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MBASIC Extensions and Modern Features

About This Implementation

This is MBASIC-2025, a modern implementation of Microsoft BASIC-80 5.21 (CP/M era) with carefully chosen extensions for modern development while maintaining 100% compatibility with original MBASIC 5.21 programs.

Extensions Beyond Original MBASIC 5.21

These features are NOT in the original CP/M MBASIC 5.21 from 1981. They are modern additions for improved development experience.

🔍 Debugging Commands

IMPORTANT: These commands are NOT in original MBASIC 5.21. They are modern extensions that may be available in different forms depending on the UI backend.

BREAK - Breakpoint Management

BREAK 100              ' Set breakpoint at line 100
BREAK                  ' List all breakpoints
BREAK CLEAR            ' Clear all breakpoints
BREAK CLEAR 100        ' Clear specific breakpoint
Status: ⚠️ MBASIC-2025 Extension (Not in MBASIC 5.21) Availability: CLI (command form), Curses ({{kbd:toggle_breakpoint:curses}}), Tk (UI controls)

STEP - Single-Step Execution

STEP                   ' Execute one statement
STEP 5                 ' Execute 5 statements
STEP INTO              ' Step into subroutines (planned)
STEP OVER              ' Step over subroutines (planned)
Status: ⚠️ MBASIC-2025 Extension (Not in MBASIC 5.21) Availability: CLI (command form), Curses ({{kbd:step:curses}}/{{kbd:step_line:curses}}), Tk (UI controls)

STACK - Call Stack Display

STACK                  ' Show full call stack
STACK GOSUB            ' Show only GOSUB stack
STACK FOR              ' Show only FOR loop stack
Status: ⚠️ MBASIC-2025 Extension (Not in MBASIC 5.21) Availability: CLI (command form), Curses (menu access), Tk (stack window)

🖥️ Multiple User Interfaces

Original MBASIC 5.21 only had a command-line interface. This implementation provides:

  1. CLI - Classic command line (closest to original)
  2. Curses - Full-screen terminal UI (⚠️ Extension)
  3. Tk - Desktop GUI with menus (⚠️ Extension)
  4. Web - Browser-based IDE (⚠️ Extension)

The GUI interfaces (Curses, Tk, Web) are NOT in MBASIC 5.21.

📝 Editor Enhancements

Features NOT in original MBASIC 5.21: - Full-screen editing (Curses, Tk, Web) - Syntax highlighting (Tk, Web) - Find and Replace (Tk only) - Cut/Copy/Paste (Tk, Web) - Mouse support (Tk, Web) - Auto-save behavior varies by UI: - CLI, Tk, Curses: Save to local filesystem (persistent) - Web UI: Files stored in server-side session memory only (lost on page refresh or session end)

🎯 Visual Debugging

Features NOT in original MBASIC 5.21: - Visual breakpoints (click line numbers) - Variable inspector windows - Real-time variable updates - Execution highlighting - Step-by-step visualization

💾 Enhanced File Handling

More permissive than MBASIC 5.21: - Accepts LF, CR, or CRLF line endings (original only CRLF) - Long filenames (original limited to 8.3) - Path support (original only drive letters) - Unicode text files (original only ASCII)

Web UI File Storage Limitations: The Web UI uses an in-memory virtual filesystem with these restrictions: - Files persist during a single browser session (lost on page refresh or when session ends) - No persistent storage across sessions or after closing the tab - 50 file limit maximum - 1MB per file maximum - No path support (simple filenames only) - Cannot access user's local filesystem (security restriction)

See Compatibility Guide for complete Web UI file storage details.

🔧 Development Tools

NOT in original MBASIC 5.21: - Parse tree visualization (utils/show_parse_tree.py) - Error analysis tools (utils/analyze_errors.py) - Automatic keyword spacing fix (utils/fix_keyword_spacing.py) - Program detokenizer (utils/detokenizer.py)

Compatibility Notes

Running Original MBASIC Programs

✅ 100% Compatible: Any program that runs on CP/M MBASIC 5.21 will run here - All statements work identically - All functions produce same results - All error codes match - Numeric precision matches

Using Extensions

⚠️ Programs using extensions will NOT run on original MBASIC 5.21

If you use: - BREAK, STEP, STACK commands - Visual debugging features - GUI-specific features - Modern file paths

Your program becomes MBASIC-2025 specific and won't run on vintage systems.

Compatibility Mode

To ensure compatibility with original MBASIC 5.21: 1. Use only CLI backend 2. Don't use debugging commands 3. Stick to 8.3 filenames 4. Use only CRLF line endings 5. Avoid GUI features

Feature Comparison

Feature MBASIC 5.21 (1981) MBASIC-2025 Notes
Core BASIC ✅ ✅ 100% compatible
BREAK command ❌ ✅ Extension
STEP command ❌ ✅ Extension
STACK command ❌ ✅ Extension
GUI interfaces ❌ ✅ Extension
Syntax highlighting ❌ ✅ Extension
Find ❌ ✅ (Tk) Extension
Replace ❌ ✅ (Tk) Extension
Visual debugging ❌ ✅ Extension
Long filenames ❌ ✅ Enhancement
Unicode ❌ ✅ Enhancement
Cross-platform ❌ ✅ Enhancement

Philosophy

This implementation follows these principles:

  1. 100% backward compatibility - All original programs must run
  2. Careful extensions - Only add features that don't break compatibility
  3. Modern development - Provide tools for productive coding
  4. Clear documentation - Always mark what's an extension
  5. Optional features - Extensions can be ignored for vintage compatibility

For Purists

If you want the authentic MBASIC 5.21 experience: 

python3 mbasic --ui cli
Then avoid using: - BREAK, STEP, STACK - Any GUI features - Long filenames - Modern paths

This gives you pure MBASIC 5.21 compatibility.

For Modern Developers

If you want all the modern conveniences: 

python3 mbasic --ui tk  # or web
Use all features freely: - Visual debugging - GUI editors - Modern file handling - Development tools

Why These Extensions?

Each extension was carefully chosen:

  • Debugging commands: Essential for development, don't affect program behavior
  • Multiple UIs: Accessibility and preference, core interpreter unchanged
  • Visual features: Productivity boost, optional use
  • File handling: Necessary for cross-platform operation

Future Extensions Under Consideration

These are being evaluated but NOT yet implemented: - DO...LOOP (structured loops) - SELECT CASE (multi-way branching) - SUB/FUNCTION procedures - Local variables - Long variable names

Each would be optional and wouldn't affect existing programs.

Reporting Compatibility Issues

If you find a program that works in CP/M MBASIC 5.21 but not here: 1. That's a bug, not a feature 2. Report it with the exact error 3. Include the original program 4. We'll fix it to match MBASIC 5.21 behavior

See Also