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Knowledge Base›Genera›Open Genera world loads and the VLOD format

Open Genera world loads and the VLOD format

Bottom line

A .vlod is not an archive and it is not a mountable disk or file-system image. It is a VLM world-load file: a purpose-built, cold-bootable image of Genera's tagged virtual memory, accompanied by a map that tells the emulator which words belong at which virtual addresses.

The closest familiar analogy is a Lisp heap or core image prepared for restart. It is also reasonable to call it snapshot-like, but it is not an arbitrary live VM suspend snapshot: Genera deliberately shuts down and makes the world cold before asking the host-side VLM to write selected virtual-memory ranges.

Question VLOD answer
Can tar, unzip, or a filesystem driver list its files? No
Does it preserve Genera code and data objects? Yes, as tagged memory words
Is it loaded into the VLM's virtual memory and booted? Yes
Does it contain a normal Unix or LMFS directory tree? No
Can a save contain only changes from another world? Yes, through Incremental Disk Save (IDS)
Can it be inspected without booting Genera? Yes, with format-aware experimental tools
Does low-level inspection immediately recover source files? No; object decoding is a separate problem

What Symbolics meant by a world load

The Open Genera User's Guide describes the Genera environment as one large virtual memory space containing the code and data objects of all loaded applications. It then defines a world load as a complete image of that environment, frozen when saved. The manual also says that a world can be composed of several files using IDS and that a saved world is not subsequently modified.

That definition makes the archive-versus-snapshot distinction fairly decisive. The VLM's -world option loads the named Lisp world into memory and boots it; a separate path names the VLM debugger image.

In the licensed Genera 8.5 source inspected locally, Genera's pathname table calls the format :V-COLD-LOAD with canonical suffix VLOD. The public VLM source calls it a “VLM world file (.VLOD).” No primary source found during this investigation gave a reliable prose expansion of the four letters, so this page does not invent one.

On-disk organization

The public VLM implementation documents and reads the format directly. A VLOD has:

  • a format cookie and version/architecture word;
  • counts and locations used to interpret the header;
  • in VLOD version 2, generation and parent timestamps for incremental worlds;
  • a load map describing destination virtual addresses, word counts, and operations;
  • page payloads containing Genera's tagged Lisp words.

A load-map entry can load words from file pages, fill memory with a constant, fill it with an incrementing constant, or copy an already loaded memory range. The header and load map use the older Ivory file packing. For VLM page payloads, each Lisp “Q” has a 32-bit data part and an 8-bit tag/type-and-CDR field. A VLM page contains 8,192 Qs; the file stores its data and tags in separate block ranges. This is why a generic file utility sees opaque data, while the emulator can map the contents into the correct tagged virtual-memory addresses.

The host-side save path receives address/extent pairs from Genera, canonicalizes them into load-map entries, writes the header, then writes the corresponding memory pages. Genera offers both a complete save of the world and an incremental save of changed pages. When loading an incremental file, the emulator locates its ancestors and merges their load maps before populating memory.

Snapshot-like, but deliberately cold

“VM snapshot” is useful shorthand only with a qualifier. Inspection of the licensed Genera 8.5 disk-save source shows that a VLOD captures the object world after a controlled save sequence. Genera stops normal activity, runs shutdown-related actions, identifies regions and active stack pages to preserve, excludes temporary and page-table areas, marks the state cold, and invokes the VLM's save-world coprocessor operation.

It therefore does not mean “every byte of the Unix process plus all host device and X11 state at an arbitrary instant.” Unix swap acquired after boot is explicitly session state and is not part of the immutable world load. Booting reconstructs the mapped Genera memory and then follows Genera's cold-start initialization path.

What can be extracted meaningfully?

Standard archive extraction: no

There is no member table containing pathnames and file payloads, and no filesystem to mount. Renaming a VLOD or feeding it to an archive program will not expose Genera sources or documentation.

