Alien (BUGFIX 1.7): Highlights

This page is for readers who want to see the interesting parts of the game without wading through 20,000 lines of Z80. Every entry below links into the full disassembly; each block has a plain-English header comment, so you can get the idea from the prose even if you skip the instructions.

Gallery — the graphics, straight off the tape

Every image below is rendered by SkoolKit directly from the bytes of the tape image — nothing has been redrawn. The captions link to the data (and the routines that draw it) in the disassembly.

The crew

Seven 24×24-pixel portraits (CrewPortraits), shown in the bottom-left corner of the room view by DrawCrewPortrait and on the crew-roster screen. Shown here at 4× scale, blue-on-white as in the game.

The alien

The 48×63-pixel body that looms over the encounter screen — a 4-frame animation blitted by BlitAlienFrame.

Jones the cat

The 5-frame run cycle that sends Jones sprinting across the encounter screen, XOR-erased and redrawn 4 pixels at a time by BlitJonesFrame.

The map markers

The two animated markers on the corridor map: the walking crew figure (AnimateCrewA, a two-frame walk cycle) and the blinking room cursor (AnimateCrewB, four shrinking boxes). All are 24×12-pixel XOR-blit sprites.

The Nostromo — three decks

The deck backgrounds of the room view, stored as plain 20×19 tile grids at ShipMapData.

The air ducts

The duct overlay drawn as the view background when the viewed position is inside the crawlways — one map per deck, at DuctMapUpper, DuctMapMiddle and DuctMapLower.

The Narcissus

The escape shuttle's cabin, stored run-length-encoded at NarcissusLayout and expanded at draw time by NarcissusBackground, with each cell coloured by its NarcissusAttrs byte.

The title screen

The ALIEN egg picture composed by DrawIntroScreen from the tile map at IntroTileMap over TileBitmaps, with the lower rows coloured by IntroAttributes — the green is the egg's glow.

The tilesets

The complete CharBitmaps tileset — font, wall pieces, doors, arrows and sprites, 8 bytes per tile. Every room, corridor and menu in the game is built from these:

And the separate title-screen tile sheet at TileBitmaps:

How the game draws things

There is no general-purpose graphics library — just a handful of tricks.

CharBitmaps — the game's entire font and tile set, 8 bytes per glyph. Squint at the 1s and 0s of each DEFB row and you can see the pixels.
RoomNameTable — the cleverest data structure on the tape: a table of 10-character strings ("Bridge", "Elctrc Prd"...) that the renderer also treats as sprite strips — the ASCII text is the drawing command list.
DrawSprite and DrawSpriteRow — the two blitters that put everything on screen, one 8×8 tile column at a time.
DrawIntroScreen — the animated ALIEN title screen.
ScreenTransition — the top-to-bottom wipe to black between screens.
BlitAlienFrame and BlitJonesFrame — the alien-encounter screen: a 4-frame alien body animation, and Jones the cat sprinting across the screen with a 5-frame run cycle, erased and redrawn with XOR.
NarcissusBackground — the shuttle screen is stored run-length-encoded and expanded at draw time.
DrawDoorArm — draws a corridor "arm" by literally walking along the wall data, steered by corner glyphs embedded in the map.
ShipMapData — the three deck maps of the Nostromo, stored as plain tile grids you can read by eye.

The alien's brain (and everyone else's)

The game has no random number generator. All "randomness" comes from walking the ZX Spectrum's ROM and using the bytes found there as a script.

ResetScriptPtr — where that trick starts: the script pointer is seeded from the ROM's 50 Hz frame counter, so behaviour is time-dependent but fully deterministic.
UpdateAlienAI — the alien's decision loop: attack room-mates, rip out a duct grille, slip into the ducts, or wander.
PickNextRoom — how any actor picks its next room from the ship's corridor and air-duct graphs.
TriggerAlienEvent — the film's Ash twist, in code: once the alien has taken enough wounds, a hidden crew member turns hostile.
UpdateAndroid — what the activated Android actually does each frame.
CrewAction5_AlienAttack — the alien's attack tick; it can even drag a corpse off into the ducts.
UpdateJones — Jones the cat's wander logic. He avoids the room the alien is in, and there's a message when the crew spots him.

Gameplay worth reading

GameEntry — the entry point; the main-loop structure is documented step by step from here.
HandleInput — one cursor, one action key: how a whole strategy game is driven with five keys (or a joystick, via self-modifying code that patches the scanner CALL).
CrewHitsAlien — the weapon table in code form: nets disable, the harpoon gun kills (and takes bystanders with it), extinguishers only scare, trackers shatter when used as clubs.
UpdateRoomActors — the ship simulation: self-destruct countdown, oxygen clock, motion trackers, engine fires.
LaunchGate — the escape-shuttle launch check (all living crew + Jones aboard). Also home to the best bug on the tape — see the Bugs page.
EndgameScreen — how your final "competence rating" is scored.
PlayMusic — beeper music: one channel, hand-timed loops.

Data tables you can read like a document

RoomAdjCorridors and RoomAdjDucts — the ship itself: which rooms connect by door and by air duct. The whole map is ~350 bytes.
ActorRecords — 8 bytes per character: room, destination, strength, courage, morale, status. The whole cast's state fits in 64 bytes.
ItemLocations — one byte per portable item says where it is, including which hand of which crew member holds it.
MessageTextTable — every status message in the game, with the trigger conditions documented alongside.
CrewNameTable — ALIEN, Dallas, Kane, Ripley, Ash, Lambert, Parker, Brett.


Frame 0	Frame 1	Frame 2	Frame 3


Frame 0	Frame 1	Frame 2	Frame 3	Frame 4


Walk 1	Walk 2	Cursor 0	Cursor 1	Cursor 2	Cursor 3


Dallas	Kane	Ripley	Ash	Lambert	Parker	Brett