Disk labels, packs, checkout, and file-system repair on the MIT Lisp Machine

What is edited

DLEDIT places a label image in a disk request buffer in Lisp memory and edits that image. Its display covers:

• pack name, drive name, and comment;
• label checkword and version, which are displayed but not ordinary editable fields;
• cylinders, heads, blocks per track, and blocks per cylinder;
• current microload and current load-band selections;
• partition count and descriptor size; and
• each partition's name, starting block, size, and comment.

The display marks the currently selected MCR and LOD partitions with an asterisk. It also reports unallocated gaps and overlaps, so the operator can see an internally inconsistent partition table before committing it.

The editor does not continuously update disk. Control-W is the explicit label write. Until that command is confirmed, edits exist only in the in-memory request buffer. Conversely, writing a changed start or size alters only the descriptor: DLEDIT does not relocate, resize, copy, erase, or validate the partition's existing payload.

The unit argument is broader than a local drive number. System 46 accepts a local unit, a machine name reached over the Chaosnet disk protocol, or the special CC debug-cable path. System 303 additionally decodes TEST as a synthetic unit and MT as a magnetic-tape path. The CC path is restricted to unit zero and has special handling for low-numbered blocks used by the remote debug mechanism.

Complete command comparison

This table inventories every source-defined command in the two DLEDIT command loops. “Same” means System 303 retains the System 46 behavior unless the notes state otherwise.

No mouse gesture is defined in either inspected command loop. Navigation and editing are keyboard-driven.

Startup changed between releases

System 46 begins by constructing a default label image in memory, displaying it, and telling the operator to use Control-R to read the label that is already on the disk. This creates a sharp safety boundary: entering DLEDIT has not yet read the medium, and an accidental Control-W could replace its label with the displayed default.

System 303 reads the existing label automatically during normal entry. It constructs an initialized label only when its initialization option is explicitly requested. It also tracks whether edits have occurred and warns at End. These are concrete later safety improvements, not merely documentation changes.

Known pack templates

System 46 supplies templates for:

The LM-3 tree retains the two Trident choices but changes their proposed partition sets, and adds a Fujitsu Eagle:

A template is a proposed layout, not evidence that a particular surviving pack has that layout. Reading and preserving its actual label remains the first evidentiary step. The changed T-80 and T-300 sets also demonstrate why a drive model alone is insufficient to infer which software release initialized a label.

Interrupted-copy marker

Before System 303 copies a partition, it replaces the target partition comment with “Incomplete Copy.” It restores the source comment only after the copying loop completes. A reset or I/O failure therefore leaves a durable warning rather than making a partial target look complete. The copy path also checks target size and, for load bands, can use measured payload size instead of blindly copying the entire partition.

This is a source-visible preservation feature absent from the maintenance manual's high-level description.

The single-drive diagnostic

System 46 lcadr/d1chk.15 explicitly warns that it tests “disk #1,” must not be run on a one-disk machine, and will clobber disk zero under that configuration. Its DCHECK operation seeks, writes test patterns to block 1, rereads them, and reports error bits. DC-WRITE-READ-TEST repeats destructive write/read cycles until keyboard input stops it. That test prompts independently for cylinder, head, block, and pattern; each address can be a single value, a case list, or a stepped range, and ALL expands to the built-in range. Its complete built-in pattern set is all zeroes, all ones, alternating bits, address-derived data, floating one, floating zero, and a fixed “gubbish” value. A trace variable can print each selected location and pattern.

FORMAT-DISK refuses unit zero in this version but otherwise formats the selected medium cylinder by cylinder and zeros cylinders. Its built-in geometries cover the T-80 and T-300, selected by its T-300-P argument. Refusing one unit is not general data protection: the other unit is intentionally erased.

The controller checkout

System 46 lmcons/dcheck.81 is aimed at a newly constructed disk controller. Its test sequence covers:

• controller-register and bus response;
• block counter and rotational-position behavior;
• recalibration and seeks;
• non-existent-memory handling;
• a write/read pattern test at block 1; and
• reads from several block ranges before directing the operator toward format and ECC tests.

It has a USE-LOCAL-DISK path, but the inspected source reaches a breakpoint whose own comment says the remainder will not work on that path. That is an implementation limit, not a supported local-disk workflow.

Neither checkout belongs on the only copy of a preservation disk. Safe emulator research requires a disposable, hash-identified clone and an explicit expectation that its contents will be destroyed.

The contemporary rebuild procedure

System 46 cadrdc/format.pack is a short operator procedure, tentatively labeled for T-80 packs. It uses two CADRs connected by the debug umbilicus: a master loads formatting microcode and performs operations on a slave whose normal pack has been replaced by the pack to rebuild. After formatting, the procedure calls for the destructive checkout and all-location write/read test, initializes the slave label through DLEDIT's CC unit, and copies selected LOD and MCR partitions from the master.

This establishes what “rebuild” meant at that boundary: format the medium, construct a new label, then repopulate boot partitions from a known source. It is not an in-place repair and does not preserve an unknown FILE partition. The document's own uncertainty about applying beyond the T-80 is retained; the procedure is not generalized to other pack models.

On-disk pack identity

An LMFILE pack header records at least:

• pack and version identity;
• volume name, pack number, and number of packs in the volume;
• an incarnation value;
• pages per LMFILE block;
• allocation maps;
• root block;
• comment and clean/dirty state;
• free-space thresholds;
• parent volume; and
• total block count.

Volumes form a parent/child tree. A child volume cannot mount without its parent, and all packs in one volume must be present. Directory pointers are constrained to the same volume or suitable descendants so a volume can be salvaged without requiring arbitrary unrelated volumes.

The three maps

The maintenance document describes a free map, a block-starts-file map, and a “bad block map.” The maintained source names the third map LOCKED-BLOCK. Its setter's own comment defines a locked block as bad and not to be used, so the code corroborates the manual's intended map rather than defining a different kind of object. The implementation adds an important nuance: it says not to lock a block unless it is already in a file, removes a newly locked block from the available/free pools, and remembers that live files can contain locked blocks. The map excludes known suspect storage from future allocation; it is not itself an automatic surface-error detector.

The block-starts-file map lets maintenance code locate every file independently of directory reachability. The free map records allocation state. For speed, LMFILE transfers free records into an in-core available table and marks them used in the durable map. A crash can therefore lose available space, but the design prevents those records from being allocated twice; garbage collection can recover them later.

INIT-FILE-PARTITION

INIT-FILE-PARTITION formats a FILE partition as a new pack. It dismounts any current use, asks for strong confirmation, initializes the header and maps, and marks the pack dirty so the first garbage collection builds its free map. It is destructive.

The complete documented keyword surface is:

REINIT-FILE-PARTITION

REINIT-FILE-PARTITION changes selected metadata while preserving the intended pack contents. It can change:

• volume name and comment;
• number of packs and parent volume;
• all four availability thresholds; and
• available-table size.

It cannot change pages per block. If the underlying partition grew, it can expand the maps to cover the new extent. It marks the pack dirty so garbage collection reconciles allocation state. Because a mistaken identity or geometry change can still make data unreachable, this remains a confirmed maintenance operation rather than a harmless preference editor.