# Port architecture and fidelity status The port is a clean, modern C implementation. The original executable is used as private behavioral evidence, not translated or linked into the Playdate program. ## Runtime layers 1. `tools.assetc` verifies and decodes the DOS archives into `LPD1` version 6. 2. Each unique level's Bayer 2x2, clustered 2x2, Bayer 4x4, solid, and steel 1600 x 160 bitplanes use deterministic PackBits compression. `lp_pack` decompresses the selected level through a bounded 1 KB streaming reader into the same fixed planes used by gameplay. Objects, original animations, terrain-edit masks, and the executable's exact 51-frame explosion-particle table remain directly addressable in the pack; identical encoded sprite sequences share atlas storage. 3. `lp_game` advances at the DOS-compatible 17 Hz rate. It owns release timing, lemming states, all eight skills, blocker interactions, steel/one-way rules, triggers, win/loss state, and terrain mutation. 4. `src/main.c` maps the portable state to Playdate input, framebuffer, menus, saves, and the callback-driven AdLib synthesizer. The package contains all 120 one-player levels in Fun, Tricky, Taxing, and Mayhem order. Two-player content is intentionally out of scope. ## Display policy The 1600 x 160 DOS terrain is converted once with world-anchored Bayer patterns, so scrolling cannot cause dither shimmer. Lemmings retain their exact source dimensions, masks, and pixel positions and render every opaque source pixel as black, keeping pale moving limbs visible against the white sky. Animated objects use a stable luminance threshold instead of dithering. Normal gameplay uses an exact 2x detail view in which every source pixel becomes a uniform 2 x 2 block and the cursor-following crop shows 200 x 104 world pixels below a fixed 32-pixel status panel. The panel reports time, released/total count, rescued/required progress, and the active skill inventory or paused state. During gameplay the crank adds one second of simulation per revolution, capped at 4x total speed; in the paused action panel it adjusts release rate. The panel exposes one two-cell release-rate editor, eight skills, pause, and nuke in a 6 x 2 grid. A enters release-rate editing, pressing or holding Up/Down changes it, and A/B accepts/cancels. The eight skill cells use clear, undithered original DOS sprite poses and only the selected skill animates. Exact native Atari Lynx crops supply the nuke art at integer 2x; it and Pause stay static. Entering the panel explicitly pauses simulation and discards menu wall time, so closing it cannot trigger a catch-up burst. A selects and B returns from the panel; on the gameplay screen, the A+B chord toggles persistent pause directly. Horizontal camera movement is limited to visible terrain/object bounds plus 24 source pixels. Gameplay remains fixed at the exact 2x view. The title-screen-only `Dither` menu selects Solid, world-anchored Bayer 2x2, Cluster 2x2, or Bayer 4x4 for terrain and special-level backgrounds before play. Lemmings and animated objects remain unchanged. The view does not draw a gameplay legend over the world. The system menu provides direct `Choose Level` and `Reset Level` commands only while a level is active. The always-available `Game` control provides Continue and Nuke. Menus and results use the native 400 x 240 display rather than emulating the DOS menu resolution. Startup shows the complete logo followed by a native difficulty and level carousel. The title credits DMA Design and the DOS port for Playdate, and B opens detailed original DOS credits. Completing Mayhem's final level shows the original DMA Design master-player salute. All four ratings are independently available; within each one, completion bits unlock the first incomplete level while every completed level remains replayable. The browser initially focuses the saved last-played rating and its current level, and also shows a compact terrain preview plus the original rescue, time, and skill requirements. Falling evenly distributes eight three-pixel and thirty-two two-pixel steps across 40 ticks, averaging 2.2 source pixels per tick. This stays below the DOS three-pixel step that appeared excessive in the default 2x view, and the speed is tuned through `LP_FALL_STEP`, `LP_FALL_PHASE`, and `LP_FALL_EXTRA` in `lp_game.h`. Falling animation cadence is paced against it. Fall distance counts every source pixel