DEC STD 167

This document, DEC STANDARD 167 REV. A, titled "Volume Identification for Removable Disk Pack Disk Systems," establishes a standard for identifying removable disk packs across various CPU families.

Key Objectives:

• To define the format and location of a volume identification block on removable disk packs.
• To enable operating systems to identify the origin and data format of a volume, helping them determine if a volume can be processed.
• To prevent data loss by ensuring correct pack identification and preventing writing on unintended packs.
• To define standard error messages for volumes that cannot be processed due to format or mode incompatibilities.

Core Components & Requirements:

1. Volume Identification Block (Logical Block 1): The standard specifies that a dedicated identification block is located at Logical Block 1 on the disk pack. This block is typically 256 words in length and contains standardized fields in 7-bit ASCII:

   • Pack Identification: A 12-character field for unique pack identity.
   • Owner Name: A 12-character field for the pack's owner.
   • Format Type: A 12-character field indicating the operating system format (e.g., TOPS-10, FILES11, SCRATCH).

2. Mode Definition: The document defines "mode" as the disk-pack data format, encompassing the number of bits per word, words per sector, and sectors per track (e.g., 16-256, 22-sector mode). Data is written in 7-bit ASCII bytes, with two bytes typically comprising a word.

3. Error Handling: Programs accessing disk packs must generate specific error messages under the following conditions:

   • Incompatible Modes: If a pack is read in a mode different from how it was written, a "mode error" message (e.g., "?The pack on drive ---- is not written in a mode supported by this software system.") must be issued. Software supporting multiple modes should attempt common modes before generating the error.
   • Incompatible File Formats: If the file structure on a pack is not supported by the accessing software, a "format error" message is required. This message must include the Pack ID, Owner Name, Format Type, and Operating System Name.

4. System-Specific Requirements:

   • For DECsystem-10 and DECsystem-20, a duplicate of the volume identification block at Logical Block 10(8) is required for backup purposes.
   • Disk system hardware is mandated to provide error indications to the operating system when an incompatible mode is detected.
   • Diagnostics must verify "SCRATCH" packs or seek explicit permission from the operator before writing to non-scratch packs.
   • The standard also details specific byte/word ordering for PDP-11 and DECsystem-10/20 systems.
   • For systems like RL01 with different sector word counts (128 words/sector vs. 256), the standard still requires the volume identification block to contain at least 256 words.

In essence, DEC STANDARD 167 creates a common framework for disk pack identification and error management, facilitating data interchange and robust operation within DEC's diverse computing environments.