Choplifter

01 · The game

The game

The player flies a rescue helicopter out of a base on the right of a long, scrolling landscape. Sixty-four hostages are held in four barracks to the left, behind a fence. The helicopter shoots a barracks open, lands beside the hostages who run out, and carries up to sixteen at a time back to the base, where they climb out and walk to the post office.

Tanks fire shells that kill hostages and a helicopter on the ground, jets fire missiles at it in the air, and at the higher levels drones home in on it. The player has three helicopters, called sorties. The game ends when all 64 hostages are rescued or killed, or when the third helicopter is lost.

The title screen: Broderbund Software presents Choplifter, by Dan Gorlin, copyright 1982

The helicopter over the battlefield, hostages running towards it

The title screen, and hostages running to the helicopter on the first sortie. Apple2TS 3.7.0 as an Apple II Plus, from the original disk (reference/).

02 · One frame, rebuilt from memory

The demonstration flight, run from the listing

With nothing pressed after the title, the game flies a demonstration sortie. Below, the game's own code from the Source tab runs that sortie in the browser, and each frame is drawn from the two screen pages in memory.

Show what each frame redrew loading the listing

The game's code runs from start ($0800) on the kit's 6502 over the listing's bytes, and stops at each call of gfx_show_page ($12E3); the page it is about to show is drawn by the site's Apple II library (reference/demo.js). The run starts from both screen pages as the disk loads them, the loading screen, because the listing leaves the screen out and the attract mode never redraws the counters' bar or the ground band. At frames 1, 2, 40, 200, 400 and 800, both pages hold the same 16,384 bytes as in Apple2TS at the same call, run from the loader's hand-over (test_demo.js). The highlight marks every byte that differs from the last time the same page was shown, two frames earlier.

The game never clears the screen. It draws on one page while the other is shown, and every shape it draws leaves a box in that page's erase list (add_erase, $7BF8). Two frames later, when the same page comes round again, erase_old ($9B4C) fills each recorded box with black above the ground line and with the ground's colour below it, and the frame is drawn again on top. The highlight shows that work: the shapes that move and the stars that twinkle are put back each frame, and the rest of the screen is left as it was.

The planet is not a shape. draw_planet ($9411) is 66 pairs of LDA # and STA a page that store its bytes straight into screen memory, so the picture lives in the code. The fourteen star bytes are stored at fixed places (store_stars1, $972D) and remade every frame from two random numbers, which makes the sky twinkle.

03 · Sound

Sound: every effect is the speaker clicked by a delay loop

The Apple II's speaker makes a click each time the program touches $C030, and a tone is clicks at a steady spacing. Choplifter makes all of its sounds that way, from three routines, and there is no engine sound and no music.

Loading the listing…

Each button runs the code that makes that effect in the game, from the listing, on the kit's 6502, and plays the clicks it makes through the site's model of the speaker (site/lib/apple2-speaker.js); the drawing shows each note's pitch and, along the foot, every click. The same code run in Apple2TS clicks at the same spacings, click for click, for all nine effects (test_sound.js). The noise of a tank or a jet firing takes its delays from the bytes of the jet's own code at $8101 onwards (Discoveries).

tone ($7370) clicks the speaker Y times, waiting X turns of a delay loop between clicks and adding A to X after each. A turn is five cycles, so a tone of period X has its clicks 5X + 30 cycles apart and sounds at 1,020,484 / (2 × (5X + 30)) Hz. noise ($7373) clicks in pairs, and click ($736D) clicks once. Every click is followed by a read of $C020, the cassette output, so a speaker plugged into the cassette socket plays the same sound.

Effect	Code	What it plays
A new game	`$0B7C`	three phrases of three notes: periods 144, 67, 97 (680, 1,398 and 991 Hz), then 112, 67, 97, then 80, 67, 97
A shot	`$0E36`	a falling tone, period 48, one longer after each of 32 clicks
A hostage climbs in	`$61E1`	three notes, periods 255, 224 and 152, once a frame at most
A hostage killed	`$79F0`	three notes, periods 48, 67 and 97, once a frame at most
A hostage home	`$7A30`	one note, period 40
A helicopter lost with hostages aboard	`$7A6B`	four falling notes, periods 64, 84, 97 and 131
A tank fires, a jet fires	`$686B`, `$8BEC`	noise
A drone drops a shell	`$8DDB`	a steady tone, period 48

Every sound routine first checks that a game is on and the sound is on (CTRL-S switches it), so the title and the demonstration are silent.

04 · The helicopter

A tilted helicopter rocks on its skids

The stick leans the helicopter, a step a frame, and the lean sets the thrust: 0, 96, 180, 264, 348 or 420 256ths of a dot a frame (thrust_table, $7418). Each frame a sixteenth of the speed is taken off as drag, so the top speed is 21 dots a frame. Up and down, the speed closes by half the difference a frame on the stick's climb rate, one of sixteen from -10 to +6 dots a frame (climb_table, $7424).

A helicopter that comes down with a lean lands tilted, and the frames after rock it back to level on its skids (rock, $7803). Each tilt has a picture of its own, and the tilted ones are taller: 13 rows level, up to 18 with the nose right down. So each step of the rocking raises or lowers the helicopter by a table of heights (land_height_table, $7895) and slides it along by another (rock_x_table, $787F). Together they keep the end of one skid on the ground while the helicopter pivots: at every tilt, facing either way, its lowest dots are on the same screen line or the one above.

