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CAN CSA Z243.310-91 (2004) and CAN CGSB-200.26-91: Standard for Volume and File Structure of CD-ROM for Information Interchange
Technical overview of the Canadian adoption of ISO 9660 for CD‑ROM data interchange and compliance considerations
Scope and Purpose
The joint standards CAN CSA Z243.310-91 (2004) and CAN CGSB-200.26-91 constitute the Canadian adoption of ISO 9660:1988 – Information processing – Volume and file structure of CD-ROM for information interchange. These standards specify the logical format for data recorded on compact disc read-only memory (CD-ROM) media, enabling interchange of data between different information processing systems.
The scope covers the arrangement of volume descriptors, file descriptors, directories, and the data files themselves, all organised into a hierarchical structure that can be read by any standard CD-ROM device. By aligning with ISO 9660, the Canadian standard ensures that a CD-ROM produced in Canada is readable worldwide, provided the software or operating system supports the ISO 9660 file system.
Key benefit: This standard eliminates ambiguity in file system implementation, allowing CD-ROMs to be exchanged across platforms – from legacy MS-DOS systems to modern Unix-like environments – without requiring conversion or additional software.
Technical Requirements: Volume and File Structure
The standard defines the data organisation at several nested levels:
- Volume level – how sectors (logical blocks) are grouped into volumes, volume sets, and partitions.
- Volume descriptor set – a sequence of descriptors (primary, supplementary, boot record, volume partition, and volume descriptor set terminators) that describe the entire disc.
- File and directory descriptors – records that encode file names, sizes, location extents, and timestamps.
- Path tables – optional tables that speed up directory traversal by recording the location of each directory.
- Data area – the contiguous storage of file content on the disc.
Primary Volume Descriptor (PVD)
The PVD is the cornerstone of the file system. It is located at logical block 16 (i.e., sector 16) and contains fixed-length fields that describe the entire volume. The table below summarises the key fields and their byte positions (offsets from the beginning of the PVD):
| Offset | Field | Size (bytes) | Description |
|---|---|---|---|
| 1 | Standard Identifier | 5 | Always the ASCII string “CD001”. |
| 6 | Volume Descriptor Version | 1 | Currently set to 1. |
| 8 | Volume Space Size | 4+4 (both endian) | Total number of logical blocks on the volume. |
| 24 | Volume Set Size | 2+2 | Number of volumes in the volume set. |
| 40 | Volume Identifier | 32 | User‑assigned label for the volume (d‑characters). |
| 72 | Volume Creation Date / Time | 17 | Date stamp in the ISO 9660 “decoded” format. |
| 156 | Root Directory Record | 34 | Directory descriptor for the root directory (includes location, length, creation time, etc.). |
| 813 | Standard Identifier (repeated) | 5 | ASCII “CD001” to confirm descriptor integrity. |
Tip: All numeric fields in ISO 9660 are recorded in both little‑endian (Intel) and big‑endian (Motorola) formats, enabling the same disc to be booted or read by architectures of either byte order.
Interchange Levels
The standard defines three levels of interchange:
- Level 1 – File names limited to 8 uppercase characters, a period, and 3 uppercase characters (similar to DOS 8.3). Directories can be nested up to eight levels deep.
- Level 2 – File names up to 30+ characters and directories up to 255 levels (the full directory path length is limited).
- Level 3 – Allows interleaved data and supplementary attributes (rarely used in practice).
All levels require the use of d‑characters (uppercase letters, digits, underscore) and allow only a specific set of characters for file names. Many modern implementations (Joliet, Rock Ridge) extend the file‑naming capabilities beyond the base standard.
Implementation Considerations for Software and Hardware Developers
Adhering to CAN CSA Z243.310-91 requires careful attention to several areas:
Logical Block Address Mapping
Each logical block (usually 2048 bytes) is addressed by its absolute position on the disc. The standard assumes a disc geometry of 75 blocks per second of playback, but the precise physical mapping is delegated to lower‑level control standards (e.g., Yellow Book). Implementers must ensure that the file system driver interprets the block numbers correctly.
