7.4  Character sets and coding

7.4.1  d-characters and a-characters

Except as specified in 7.4.4, the characters in the descriptors shall
be coded according to ISO 646 (see annex A).

The 37 characters in the following positions of the International
Reference Version are referred to as d-characters:

  3/0 to 3/9

  4/1 to 5/10

  5/15

The 57 characters in the following positions of the International
Reference Version are referred to as a-characters:

  2/0 to 2/2

  2/5 to 2/15

  3/0 to 3/15

  4/1 to 4/15

  5/0 to 5/10

  5/15

The applicable set of characters is specified in the description of
the descriptor fields.

7.4.2  c-characters

The characters of the coded graphic character sets identified by the
escape sequences in a Supplementary Volume Descriptor are referred to
as c-characters.

7.4.2.1  a1-characters

A subset of the c-characters will be referred to as a1-characters.
This subset shall be subject to agreement between the originator and
the recipient of the volume.

7.4.2.2  d1-characters

A subset of the a1-characters will be referred to as d1-characters.
This subset shall be the subject of agreement between the originator
and the recipient of the volume.

7.4.3  Separators

The characters separating the components of a File Identifier shall be

SEPARATOR 1  represented by the bit combination (2E)

SEPARATOR 2  represented by the bit combination (3B)

7.4.4  Use of characters in descriptor fields

The characters in the fields of the following descriptors shall be
a-characters or d-characters as specified in clause 9.

- Directory records within a Directory Hierarchy that is identified in
  a Primary Volume Descriptor;

- Path Table records within a Path Table Group identified in a Primary
  Volume Descriptor;

- Extended Attribute records identified in a directory of a Directory
  Hierarchy that is identified in a Primary Volume Descriptor.

The characters in the fields of the following descriptors shall be
a1-characters or d1-characters as specified in clause 9.

- Directory records within a Directory Hierarchy that is identified in
  a Supplementary Volume Descriptor;

- Path Table records within a Path Table Group identified in a
  Supplementary Volume Descriptor;

- Extended Attribute records identified ina directory of a Directory
  Hierarchy that is identified in a Supplementary Volume Descriptor.

7.4.5  Justification of Characters

In each fixed-length field the content of which is specified by this
International Standard to be characters, the characters shall be
left-justified and any remaining positions on the right shall be set
to (20).

7.5  File Identifier

7.5.1  File Identifier format

A File Identifier shall consist of the following sequence:

- File Name: A sequence of zero or more d-characters or d1-characters;

- SEPARATOR 1;

- File Name Extension: A sequence of zero or more d-characters or
  d1-characters;

- SEPARATOR 2;

- File Version Number: Digits representing a number from 1 to 32 767.

This sequence shall meet the following requirements:

- If no characters are specified for the File Name then the File Name
  Extension shall consist of at least one character.

- If no characters are specified for the File name Extension then the
  File Name shall consist of at least one character.

- The sum of the following shall not exceed 30:

  if there is a File Name, the length of the File name;

  if there is a File Name Extension, the length of the File Name
  Extension.

7.5.2  File Identifier length

The length of the File Identifier shall be the sum of the following: 

- if there is a File Name, the length of the File Name,

- if there is a File Name Extension, the length of the File Name
  Extension,

- if there is a File Version Number, the number of digits in the File
  Version Number,

- the number of SEPARATORs.


7.6  Directory Identifier

7.6.1  Directory Identifier Format

A Directory Identifier shall consist of a sequence of one or more
d-characters or d1-characters (see 7.4.4), except as specified in
7.6.2.

7.6.2  Reserved Directory Identifiers

The Directory Identifier of a Directory Record describing the Root
Directory shall consist of a single (00) byte.

The Directory Identifier of the first Directory Record of each
directory shall consist of a single (00) byte.

The directory identifier of the second Directory Record of each
directory shall consist of a single (01) byte.

7.6.3  Directory Identifier Length

The length of a Directory Identifier shall not exceed 31.