DUMPER tape format

The original TENEX tape backup program is called MINI-DUMPER or just DUMPER. It was written by Ray Tomlinson April 1971, in FAIL assembly language. The oldest version on record is from December 1971, and the newest is from February 1982. There is also a more elaborate program called BSYS, written at Stanford.

(As a historical aside, the MINI-DUMPER user interface was considered atrocious which inspired making a command line library similar to the EXEC facility. This was later moved inside the TOPS-20 monitor.)

This was written primarily to document the reverse engineered MINI-DUMPER format. The format is an older version of what TOPS-20 DUMPER uses, and some information about the newer formats is included. This describes DUMPER format 4: http://bitsavers.org/pdf/dec/pdp10/Dumper_And_Backup_Tape_Formats_Oct82.pdf

OVERVIEW

A DUMPER tape consists of a series of 518-word records, where the first six words is a header and the following 512 words are data. The first record is a tape header. After this follows files, or optionally, directory data. A file is started by a file header record, followed by data records, and ended by a file trailer. The tape is terminated by a tape trailer.

DUMPER normally doesn't write any tape marks except to terminate a saveset; marks are mostly ignored when read. MINI-DUMPER is much more particular about marks and requires them after tape headers and file trailers, and nowhere else.

TAPE FORMATS

There is a tape format number stored in the tape header. MINI-DUMPER is retroactively assigned format number 0, and doesn't have the format number stored in the tape header.

Format	Description
0	BBN original format.
3	"Full dir spec".
4	Lowest legal format for latest DUMPER.
5	Encrypted passwords.
6	"Tonext" record type.

RECORD HEADER

All records have a six-word header.

Offset	Type	Description
0	Checksum	See below.
1	Access	Unused, set to 0.
2	Tape number	Starts from 1.
3	Page number	Starts from 0 in data pages; other records set to 0.
4	Record type	See below.
5	Sequence number	See below.

The checksum is computed over all 518 words in the record, with the checksum set to 0. For formats 0-4, it's a one's complement sum. For format 5-6, it the sum is first rotated one step to the left and then adding in the data. The finished checksum is bitwise inversed before being stored in the header.

MINI-DUMPER expects the sequence number to start with 2, whereas later versions begin with 1. Normally each record increments the sequence number by 1, but a MINI-DUMPER tape header increments it by 2.

Record Type	Number
Data	0
Tape header	1
File header	2
File trailer	3
Tape trailer	4
User directory	5

TAPE HEADER

In format 0, the data is just an ASCIZ saveset name.

If format 4, the data is:

Record offset	Description
6	Tape format number.
7	"BFMSG" - offset from header to start of saveset name?
8	Timestamp for saveset.
6+BFMSG	Saveset name.

FILE HEADER

In format 0, the only file header data is an ASCIZ string in the format DEV:<DIRECTORY>NAME.TYPE;VERSION;P123456;AUSER.

Other formats have an FDB, file descriptor block at offset 200 (octal) from the record header.

USER DIRECTORY

These are optional in all formats.