Directory Records and Extents
File extents (the actual data) are stored as contiguous block runs. Each directory record for a file contains the extent location, length, and the file identifier. If a file is fragmented, the file system must use a multi‑extent record – a feature that is often omitted in simpler readers. The standard mandates that files be contiguous unless interleaving is used.
Warning: Some older drives or operating systems cannot handle multi‑extent files or long directory paths. For maximum compatibility, authors should use Level 1 restrictions and avoid multi‑extent records.
Character Set Enforcement
The base standard restricts file names to a subset of ASCII that is supported by all common platforms. When using extensions such as Joliet (for Unicode) or Rock Ridge (for POSIX‑style attributes), the underlying ISO 9660 structure must remain valid so that standard readers can at least access files with their short names.
Implementation tip: Always write the Primary Volume Descriptor first, followed by the Volume Descriptor Set Terminator. If extension descriptors (e.g., Joliet) are added, they should appear in a supplementary volume descriptor, and the base PVD must still point to a valid root directory for basic ISO 9660 access.
Compliance Notes and Certification
Products claiming compliance with CAN CSA Z243.310-91 (2004) or CAN CGSB-200.26-91 must pass a series of tests that verify:
- Correct placement and content of volume descriptors.
- Proper encoding of file names, dates, and permissions (if extended).
- Consistent use of both little‑endian and big‑endian numeric representations.
- Accurate file extent addressing and absence of overlap.
- Conformity to the chosen interchange level.
Non‑compliance risks: A CD‑ROM that does not respect the volume descriptor order or that places the root directory record at an invalid block will not be readable in many operating systems. Deficiencies in endian handling can cause all file offsets to be misinterpreted, leading to “Data error” (cyclic redundancy check) messages.
In practice, compliance is usually self‑declared based on known reference implementations (e.g., the ISO 9660 filesystem module in Linux). No formal certification body is mandated in Canada, but the standard itself provides a rigorous specification against which implementers can validate their products.
Q: Does CAN CSA Z243.310-91 differ from ISO 9660:1988?
A: No. The Canadian standards are direct adoptions of the ISO document, with identical technical content. The dual numbering reflects the involvement of both the Canadian Standards Association and the Canadian General Standards Board.
A: No. The Canadian standards are direct adoptions of the ISO document, with identical technical content. The dual numbering reflects the involvement of both the Canadian Standards Association and the Canadian General Standards Board.
Q: Can this standard be used for DVD-ROM or Blu-ray media?
A: Yes, with caveats. The volume and file structure described by ISO 9660 has been extended by the UDF (Universal Disk Format) standard for higher‑capacity discs. However, the basic ISO 9660 structure is still used in many hybrid discs (e.g., “El Torito” bootable CDs) and can be applied to other optical media as long as the logical block size is 2048 bytes.
A: Yes, with caveats. The volume and file structure described by ISO 9660 has been extended by the UDF (Universal Disk Format) standard for higher‑capacity discs. However, the basic ISO 9660 structure is still used in many hybrid discs (e.g., “El Torito” bootable CDs) and can be applied to other optical media as long as the logical block size is 2048 bytes.
Q: What is the difference between CSA Z243.310‑91 (2004) and the 2004 reaffirmation?
A: The “(2004)” indicates that the 1991 edition was reaffirmed without technical changes in 2004. The content remains identical to the 1991 release and to ISO 9660:1988.
A: The “(2004)” indicates that the 1991 edition was reaffirmed without technical changes in 2004. The content remains identical to the 1991 release and to ISO 9660:1988.
Q: Is there a newer version of this standard?
A: ISO 9660 has not been revised since 1988, but the related standard ISO 13490 (for CD‑RW) and later UDF provide additional functionality. For most modern optical media, UDF (ISO 13346) is preferred.
A: ISO 9660 has not been revised since 1988, but the related standard ISO 13490 (for CD‑RW) and later UDF provide additional functionality. For most modern optical media, UDF (ISO 13346) is preferred.