Z-Machine Interpreter

Things to notice

• The interpreter doesn't understand a word you type. It turns your line to lower case, splits it at spaces and at the few punctuation marks the game lists as separators, and looks each piece up in the game's dictionary. The game gets back a list: where each word sits in its dictionary, how long it is, and where it was in your line. Working out what put the crank in the socket means is the game's own code.
• Only the start of a long word counts. A Version 3 game keeps six Z-characters of each dictionary word, and Version 4 and later keep nine. So in After Hours, examine odometry examines the odometer, since both pack to odomet.
• In Version 3, the status line is the interpreter's job. The game only keeps the current room in its first global variable, and the score and moves in the next two. The interpreter draws the bar from those just before it reads a line, or when the game asks with show_status. From Version 4 on, games draw their own status line in an upper window.
• The map knows where you are from the status line. In Version 3 it reads the same global the status line does. Later games draw their own status line, so the map takes the first object a game names there, which is the room in Inform and ZIL games. When the room changes after a command, the map draws that command as the way between the two rooms. When a direction leaves you where you were and the game describes the room again, as in Zork's maze, the map draws a small loop on that side of the room. A way you can't take says so without naming the room, so it draws nothing. Dying in the Zorks moves you too, to the forest or the gates of Hell, so when the game says “You have died” the map draws no way at all. A game that writes its room name some other way gets no map. Things go on the map once the game names them, or prints one of their own descriptions, while they're in your room, so whatever is still hidden stays off it.
• Enhanced reads what the Zorks keep to themselves. The games know whether a room has light of its own, how long your lamp has left, how much more you can carry, and how long your wounds take to heal, and only hint at them: “It is pitch black.” once your light is gone, “The lamp appears a bit dimmer.”, “Your load is too heavy.”, or the numbers diagnose gives in Zork I. The line under the status line reads them from the game's memory each turn. A room with light of its own carries a flag, ONBIT, and the game checks it before looking for anything lit you carry, so the line can tell you a room is dark while your lamp is on, or lit well enough to turn the lamp off and save it. Lights come from the clock, a table holding every timer: whether it's running, how many turns until it goes off, and which routine it calls then. The lamp's routine runs at each stage of a second table (in Zork I, 200 turns, then 100, 70, and 15), so the turns left are the turns to the next stage plus every stage after it, and turning the lamp off stops the clock. Load adds up the size of everything you carry, the way the game does before it lets you take something, and the limit drops while you're hurt. The page finds all this by the names ZILF lists when it builds each game, so Enhanced comes only with the built-in Zorks. Any command the game understands takes a turn, even walking into a wall, and wait takes three. Only a few, like save, score, and verbose, are free.
• A cheat answers for one of the game's own routines. Each time the game calls a routine, the interpreter checks its address, and when a cheat that's on covers that routine, it hands back an answer without running any of it. Zork asks a routine called LIT? whether a room has light, and Well lit always answers yes, the same answer the game gives in Zork I once you've died and wander as a spirit. The game's clock calls I-THIEF every turn to move the thief and let him rob, and Thief stays home answers for it, so he never wanders. While you're elsewhere, the page does what the game does each time he gets home: he puts down the treasures he's carrying, opening the jeweled egg if he has it. Walk into his Treasure Room and the game's own code takes over: he's there on guard, his treasures vanish, and you have to beat him to see them again. With the cheat on he never runs from a fight. Stable objects answers for the three routines that take things from a room, and leaves alone the one that robs you. The lamp's routine runs at the end of each stage of its life to dim it, and at the last stage to put it out, so Endless lights answers by setting the same stage going again. No wizard is choosier: the Wizard's routine also ends a spell that has worn off, and brings him for the demon near the end of Zork II, so the cheat lets it run for those and only stops his visits. Turn a cheat off and those routines run again. Only the thief opens the jeweled egg without breaking it. Give it to him in his Treasure Room, and he stops to admire it long enough for you to leave; when you come back, it's open, with the canary whole, and his to defend.
• One command is thousands of instructions. Typing take leaflet at the start of Zork I runs about 1,650 of them, through 37 routines: the parser matching your words, the checks on whether you can reach the leaflet, the take itself, and then each timer and creature on the game's clock, the thief among them. In the Inspector, This turn lists the routines in the order they were first called, and what the command moved, switched, and changed.
• x means examine, even where the game never heard of it. The Zorks want examine lamp or look at lamp, and many players now type x lamp without thinking. When a game's dictionary has no x, the interpreter passes examine in its place at the start of each command. The game itself is unchanged.
• A game can keep time while you type. From Version 4, a game asking for a line or a key can also name a routine and a time, in tenths of a second. Each time that runs out, the interpreter calls the routine while your half-typed line waits. The routine may print something, and you carry on typing below it, or it may end your line where it stands. A Version 5 game can then hand back what you'd typed, and this page puts it back in the input for you to finish.
• Each screen shows colours its own way. From Version 5 a game can ask for eight named colours, or any of 32,768 more. The desk shows them as the Standard lists them, and the Atari 800 uses its own palette, with its own blue. The green screen shows the same colours with a touch of its green mixed in. The printout prints letters in coloured inks and leaves backgrounds as paper, since a ribbon can't print white. A game that clears the screen in its colours gets the whole screen in them, the way Photopia changes colour from scene to scene.
• A save is small. Everything a game can change sits in its dynamic memory, so a save needs only that, the stack, and where the game was in its code. This page stores only the bytes that differ from the story file as it was loaded, and says how many bytes each save took.
• A save file works in other interpreters. Download in Saves writes a Quetzal file, the save format Frotz and many other interpreters read and write. It says which story it's for by release, serial number, and checksum, then holds dynamic memory packed the same way this page keeps it, and the call stack, oldest call first. The page adds the save's name, your notes, and your map in chunks of its own, and other interpreters skip chunks they don't know. Open the file on another computer, phone, or tablet, and the save joins the list there. Restore it, and you choose whether its notes and map replace the ones there. A save for one of the built-in stories opens that story first.
• Restart doesn't reload the file. The game can't change static or high memory, so restarting copies the story file's dynamic memory back over the changed copy and starts at the first instruction again. Only the transcript and fixed-pitch bits of the header survive.