The Genera Debugger and Display Debugger

Evidence and rights boundary

The user-facing manual is the publicly readable Symbolics Program Development Utilities manual for Genera 8. It describes the standard Debugger in depth and contains a printed alphabetical summary of its commands. It mentions and enters the Display Debugger, but does not supply a comparable pane-and-gesture reference. That missing visible behavior was reconstructed from the licensed Genera 8.5 source and then checked in the user's licensed world.

No licensed source or help payload is reproduced here. The following table records only filenames, byte counts, hashes, and original analysis sufficient to identify the local evidence.

The downloaded reference PDF was 2,292,247 bytes with SHA-256 2c6e23e4a0f969cb7962b71f0ed0eb390b47e394195f6ed9e7a862ba33914d6f. It was verified through 2026-07-19. The purchased archive, world, extracted source, and raw screenshots remain untracked.

From condition to debugger

The condition system decides whether the Debugger is needed. Handlers see a condition object; if none handles it, the debugger receives that object along with the stack and the available restart/proceed machinery. Proceed choices are assembled dynamically from the condition, handlers, and special commands. That is why a Debugger opening prints a set of choices whose meanings vary by error rather than relying on one fixed restart menu.

The Debugger then:

1. invokes the one-shot *debugger-hook*, rebinding it to prevent accidental recursion through the same hook;
2. chooses *debug-io-override* if present, otherwise the failing program's debug-io, and tests whether the resulting stream and window are usable;
3. falls back to the cold-load stream for noninteractive, recursive, or damaged window cases;
4. selects the innermost and current language-specific stack frames;
5. constructs condition-specific proceed accelerators and special commands;
6. reads either accelerated commands, full CP commands, or forms;
7. evaluates forms in the current frame's lexical environment by default and preserves all returned values; and
8. ends only when a proceed/restart, return, reinvocation, throw, abort, or outer recovery operation transfers control.

Before the full command loop, selected conditions can be offered through a small temporary proceed menu. The source excludes unsuitable cases such as the cold-load stream, remote terminals, break conditions, and recursive debugging. This is a convenience path over the same condition choices, not a separate debugger.

Entry and exit paths

Suspend, Abort, Resume, End, and Function are Symbolics keyboard character names. Host X11 keysyms in the museum harness are a separate layer and are reported with the runtime evidence.

The standard Debugger interaction model

The standard prompt is a right arrow. Colon or Meta-X starts a full CP command such as :Show Backtrace; an accelerator can invoke the same command directly; a Lisp form is evaluated in the selected frame. A numeric prefix on colon or Meta-X is an accelerator error because that entry accepts no argument. Output is presentation-sensitive, so stack frames, functions, values, and source objects can carry pointer gestures appropriate to their types.

Evaluation normally inherits the current frame's lexical environment. The :Use Dynamic Environment / :Use Lexical Environment toggle changes that choice. The Debugger protects its own streams, history variables, hooks, and condition machinery from the borrowed environment. Recursive errors from a typed form create another debugger level rather than silently corrupting the first.

The manual says Control-Help lists all commands and accelerators; plain Help is a shorter introduction. Source further routes Help through a separate Dynamic Windows help display when the Display Debugger is active.

Complete printed Debugger command inventory

This table covers every command in the Genera 8 printed “Summary of Debugger Commands.” Accelerators are stated where the summary gives one. Commands without a fixed key remain available by their colon-prefixed CP names and, in some contexts, presentations or Display Debugger menus.

Control-Space
├─ no prefix -> push current frame
├─ modified numeric 0 -> show frame PDL
├─ exactly Control-U -> pop and select
├─ exactly Control-U Control-U -> pop and discard
└─ every other prefix, including typed modified 4 -> beep

Debugger evaluation functions

The manual exposes functions specifically for forms evaluated in the Debugger:

These APIs make the debugger an execution-control environment, not a passive viewer. Museum runtime checks avoid all mutation and monitoring operations.

Display Debugger architecture and layout

The frame has eight named pane specifications arranged as two columns:

The title identifies the activity whose window is shadowed. Selecting a backtrace frame updates the shared current-frame state; the proceed, code, and argument panes are then incrementally redisplayed. The inspect history retains complex objects reached through presentations or description. When source is available, forms in the code pane are themselves presentations; otherwise the pane shows disassembly.

