A.2 Logical Format
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## 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 file system 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:

Table A.2-1. Boot Sector

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

### Note

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.

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