Dumper And Backup Tape Formats Oct82

This document, "Dumper and Backup Tape Formats" (dated October 7, 1982), is a detailed technical reference describing the tape record formats utilized by Digital Equipment Corporation's (DEC) Backup and Dumper utilities on TOPS-10 and TOPS-20 operating systems. It is explicitly aimed at experienced MACRO programmers familiar with the internal workings of their operating systems, serving as a reference guide rather than a beginner's primer.

The document first establishes fundamental concepts, defining a record as a buffer composed of a header (containing metadata like checksums, sequence numbers, and record type) and a data area (where file contents or file-specific information blocks are stored). It explains sequence numbers (for individual records) and relative tape numbers (for files across tape volumes), as well as the initial tape label.

A key distinction is made between labeled tapes (primarily used by TOPS-20, including "extra" transparent records for volume/saveset management) and unlabeled tapes (primarily by TOPS-10, simpler, and the main focus for Dumper's standard operation).

The document details two primary format categories:

1. Backup Tape Format:

   • Records have a fixed length of 544 words (32-word header, 512-word data area).
   • The header contains general record information (type, sequence, flags, checksum).
   • The data area can contain either disk file data or various non-data information blocks that describe the file, such as 0$NAME (path identification), 0$FILE (file attributes, including protection codes), 0$DIRT (directory attributes), 0$SYSN (system header), and 0$SSNM (saveset name).
   • Specific record types include Tape Label (T$LBL), Saveset Header (T$BEG), Saveset Trailer (T$END), File Data (T$FIL), User File Directory (T$UFD), End of Volume (T$EOV), Comment (T$COM), and Saveset Continuation (T$CON).

2. Dumper Tape Formats:

   • Dumper employs a standard Dumper format and an Interchange format, the latter aiming for compatibility with Backup for data transfer between DECsystem-10 and -20.
   • Standard Dumper physical records are 518 words long (6-word header, 512-word data). Its logical record types include DATA (file page), Saveset Header (TPHD), File Header (FLHD), File Trailer (FLTR), Tape Trailer (TFTR), User-directory Information (USR), Continuation Header (CTPH), and Filler (FILL). It also specifies save types (Normal, Collection, Archive, Migration) and tape format flags.
   • The Interchange Format Dumper Tapes attempt to convert Dumper's internal records to Backup's Interchange format. However, it notes limitations: not all Backup fields are filled (e.g., physical device, CPU number), and system dependencies can cause "glitches." While Dumper sets an "ignore checksum" flag (GF$NCH) in Interchange mode, it still provides a checksum. Some Interchange record types are analogous to Backup records, while others are unique or entirely omitted due to containing system-dependent information.

The document provides extensive tables detailing the word-by-word contents of headers and data blocks for all discussed record types and formats, including the meaning of various flags and attributes.