Tilt Show it level loading the listing

Every picture here is drawn by the game's own routines from the listing, on the kit's 6502 (reference/tilt.js). The demonstration flight runs to its first frame, with the helicopter level on its pad. Then rock tips it to the tilt, and the helicopter's object (heli_object, $8276) draws its body and both rotors on that frame's screen. The amber helicopter behind is the same one level. Eighteen frames of helicopters landing with a lean in Apple2TS, facing either way, hold the same bytes as these routines draw from the same state (test_tilt.js); there, a helicopter that landed at a tilt of 4 was level two frames later. Only a level helicopter opens its door (door_open, $0D), so a tilted landing neither picks up nor crushes anyone.

On the ground and level, the door opens and the hostages come. One that reaches 8 or 9 dots right of the helicopter's X, or 10 or 11 dots left, climbs in, up to sixteen aboard (boarding, $6139). One that is closer, from 7 dots right to 9 dots left, is under the helicopter and is crushed (crush, $62D7). Coming down at more than about 4.4 dots a frame bounces the helicopter, and touching the ground at full speed crashes it.

05 · Enemies

The level goes up with each trip home

The level starts each game at 0 and goes up by one, to 3 at most, the first time a hostage is set down at the base after a pickup (trip_flag, $731E). Eight frames in every 256 an enemy may come (spawn, $6884): at level 0 only tanks; at level 1 a jet half the time, otherwise a tank; at levels 2 and 3 a drone one time in four, otherwise a jet half the time, otherwise a tank. Each kind comes only while fewer than its most are in play.

The most of each kind in play, read from max_tanks ($68D9), max_jets ($68DD) and max_drones ($68E1) in the listing, and the share of jets that are bold from bold_masks ($6536). In Apple2TS the most of each kind seen at each level matched the table (facts.md, "Live tests").

Tanks drive towards the helicopter until they are 128 dots away, then decide every 8 to 23 frames: fire if the helicopter is inside the arc of the gun's aim, and stop, come on or back off (tank_impl, $65C3). The gun turns a step a frame towards the helicopter, through five aims. Only a blast in front of the ground's depth destroys a tank, and only a shot fired while the helicopter faces the viewer or is turning comes forward as it falls, so a side-on shot never reaches one.

Jets follow scripts of four-byte steps, each a picture and the steps in X, height and depth: a cruise, a turn towards the viewer, a pass coming forward, and the attack run, with missiles at a helicopter in the air and a bomb at one on the ground (jet_scripts, $8262). Drones turn their speed a dot a frame towards the helicopter and close on its height; one that touches it destroys it, and at level 3 they drop shells (drone, $8C42).

06 · Controls

Controls

The joystick is two paddles read through the Monitor ROM's PREAD. Up and down is sixteen steps of climb rate (read_stick_y, $7663); left and right is eleven steps of lean, -5 to +5, with a dead band at each end (read_stick_x, $769C). Button 0 fires, one shot a press and at most five in flight (fire, $0E0F). Button 1 turns: a short press brings a side-on helicopter round to face the viewer, and a press held until the turn has passed the front carries it on to face the other way (turn_button, $0E3B).

Key	What it does
ESC	pause, until a second ESC
CTRL-S	sound off and on
CTRL-V	up and down reversed
CTRL-A	left and right reversed
CTRL-L, then a key	the level set to the key's low two bits (Discoveries)
any other key	a new game, from the title or the demonstration

The keys are read by read_keys ($0D92).

07 · The Commodore 64 version

The C64 version is the same program, rewritten

The Commodore 64 cartridge keeps this game's design down to its numbers: the same object types, the jets' four-byte script steps, the arguments written inline after a call, the crushing and boarding distances, the random gap between hostages, and the slip that loses the tanks meant for the helicopter's right. It changes what the machine changes, and a few of the numbers.

	Apple II (1982 disk)	Commodore 64 (cartridge)
The screen	hi-res, two pages flipped, 280 dots across	multicolour bitmap, two screens flipped, no sprites
The loop	no interrupts; about 12 frames a second in play	a raster interrupt for timers; about 21 passes a second
Sound	the speaker: nine effects, no engine sound	the SID: two noise voices for the engine, a third for effects
Fire and turn	button 0 fires, button 1 turns	one button: a tap fires, a hold turns
Barracks, base	$0380-$0680, base $1274	$037D-$067D, base $1254
Crushed, boarding	-9 to +7; +8, +9, -10, -11	-9 to +7; +8, +9, -10, -11
Hostage gap at the start	an even 32 to 158 dots	an even 32 to 158
Most tanks, levels 0-3	1, 2, 2, 2	1, 2, 1, 1
Most jets, levels 0-3	0, 1, 2, 2	0, 1, 2, 2
Steps in a jet's attack run	24	10
The homing enemy	drones, up to two at level 3	one mine at a time
The level goes up	at the first unloading after a pickup	at the first unloading after a pickup
Tanks meant for the right	put at the world's far left; dropped unless the helicopter is in its first 128 dots	put at the world's far left and deleted

The Apple II column is this game's facts.md; the C64 column is the C64 page's facts.md. Both games measure the world in their own units, so positions agree only roughly.