System Menu and program selection on the MIT Lisp Machine

Evidence sets

The public MIT CADR System 46 source is pinned at Git revision 8e978d7. The maintained LM-3 System tree is pinned separately at Fossil check-in 4df393c, tag system-303.

The maintained LM-3 files preserve and extend older MIT window-system code. They are not silently treated as the missing continuation of every System 46 file, and a live System 303 registry is not projected backward onto System 46.

Three different selection mechanisms

How the System Menu is invoked

The System 46 manual says generic windows normally summon the System Menu with a double right click. When the pointer is the ordinary north-by-northwest arrow, as in a Lisp Listener, either a single or double right click works. Moving off a momentary menu dismisses it without a choice.

The menu appears at the pointer, but several actions operate on the window that was under the pointer when the menu was requested, not necessarily on the selected window. System 46's Other command explicitly restores that pointer position before showing its auxiliary page so the eventual operation still has the same target.

System 46 menu inventory

System 46 uses one eight-item menu and an auxiliary seven-item Other menu.

The exact System 46 Create menu is Supdup, Telnet, Lisp, Edit, Peek, and Any. Any prompts for a window flavor instead of limiting creation to the built-in list.

The auxiliary System 46 implementations of Reset and Kill do not contain the confirmation dialogs added to the corresponding System 303 menu paths. The manual describes intent, but the pinned code establishes this potentially important safety difference.

System 303 menu inventory

The LM-3 source reorganizes the menu into three visible columns. A fresh System 303-0 runtime showed these exact labels:

The operations are:

• Create and Select retain the roles described above.
• Split Screen builds a chosen set of new or existing windows, optionally in a named frame with its own System key.
• Layouts saves or restores live window objects, exposure status, geometry, ordering, and the selected window.
• Edit Screen edits the current screen with left or middle; its right-button form offers frames to edit.
• Set Mouse Screen chooses a default/cycled screen with left or middle and always offers the screen menu with right.
• Kill and Reset ask for pointer confirmation before acting in the inspected System 303 source.
• Refresh, Bury, Attributes, Arrest, and Un-Arrest act on the window under the pointer. Attributes opens a live window-attribute editor.
• Lisp, Edit, Inspect, and Peek select an existing matching frame or create one.
• Trace starts its menu-driven setup, Mail selects or creates ZMail, and Emergency Break opens the cold-load-stream interaction.

System 303's exact default Create choices are Supdup, Telnet, Lisp, Edit, Peek, Inspect, Font Edit, Lisp (Edit), and Any. Lisp (Edit) is the ZWEI editor-stream top level, not a second label for the ordinary Listener.

The Programs column is mutable through ADD-TO-SYSTEM-MENU-PROGRAMS-COLUMN; adding an existing name replaces its form and documentation. ZMail uses this integration point in the maintained tree. The static list and a loaded band's effective list therefore need not be identical.

Split Screen and saved layouts

Both source lines expose more than a simple “tile windows” command. The Split Screen menu can include:

• any normal Create-menu flavor;
• an existing Listener or arbitrary existing selectable window;
• plain output, trace, error, or combined trace/error windows;
• an optional frame with a name and System key;
• pointer-selected corners instead of the whole screen;
• Undo, Do It, and Abort while constructing the proposal.

System 303's layout algorithm stacks fewer than four windows vertically. For four or more it uses two columns and ceiling(n/2) rows; when n is odd, the last bottom window spans the full width. This concrete geometry rule is in the implementation, not the short menu label.

A saved layout stores live window references and their status and edges; it is not a serialized restartable desktop. Restoring it reshapes and exposes those same objects, restores their ordering, selects the first saved window, and asks the screen manager to reconcile the display. Saving can also register a new System selector key for that layout.

The source preserves known limitations: Split Screen may thrash the disk, saved layouts cannot be edited through this interface, the proposal diagram is not itself mouse-editable, and the frame value chooser sometimes fails to appear or use a requested frame. SYSTEM-MENU-LAYOUTS is also preceded by a comment that it needs substantially more error checking. These are explicit implementation notes, not inferred defects.

System-key inventories and a source/manual disagreement

Findings the menu labels do not reveal

• The System Menu Programs registry and the System selector registry are independent. The observed presence of Mail in one and absence of M in the other is expected under this design.
• A selector entry may run an arbitrary creation expression; it is not limited to calling a window constructor.
• Normal System selection cycles an existing flavor, while Control-System requests creation. The visible Help screen states both behaviors.
• Saved layouts hold live object references. They are not world snapshots and cannot reconstruct killed windows.
• Window selection deliberately moves typeahead to the old window before the keyboard focus changes.
• The process/window startup code itself says a future “program system” should replace current selection-time process handling. Program selection is a window-system convention here, not a separate application manager.

Runtime observation: System 303-0

The fresh Xvfb session shared with the Listener study recorded:

After the boot date/time responses and a Listener evaluation, the relevant ordered actions were: click right at framebuffer coordinate (400, 500), capture the three-column menu, dismiss it, type System Help, capture the five live entries, type Space to flush the Help window, then continue to Inspector and Peek. No System Menu operation was selected.

The image-specific review selected only the sparse System Menu capture for the curated CADR screenshot catalog. The fuller System Help capture remains ignored because the menu image is sufficient for the layout claim and avoids publishing an additional explanatory-text screen.

Runtime observation: the LM-3 System 303 System Menu after a right click in the Listener, captured 2026-07-18. Underlying software and display material remain the property of their respective rightsholders; reproduced here for criticism, scholarship, and historical documentation under 17 U.S.C. section 107. No affiliation or endorsement is implied.

The 6,781-byte run record has SHA-256 2d877843f58a8a261bafe68afca846919963dec5584a39bbb5275fcbe6615e22. Shutdown was clean: emulator and Xvfb status zero, forced_stop=false, state_may_be_incomplete=false, and the base disk unchanged.

Open questions

• The System 46 menu and static selector were not exercised in a runnable System 46 band. Pointer geometry and any load-band patches remain source/manual evidence.
• The observed System 303 run did not open every subsidiary menu or perform destructive window operations. Their inventories and confirmation behavior are established by the pinned implementation, not by destructive testing.
• The exact load-time reason that the observed selector lacks optional entries such as M was not traced to a saved-world build log. The live absence is confirmed; its historical build decision remains unknown.

Sources

• MIT CADR System 46, sysmen.105, basstr.163, and operations manual source, revision 8e978d7; verified 2026-07-18.
• LM-3 System 303, sysmen.lisp and basstr.lisp, check-in 4df393c; verified 2026-07-18.