travelled, landing step included, so the fatal distance in `LP_FATAL_FALL_PIXELS` stays a real distance when the speed changes: a 62-pixel drop is survivable and a 63-pixel drop is fatal. A level opens centred on its entrance hatch. The DOS start scroll assumes a 320-pixel viewport and leaves the hatch out of frame on 13 of the 120 levels in the 200-wide 2x detail view, so it does not place the opening view. Levels whose hatch sits near a world edge keep the hatch on screen but off centre, because the camera stays inside the level. The first lemming is released a fixed `LP_FIRST_RELEASE_TICKS` after the level starts. Only the gap between later lemmings follows the level's release rate, so a slow level does not wait an entire release interval for its first lemming. The visible level clock remains at its initial value during the spoken intro and entrance-hatch opening. It begins on the first lemming release, alongside the level music, so setup animation does not consume playable time. ## Audio policy The authoritative `ADLIB.DAT` is decompressed to its 22,125-byte Sound Images memory image during asset compilation. A clean C port of the command interpreter drives the pinned DOSBox DBOPL core in nine-channel OPL2 mode. The reference and Simulator paths synthesize at 49,716 Hz. On physical Playdate, DBOPL synthesizes at 22,050 Hz above the final 8.5 kHz passband, then uses deterministic 2x linear interpolation for the 44.1 kHz callback. A fractional scheduler preserves the approved 49,716 Hz tick durations: Fun 1 differs by only two output frames over its complete 59.04-second cycle. Two gentle fixed-point low-pass poles run after rate conversion. Songs repeat through their original control flow, including independently looping voices, without stored PCM seams. The main update loop renders ahead toward a 6,144-frame (139 ms) high-water mark in a lock-free 8,192-frame ring. The SDK audio thread only copies published samples, so it performs no synthesis, allocation, or file I/O; atomic producer/consumer indices make track changes and underrun zero-fills safe. Gameplay emits semantic events into a bounded queue. Fifteen short IMA ADPCM effects are preloaded and played through a four-voice pool, eliminating trigger-time file I/O. Action effects retain the DOS AdLib character, while the three iconic spoken lines use original Amiga samples. Per-event gains compensate for the captures' different mastering levels, so voices and transient effects no longer overpower the live OPL mix. DBOPL is copyright the DOSBox Team and licensed under GPL-2.0-or-later. The pinned adapted source and complete license are retained in `third_party/dbopl`, and the license is copied into PDX builds. Public binary distribution must make the corresponding engine source and build instructions available under GPL-compatible terms. The copyrighted DOS data is separate, remains user-supplied, and is not covered by that license. The Python interpreter and JavaScript DBOPL player remain independent research oracles. All 39 clean-interpreter register streams were verified byte-for-byte against the independent player for 10,000 timer steps. A one-second Fun 1 native render is also pinned PCM-exactly against that oracle. ## Validation and remaining fidelity work Automated checks cover archive checksums, decompression failures, known section sizes, canonical level ordering, pack structure, all-level loading and 300-tick smoke simulations, sprite/mask loading, spawning, movement, all eight skills, steel preservation, traps, object animation, sound-event transitions, nuke and win conditions, host compilation, and both Playdate compiler targets. Bombers display a five-to-one countdown over five real simulation seconds. Nuking follows DOS slot order and arms one lemming per tick with a complete fuse. The Oh-no pose, static burst frame, 51 frames of 80 signed particle positions, terrain mask, sound transition, and delayed removal are preserved. The port is playable, but “DOS-exact” remains a validation target rather than a claim. Original object animation frames, masks, and trap activation are now implemented. Named gameplay sound transitions are implemented; exact frame-level comparison against a golden DOS replay remains future work. The private Ghidra workspace is intended to close those gaps without contaminating the clean C implementation. Physical-device profiling also remains necessary.