PDP-1-X microtape format - Revision history

Jnc: Add cat, some links

2025-04-07T14:39:55Z

Add cat, some links

Larsbrinkhoff: How to tell the two versions apart.

2025-04-07T09:45:10Z

How to tell the two versions apart.

Larsbrinkhoff: /* Block table */ Tape name.

2025-04-07T09:21:11Z

‎Block table: Tape name.

Larsbrinkhoff: /* Directory */ File entry description.

2025-04-07T08:43:52Z

‎Directory: File entry description.

Larsbrinkhoff: Expanding description, leaving the file entry for now.

2025-04-07T08:25:08Z

Expanding description, leaving the file entry for now.

Larsbrinkhoff: /* External links */ Program for decoding.

2025-04-07T08:00:42Z

‎External links: Program for decoding.

Larsbrinkhoff: Initial overview.

2025-04-07T07:56:36Z

Initial overview.

New page

The [[PDP-1-X]] [[time-sharing]] system used a [[file system]] for storing data on [[DECtape|microtapes]]. Microtapes could also be stored in a [[drum]] area, from which they could be written to 7-track [[magnetic tape]]s.

The two main data structures on a tape are the directory and the block table. There is just one directory listing all file names and pointing to the first data block in the file. However, the files are arranged in a hierarchical manner which can be viewed as a system with subdirectories. The block table has a link to the next block for all blocks on the tape.

There are two versions of the file system. The older version only stored 512 blocks on a tape. The newer version stores 576 blocks; for this to work, the directory and block table are expanded in size and moved to another location.

==External links==

* http://www.bitsavers.org/bits/MIT/rle_pdp1x/dectape/ - Microtape images.
* http://www.bitsavers.org/bits/MIT/rle_pdp1x/magtape/ - Magtape images.

@@ Line 1: / Line 1: @@
 The [[PDP-1-X]] [[time-sharing]] system used a [[file system]] for storing data on [[DECtape|microtapes]].  Microtapes could also be stored in a [[drum]] area, from which they could be written to 7-track [[magnetic tape]]s.
-The two main data structures on a tape are the directory and the block table.  There is just one directory listing all file names and pointing to the first data block in the file.  However, the files are arranged in a hierarchical manner which can be viewed as a system with subdirectories.  The block table has a link to the next block for all blocks on the tape.
+The two main data structures on a tape are the [[directory]] and the block [[array|table]].  There is just one directory, listing all [[file]] names and pointing to the first data block in the file.  However, the files are arranged in a hierarchical manner which can be viewed as a system with sub-directories.  The block table has a link to the next block for all blocks on the tape.
 There are two versions of the file system.  The older version only stored 512 blocks on a tape.  The newer version stores 576 blocks; for this to work, the directory and block table are expanded in size and moved to another location.  At this editorially awkward position we should mention that blocks are 256 words of 18 bits.
-The file system always refers to logical block numbers, which start from 0.  Even and odd blocks are interleaved in an alternating fashion.  Even blocks start at physical block 1 and goes up to 255 or 287, which is the last physical block.  Odd blocks go in the other direction and thus start at the penultimate physical block, and go back to physical block 0.
+The file system always refers to logical block numbers, which start from 0.  Even and odd blocks are [[sector interleaving|interleaved]] in an alternating fashion.  Even blocks start at physical block 1 and goes up to 255 or 287, which is the last physical block.  Odd blocks go in the other direction and thus start at the penultimate physical block, and go back to physical block 0.
 The first 11 logical blocks are reserved by the system for storing the directory, block table, and file system code.
@@ Line 54: / Line 54: @@
 * http://www.bitsavers.org/bits/MIT/rle_pdp1x/magtape/ - Magtape images.
 * https://github.com/PDP-1X/decode-pdp1x-tape - Program for decoding microtapes and magtapes.

@@ Line 6: / Line 6: @@
 The file system always refers to logical block numbers, which start from 0.  Even and odd blocks are interleaved in an alternating fashion.  Even blocks start at physical block 1 and goes up to 255 or 287, which is the last physical block.  Odd blocks go in the other direction and thus start at the penultimate physical block, and go back to physical block 0.
 ==Versions==
@@ Line 14: / Line 16: @@
 The formats of the directory and block tables are the same between the two versions, just differing in block table size.
 ==Directory==
@@ Line 43: / Line 47: @@
 Each word in the block table correspond to a logical block on the tape.  The contents points to the next block in a file.
-Since the first few blocks on a tape are reserved by the system, the tape name can be encoded in the first three words of the table.
+Since the first few blocks on a tape are reserved by the system, the tape name can be encoded in the first three words of the table.  The fourth word stores the number of usable blocks on the tape, i.e. the physical maximum minus 11.
 ==External links==

← Older revision		Revision as of 09:21, 7 April 2025
Line 42:		Line 42:

	Each word in the block table correspond to a logical block on the tape. The contents points to the next block in a file.		Each word in the block table correspond to a logical block on the tape. The contents points to the next block in a file.
		+
		+	Since the first few blocks on a tape are reserved by the system, the tape name can be encoded in the first three words of the table.

	==External links==		==External links==

← Older revision		Revision as of 08:43, 7 April 2025
Line 22:		Line 22:

	Each file entry looks like this:		Each file entry looks like this:
		+
		+	{\| class="wikitable"
		+	! Offset
		+	! Description
		+	\|-
		+	\| 0
		+	\| Offset to next (sibling) file, or 0 for last file
		+	\|-
		+	\| 1-3
		+	\| File name, using the FIODEC character set
		+	\|-
		+	\| 4
		+	\| First data block, or offset to subname (subdirectory file)
		+	\|}
		+
		+	Word five in each entry has bit 0 set (PDP-1 numbering) to indicate a regular data file; the low 9 or 10 bits indicate the first data block. If clear, this is a "subname" (or directory) and the low bits point to the subfiles.

	==Block table==		==Block table==

← Older revision		Revision as of 08:00, 7 April 2025
Line 9:		Line 9:
	* http://www.bitsavers.org/bits/MIT/rle_pdp1x/dectape/ - Microtape images.		* http://www.bitsavers.org/bits/MIT/rle_pdp1x/dectape/ - Microtape images.
	* http://www.bitsavers.org/bits/MIT/rle_pdp1x/magtape/ - Magtape images.		* http://www.bitsavers.org/bits/MIT/rle_pdp1x/magtape/ - Magtape images.
		+	* https://github.com/PDP-1X/decode-pdp1x-tape - Program for decoding microtapes and magtapes.