Open Genera release media keeps these concerns separate. The world file is one artifact; the sys.sct hierarchy contains separately stored Lisp sources, compiled systems, and documentation. If the goal is to study distributed source or manuals, the extracted sys.sct tree is far more useful than mining the heap image.

The licensed release inspected for this museum produced a sys.sct tree of 5,075 files totaling about 174.5 MB (166.4 MiB), including 1,794 Lisp-source files, 1,453 compiled .vbin files, and 801 .sab documentation files. These are ordinary host files extracted from the release media around the world image, not members extracted from the VLOD.

Structural and object-oriented inspection: yes

The format is sufficiently documented in VLM source to parse. The experimental Common Lisp project worldtool currently supports operations including:

  • dumping VLOD/ILOD headers and load maps;
  • reading tagged Qs by virtual address;
  • producing annotated architectural-memory dumps when given layout data;
  • scanning strings, packed symbol print names, and symbol values;
  • exporting a parsed world and round-tripping it through a writer;
  • emitting and experimenting with newly constructed cold worlds.

This is meaningful extraction, but at the level of a language runtime image. Turning tagged words into packages, symbols, arrays, functions, classes, source locations, and other high-level objects requires knowledge of Genera's object layouts. A tool can recover many strings and symbols before it can reconstruct every object graph. Booting the world and using Genera's own introspection and export facilities remains the most mature route for high-level exploration.

For the specific boundary between original source, symbolic disassembly, executable low-level Lisp, and recoverable font data, see Recovering code and assets from a Genera world.

worldtool is active experimental research rather than a stable archival standard. Pin a revision and validate results before building preservation workflows around it.

Observation of the museum's Genera 8.5 world

The licensed Genera-8-5.vlod inspected locally has this reproducibility record:

  • size: 54,804,480 bytes;
  • SHA-256: a8ee5e86cc7e322f7385af3e0cd579d7650d4dcfc3ce328acbf8b25515dd0672;
  • VLOD cookie: 0xA38A8988;
  • VLM format version/architecture value: 0x00800081 (version 2);
  • 6,690 VLM blocks and 114 wired load-map entries;
  • 10,952,704 tagged Qs described by those entries, all using raw data-page loads;
  • base-world generation number 0, so this file does not require a parent world.

Using worldtool revision a2fdcfd, the dump command decoded the header and virtual-address map. Its symbols --min 24 scan found 42,258 character-array strings and 1,126 packed print-name runs, including recognizable build messages, command prompts, and documentation strings. That is direct evidence that useful semantic material can be recovered offline, even though it is not file extraction.

The distribution also includes smaller inc-...from-Genera-8-5.vlod files. Those are incremental worlds whose names and headers associate them with a base world. They cannot in general be treated as standalone complete images. For example, the bundled inc-clim-from-Genera-8-5.vlod reports generation 1, and its recorded parent timestamp exactly matches the inspected base world's timestamp.

Sources

  • Symbolics, Open Genera User's Guide, “Controlling Open Genera,” especially PDF pages 5–7.
  • Symbolics, Open Genera Installation Guide, “Disk Space Requirements for Open Genera” and “Machines and Worlds.”
  • VLM source, world_tools.h at revision 55b2a3b, for cookies, page sizes, load-map operations, and world metadata.
  • VLM source, world_tools.c at revision 55b2a3b, for loading, saving, parent-map merging, and page layout.
  • Experimental worldtool, vlod.lisp at revision a2fdcfd and cli.lisp at the same revision.
  • Licensed Genera 8.5 source inspected locally: sys.sct/io/pathnm.lisp.~882~ (pathname type), sys.sct/l-sys/disk-save.lisp.~158~ (complete and incremental save flow and memory-range selection), and sys.sct/i-sys/sysdf1.lisp.~156~ (save-world coprocessor structures and world metadata). These artifact-relative names record provenance without redistributing or linking the proprietary files.

Last verified: 2026-07-16.

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