shown in Table A.2-1. The FAT and related file structures are compatible with the DOS 4.0 and later file systems, and are described in detail in the Microsoft MS-DOS Programmer's Reference. Two byte integers shall be encoded in little endian.

Note: A PC File system may be either unpartitioned or partitioned. Traditionally, removable media such as floppy disks have been formatted as unpartitioned, and fixed media like hard disks have been formatted with a different form of Master Boot Record that specifies several partitions, each of which has the format of a complete unpartitioned system. When forms of removable media with larger capacity were introduced, some driver vendors chose to format them as unpartitioned, and others as partitioned. In order to facilitate interoperability with existing implementations this Part of the DICOM standard currently specifies one format, the unpartitioned format. Some implementations of the PC DOS filesystem may experience difficulty reading or writing to large capacity unpartitioned removable media, and require special drivers.

The boot sector, sector 0 of track 0, shall be formatted as follows:


Byte(s) Value Description
00 - 02 varies Jump instruction to loader (NOPs) (see note 1)
03 - 10 “dddddddd” The formatting DOS( vendor specific) (see note 2)
11 -12 see note 5 bytes/sector
13 see note 5 sectors/cluster
14 - 15 0001H 1 sector in boot record
16 02H 2 File Allocation Tables (FAT) (see note 3)
17 - 18 200H 512 root directory entries
19 - 20 0000H Flag for more than 65536 sector/disk. Use offset 32 value
21 see note 5 Flag for disk type; F0H if not otherwise specified
22 -23 varies sectors/FAT
24 - 25 see note 6 sectors/track
26 - 27 see note 6 side (head) per disk
28 - 31 00000000 0 reserved or hidden sectors
32 - 35 varies Total sector/disk. Varies from disk to disk
36 - 37 0000 Physical Drive number = 0
38 29H Extended boot record signature = 41
39 - 42 undefined Volume serial number. (see note 4)
43 - 53 varies The volume ID (vendor specific)
54 - 61 varies The file system label
62 - 509 varies Don't care. Any contents acceptable
510 55H Signature flag - first byte
511 AAH Signature flag - second byte

Notes: 1. These three bytes should either be EBH,00H,90H (indicating a relative jump) or 909090H indicating NOPs. The bytes are for booting off the optical drive which DICOM does not standardize. Some programs use them to validate the disk. The use of EB0090H is known to be more commonly used and is the recommended choice. Readers of DICOM disks that use the PC File System should ignore this field.

2. While eight characters appear to be valid in this field, the use of “MSDOS4.0” is known to be the preferred choice for this string. Some systems, upon finding this field not set to “MSDOS4.0” will ignore the sectors/FAT field and use their own calculation. This may cause an error due to the calculation resulting in a different value than the sectors/FAT field. (MS-DOS is a trademark of Microsoft)

3. Two FATs are recommended. One FAT could also be used but again may cause some incompatibility.

4. The serial number may be any four bytes. A random or sequential number is preferred but is not required.

5. These values are specified in the annex for each particular type of media.

6. These values are nominally specified in the Annex for each particular type of media, but vary considerably between implementations, and should not affect interoperability.