DEC STD 125

This document, DEC STD 125, Revision B (reaffirmed June 1981), is a standard defining the format and labeling conventions for files stored on Digital Equipment Corporation (DEC) magnetic cassettes.

Its primary purpose is to ensure compatibility and interchangeability of data between various DEC systems (e.g., PDP-8, PDP-11 families) that utilize cassette storage.

Key aspects of the standard include:

1. File Types: It defines formats for both labelled and unlabelled files.
2. Labelled Files: These are the primary focus for interchange.
   • They begin with a 32-byte file header block containing essential metadata such as filename, file extension, data type, block length, file sequence number, and creation date.
   • The standard introduces a tiered system of support levels (Level 0, Level 1, Level 2).
     • Level 0 is the fundamental, guaranteed interchange level, typically using fixed 128-byte blocks and ASCII data.
     • Higher levels (1 and 2) add features like multi-volume file support, longer filenames, and variable block lengths.
   • It specifies conventions for file names, data types (including various binary and ASCII formats), and how logical end-of-file and end-of-tape conditions are indicated.
3. Unlabelled Files: These are simpler, consisting of raw data blocks separated by gaps without a formal header. The standard notes they are less recommended for interchange due to unspecified data formats.
4. Physical/Logical Structures: It defines elements like files, data blocks, file gaps (separating files), and block gaps (separating data blocks within a file).
5. Multi-Volume Support: Details are provided for how files can span multiple cassettes, especially for higher support levels.

The document emphasizes adherence to these formats for any cassette intended for interchange between systems, with Level 0 being the baseline for universal compatibility.