Gutenberg Filesystem

• Reverse engineering, primarily by David Schmidt

The Gutenberg word processor (reviewed here) is a somewhat obtuse program with some very advanced features. For example, you could draw graphics, insert them into a document, and flow columns around them. The full version, and a more limited version called Gutenberg, Jr., used a custom filesystem.

The disk directory is held in a file called DIR. To get a list of files on a disk from within the program, you open the file like you would any other document. (It's not exactly the same, as the volume name header is skipped over, but it's very close to being formatted text.)

The directory holds the filename, type, and initial track/sector of each file. References to track or sector zero are stored as $40, probably because $00 has a special meaning in a document. The software displays the full contents of the directory file on screen, including the track/sector numbers; this works because bytes with the high bit clear are considered printable "alternate" character values.

There are two types of files: programs and documents. Fonts and non-resident commands are programs, and use file type 'P', or sometimes 'M'. Documents use file type ' ' (space), or 'L' if they have been locked.

Disks have a volume name that can be up to nine characters long. Filenames can be up to 12 characters, and may be any high-ASCII value other than slashes ('/'), spaces (' '), and control characters. When a file is deleted the entire entry is cleared to high-ASCII spaces, so empty slots can be identified by checking the first byte of the filename. (Having slashes in the volume name is apparently allowed, since the program disks do this themselves.)

The program doesn't appear to allow lower-case characters to be used in filenames.

The filesystem used an unusual approach: every 256-byte sector that is part of a file is arranged in a doubly-linked list. The first six bytes of every file sector hold the track/sector numbers of the previous, current, and next sector in the list. The list is circular. The high bit of the next track number is set when it points to the first sector in the file, and the high bit of the current sector number is set when the sector is the first in the file.

For example, consider a two-sector document that lives in track 3, sectors 1 and 3:

T3 S1:
  03 03 03 81 03 03 ...
T3 S3:
  03 01 03 03 83 01 ...

The self-referencing sector in T3 S1, and forward-referencing track in T3 S3, have the high bit set, because traversing those links would take you to the start of the list. The backward reference doesn't have the high bit set, though it probably should. (In some files, such as the directories on the Gutenberg program disks, it looks like the backward reference in the first sector didn't get updated when the file expanded. Use backward links with caution.)

Some of the Gutenberg disks refer to files in track 0. Some of the track/sector links for these files follow the directory convention of recording 0x00 as 0x40.

The first sector of the disk directory lives in track 17, sector 7. The first entry in the directory is itself, represented as a locked file called DIR. Additional directory sectors can be found by traversing the linked list pointers.

Each directory sector holds 15 entries, and looks like this:

+$00 / 1: previous track
+$01 / 1: previous sector
+$02 / 1: current track
+$03 / 1: current sector
+$04 / 1: next track
+$05 / 1: next sector
+$06 / 9: volume name, high ASCII padded with spaces
+$0f / 1: $8d (high-ASCII carriage return)
+$10 /16: entry #0
 ...
+$f0 /16: entry #14

The volume name is found in all directory sectors. The individual entries have the form:

+$00 /12: filename, high ASCII padded with spaces
+$0c / 1: file start track number
+$0d / 1: file start sector number
+$0e / 1: file type: high-ASCII ' ', 'L', 'M', or 'P'
+$0f / 1: $8d (high-ASCII carriage return)

Directory sectors are initialized to $a0, with $8d every 16 bytes. This comes in handy when the DIR file is loaded and displayed within the program's text editor.

The length of a file is not stored in the directory. The only way to determine it is to walk through the file, and that only yields a multiple of 250.

The boot area, which spans the first 3 tracks, is not represented by a file. Some of the sectors seem to have the six-byte headers, and the program disk references files in that area. On a newly-formatted Gutenberg, Jr. data disk, the first file is created in T3 S0.

Track 17, sector 6 appears to hold a sector allocation bitmap. The format details are unknown, but allocations seem to start at track 3.

Document files use an extended ASCII character set, and end when the first $00 byte is encountered. The program supports custom character sets, so it's only possible to display the document correctly if the character set can be identified. "Standard" characters have the high bit set, "alternate" characters have the high bit clear. Only 0x80-0x9f are considered control characters. Open the file FONTS.TABLE on the Gutenberg, Jr. program disk to see the full set of characters available.

Carriage return ($8d) is used as the end-of-line character.

The structure of "program" files is unknown. Gutenberg executables generally begin with the bytes 00 01 02 03 04 05 06 07 d3, while Gutenberg, Jr.'s start with 00 01 02 03 04 05 06 07 08 d3. The following byte appears to hold flags. This is then followed by 6502 code, or by the program name between '/' characters and then the code.

Graphics and font files have a different structure. Files of type 'M' seem to be rare; the only instance is the file FONTD.B on the Gutenberg, Jr. boot disk. (It's possible this is actually disk corruption, and there is no type 'M'.)

The filesystem does not impose a maximum file length, though exceeding (((35 - 3) * 16) - 2) = 510 sectors is impossible on a 140KB floppy disk (unless the file dips into the boot tracks).

The Gutenberg and Gutenberg, Jr. disks found online have some damaged files. For example, if you boot Gutenberg, Jr. and load CHAP.VI from side 2 into the editor, and then scroll down to the bottom, you will find a bit of text that says, "--> BOOT OTHER SIDE <--". The string comes from the boot sector, T0 S0, which is treated as part of the file due to a bad sector link in the document. Other files have overlapping regions or other issues.