The interactor is the ordinary Debugger reader. Every normal Debugger keyboard command remains usable there. Display Debugger entry is restricted to the main console and rejected recursively. While the frame is active, its default policy disables Abort to reduce accidental loss of the suspended computation; the button path can still invoke the Debugger's applicable abort choice.

Complete Display Debugger button and gesture inventory

The following matrix is exact for the inspected Genera 8.5 source. “Middle” and “right” denote distinct Dynamic Windows gestures, not generic context-menu conventions inferred from modern systems.

The right-button submenus are also source-complete:

What changed from the MIT window debugger

The strongest continuity is the live-frame model: in every generation, the UI looks at a suspended computation and can change it. The strongest interface change is that Genera turns that model into typed CP commands and Dynamic Windows presentations rather than one fixed character table and menu pane.

Findings not evident from the public manual

• The Display Debugger is declared nonselectable and has no process or Select key of its own. It temporarily shadows the failing activity.
• Its panes share the ordinary Debugger state and command loop; it is not a second debugging engine.
• Main-console-only and no-recursion checks are explicit at entry.
• The sixteen button labels hide twenty-nine distinct gestures before counting submenu entries.
• The source/compiled, ordinary/invisible backtrace, trap, breakpoint, and monitor choosers are separate command menus.
• End exits only the Display Debugger. This is the Genera counterpart of System 303's Exit Window EH, not an Abort.
• Arguments and locals in the pane are mutable through a Control-Meta-right presentation gesture.
• The source maintains per-pane redisplay ticks and cached displayed-code state.
• The standard manual's Show Arglist accelerator disagrees with the inspected source, as documented above.
• The recursive-failure thresholds form an explicit degradation ladder ending in a halt.

Runtime observation: Genera 8.5

The fresh debuggers-d12-genera-20260718 session booted the licensed Genera 8.5 world into Dynamic Lisp Listener 1. The listener evaluated the project-authored probe ,(error "Museum debugger probe D12"). The ordinary Debugger then displayed the exact error text, selected CP::COMMAND-LOOP-EVAL-FUNCTION, printed its arguments and defaulted environment, and constructed two condition-specific Super choices: return to the listener's Lisp top level, or restart the listener process. This directly confirms that the opening choices are derived from the live condition rather than one fixed menu.

Entering (+ 40 2) at the arrow selected the Debugger's Eval operation, printed 42, and advanced the visible frame display to SI::EVAL. No guest state was changed by the form. A later clean :Abort returned to Dynamic Lisp Listener 1 and printed the transfer back to its Lisp top level.

Runtime observation: this exact Genera 8.5 display records the ordinary Debugger before any proceed or mutation on 2026-07-18. The Dynamic Lisp Listener's top edge is visibly ragged while its side edges remain straight, a bounded runtime example of the viewport-continuation border. It is published as the minimum visual evidence for those visible claims and the Debugger's live condition choices and text-oriented frame view; Symbolics retains interests in the licensed historical software, and inclusion implies no endorsement.

Limits and open questions

• “Complete” for the standard Debugger means the printed Genera 8 command summary plus the explicitly enumerated source additions. Condition-specific proceed commands are dynamic by design and cannot be assigned one fixed list.
• No licensed source or decoded help text is tracked. Hashes identify the local evidence without redistributing it.
• The Display Debugger manual reference does not fully describe its layout or gestures; those claims depend on local source and the bounded runtime check.
• The debugger-specific translator base is enumerated, but inherited Dynamic Windows, language/type, loaded-world, and site overlays can extend effective Right menus. A live translator dump remains required before those menus can be called closed.
• The Show Arglist accelerator discrepancy requires a dedicated direct keystroke test before either mapping can be called runtime-confirmed.
• Mutation, monitoring, breakpoints, traps, returns, and reinvocations are source/manual-established but intentionally not exercised on the preserved world.
• The two selected captures are publication-reviewed for these documentary uses only. Remaining raw captures are not thereby publication-ready.

Sources

Public documentation link verified through 2026-07-19.

• Symbolics, Program Development Utilities, Genera 8, printed pages 182–245, especially the overview, command descriptions, printed summary, and Debugger evaluation functions.
• Licensed Genera 8.5 source files identified by filename, byte count, and hash in the evidence table above; inspected locally through 2026-07-19 and not redistributed.