lem_port

lem_port brings the 1991 DOS release of Lemmings to modern systems – natively, without an emulator.

It is not a remake and not a reinterpretation. The aim is to play the original game exactly as it was: the same 120 levels, the same rules and the same timing, so that every solution that worked in 1991 still works today.

Why another engine?

The best-known faithful recreation of DOS Lemmings is Lemmix by Eric Langedijk – a remarkable piece of work that captures the original down to its quirks. Lemmix, however, is written in Delphi and runs only on Windows.

lem_port follows the same path with the same goal of accuracy, written in portable C++20 on top of SDL3, so the game can be enjoyed on:

• Linux
• macOS
• iOS and iPadOS
• and later in the browser (WebAssembly) and on Windows

The engine core is free of platform code; each platform only provides a thin layer for the window, input and sound.

Game data

The original game files are not included and never will be. You need your own copy of the DOS version of Lemmings. Point lem_port at the folder that contains main.dat, level000.dat, vgagr0.dat and the other data files – upper or lower case names both work.

Building

You need CMake 3.20 or newer, a C++20 compiler and, for the game itself, SDL3.

cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build
ctest --test-dir build

macOS app to share

tools/mac_package.sh build-mac

builds a self-contained universal Lem_Port.app (Apple Silicon and Intel, macOS 11 or newer, SDL3 built in) and packs it as build-mac/Lem_Port-macos.zip. The app is signed ad hoc; on another Mac open it the first time with right click → Open.

iOS and iPadOS

Works on iOS 12 and newer (tested on an iPad mini 2 with iOS 12.5). You need Xcode, an Apple ID added in Xcode → Settings → Accounts (a free one is fine) and, for installing from the command line, libimobiledevice (sudo port install ideviceinstaller or brew install ideviceinstaller).

# Xcode project; SDL3 is downloaded and built for iOS
cmake -S . -B build-ios -G Xcode -DCMAKE_SYSTEM_NAME=iOS \
      -DCMAKE_OSX_ARCHITECTURES=arm64

# signed build for devices (TEAM = your team id, see below)
cmake --build build-ios --config Release --target lem_port -- \
      -sdk iphoneos -allowProvisioningUpdates DEVELOPMENT_TEAM=TEAM

# install on the USB-connected device and copy the game data
tools/ios_install.sh build-ios/Release-iphoneos/Lem_Port.app /path/to/lemmings

• The team id is the OU= value of your signing certificate: security find-certificate -c "Apple Development" -p | openssl x509 -noout -subject.
• A device must be registered with the team once: open build-ios/lem_port.xcodeproj in Xcode, choose the device and press Run. Afterwards the build and ios_install.sh are enough.
• With a free Apple ID an installed app runs for 7 days; build and install again to renew it. Updates keep the game data.
• iOS 12–14 devices need Xcode's support files for that version in Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport.
• On the first start, trust the developer on the device: Settings → General → Device Management.
• The game data can also be copied with Finder (device → Files) or the Files app into the Lem_Port folder; the game starts once the files are there.

Touch controls: tap = click (tap a lemming to assign the selected skill), long press = assign preferring walking lemmings (right button), drag the level to scroll, double tap TIME for fast forward (it then reads FAST), three-finger tap ends a level, two-finger tap returns to the menu from the preview and result screens.

Tests

The regression tests in tests/data_tests.cpp need the original data and run only when LEM_DATA points to it (LEM_DATA=/path/to/lemmings ctest ...). They play the replays in tests/replays and check every level against tests/golden.txt.

On macOS with MacPorts, add -DCMAKE_PREFIX_PATH=/opt/local to the first command. If SDL3 is not installed (many Linux distributions do not package it yet), add -DLEM_FETCH_SDL=ON to download and build it.

Playing

Lem_Port --data /path/to/lemmings      # remembered for next time

On first start without --data, a folder picker asks for the data folder.

Input	Action
Left click on a lemming	assign the selected skill
Click a panel button, F3–F10, 1–8	select a skill
F1 / F2, − / +	release rate down / up
F11, P	pause
Double-click nuke, F12	nuke
F, double click on TIME	fast forward
M / X	music / sound effects on or off
Arrow keys, mouse at screen edge, minimap	scroll
Hold right button	prefer walking lemmings when assigning
Alt+Enter	fullscreen
Esc	end the level

Main menu: F1 (or click the sign) plays, F2 enters an access code, F4 shows the controls, F3 cycles music + effects / effects only / silence, the arrow keys or the rating sign change the rating, Esc quits. On the level preview screen the arrow keys and Page Up/Down (on touch screens a swipe) also choose any level. After a successful level the result screen shows the access code for the next one; the right mouse button returns to the menu.

Tools

lemtool lets you look inside the original data files:

lemtool dat    <file.dat> [outprefix]   # list or extract sections
lemtool gfx    <datadir> <outdir>       # graphics sets as PNG sheets
lemtool levels <datadir> <outdir>       # every level record as PNG + summary
lemtool main   <datadir> <outdir>       # sprites, panel, fonts, menu art
lemtool list   <datadir> [outdir]       # the 120 levels in game order (+ PNGs)
lemtool audio  <datadir> <music|sfx> <n> <out.wav> [seconds]
lemtool replay <datadir> <file.lrp>     # play a replay, print the result
lemtool sim    <datadir> <rating> <n> <outdir> [ticks] [every] [script]
                                        # play a level headless, save snapshots

Documentation

• Roadmap – the plan and current progress
• File formats – how the original data is stored
• Game mechanics – how the simulation behaves

Acknowledgements

lem_port stands on the shoulders of others:

• DMA Design created Lemmings, published by Psygnosis in 1991. The game and its data remain the property of their copyright holders.
• Eric Langedijk wrote Lemmix, the inspiration for this project and the yardstick for faithful behaviour.
• ccexplore and namida refined Lemmix to match the DOS game even more closely; Aaron Kelley maintains that work as Lemmix Player.
• Aaron Giles wrote ymfm (BSD-3-Clause), which emulates the AdLib's FM chip; it is included in third_party/ymfm.
• ccexplore, rt, Mindless and Simon documented the original file formats and shared that knowledge with the Lemmings community.

Thank you all. Apart from ymfm, no code from these projects is included here.

License

Copyright © 2026 Michał Majchrowicz

lem_port is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version. See LICENSE for details.

The original game data is not part of this project and is not covered by this license.