DEC STD 154

This document, DEC Standard 154 (Revision A, May 19, 1977), defines the data recording conventions for RX01 floppy disks. Its primary purpose is to establish a standard for volume identification and data interchange across various Digital Equipment Corporation (DEC) systems, as well as with other vendor systems, notably IBM.

The standard specifies three levels of conformance:

1. Level 0 (Basic Physical I/O and Volume ID): Requires fundamental physical I/O compatibility, enabling disks to be exchanged physically between systems without data interpretation. It mandates a standard 4-byte Volume ID (e.g., "VOL1") which is preferably ASCII but can also be EBCDIC.
2. Level 1 (Universal Interchange): Builds on Level 0 by defining a universal interchange format. For interchange, ASCII is the preferred character code for volume identification, headers, and data, though EBCDIC is also permitted. This level supports fixed-length records (1-128 bytes) and defines structures for data set labels (directory entries) and error mapping.
3. Level 2 (DEC Native Formats): Applicable to DEC-to-DEC interchange, this level specifies the use of approved DEC native file formats. All data, headers, and Volume IDs at this level must be in ASCII (EBCDIC is not permitted), and it defines specific virtual block sizes and interlace patterns for optimized performance on DEC systems.

The document details the physical and logical layouts of the disk, including the structure of the "Index Track" (Track 00) which contains volume labels and data set labels. It covers aspects like disk initialization, data recording, access methods, and error mapping. Appendices provide comprehensive information on universal interchange layouts, data set label formats, DEC-specific interlace patterns, overall disk layouts, approved DEC native formats, and EBCDIC/ASCII conversion tables.