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doc/sysdoc/dump.format

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+-*- Text -*-			This is the file AI:SYSDOC;DUMP FORMAT
+
+This file attempts to document the format of DUMP tapes.
+It probably contains inaccuracies.
+
+Each tape record contains 1,024 36-bit words in "core dump" format;
+thus on 7-track tape each 36-bit word takes 6 tape frames.  Data
+formats are standard pdp-10 data formats such as sixbit (6 characters
+per word), 7-bit ascii (5 characters per word, left-justified),
+halfwords, and so forth.
+
+Each file dumped to tape consists of a header, then the 36-bit data of
+the file, and ends with an end-of-file mark.  The last record in a file
+will generally be shorter than 1,024 words, depending on the length
+of the file.  There is no padding, other than the usual control-C's
+padding a text file out to a word boundary.  The tape ends with a
+double end-of-file mark.
+
+The first file on the tape is preceded by a tape header in front
+of the file header, with no end-of-file mark in between.
+
+Each type of header starts with a 36-bit word whose left half is minus
+the number of words in the header, including this word.  The right
+half of this word is zero.
+
+Tape header:
+	-LTHBLK,,0
+	tape number,,reel number
+	tape creation date in sixbit
+	type of tape (0 random, >0 full dump, <0 incremental dump)
+
+File header:
+	-LHBLK,,0
+	directory name in sixbit
+	first file name in sixbit
+	second file name in sixbit
+	disk pack number where file was
+	creation date of file (same format as in ITS directory)
+
+If the file is really a link, then the left half of the
+pack number is non-zero and the data of the file consists of
+three words, containing the sixbit file name the link points to.
+
+In addition to files, directories also appear on some dump tapes.  The
+master file directory has name M.F.D. (FILE) and a user file directory
+has name .FILE. (DIR).  The data for the master directory is an exact
+binary copy of the directory as stored on disk.  The data for a user
+directory is almost an exact binary copy except that unused space
+between the descriptor area and the name area is squished out.