Little Man Computer for iOS and iPadOS

Write Little Man Computer assembly, assemble it into the 100 mailboxes, then step or run the program while watching the accumulator, the program counter, the baskets and every mailbox change. Built on CoreLittleManComputer, the same engine behind the lmc command-line tool.

Learn about the Little Man Computer on Wikipedia.

Using the app

• Write a program in the editor, then tap Assemble. Problems are listed under the editor with their line numbers.
• Step runs one instruction; Run keeps going at the speed chosen in the speed menu or Settings. When the program needs input the app asks for a card.
• Tap a mailbox to change its value. Press and hold one to set a breakpoint.
• Programs saves what is in the editor and loads saved programs or the built-in samples. Help explains the instruction set and how the machine behaves.

Sample to try:

     INP
     OUT
LOOP BRZ QUIT
     SUB ONE
     OUT
     BRA LOOP
QUIT HLT
ONE  DAT 1

Enter 3 and the out-basket fills with 3, 2, 1, 0.

Architecture

SwiftUI throughout, Swift 6 with main-actor default isolation, and SwiftData for saved programs.

• System – the App and AppState, which owns routing, sheets and alerts.
• States – EditorState, a @MainActor @Observable object that owns the editor text, the assembled program and an ObservableMachine from the engine. Engine work runs through TaskTriggers attached in the presenter, never in free-floating tasks.
• Presenters – AppPresenter and EditorPresenter, which handle navigation, sheets, alerts and the task modifiers.
• Views – the editor, memory grid, registers, baskets, listing, and the help, library, settings and input sheets.
• Model – the SwiftData schema, SavedProgram, and the preview trait that seeds sample data.

The Xcode project uses synchronized folders, so adding a file on disk adds it to the target.

Building

Requires Xcode 26 and macOS 26.

• Clone the repo
• cp User.xcconfig.template User.xcconfig
• Set your team and bundle prefix in User.xcconfig
• Build and run with Xcode

Unit tests use Swift Testing and cover EditorState and the SwiftData model.

Contributing

It is always a good idea to discuss before taking on a significant task. That said, I have a strong bias towards enthusiasm. If you are excited about doing something, I'll do my best to get out of your way.

By participating in this project you agree to abide by the Contributor Code of Conduct.

Project State

The code in this repo is live in the Apple iOS App Store.