Cosmic Ark (1982)#

Cosmic Ark — Rob Fulop, Imagic — casts you as a sort of Noah in orbit: a great ark sweeping the galaxy to rescue pairs of animals from dying worlds. It plays in two alternating acts — fend off a meteor storm from the center of the screen, then drop to a planet and lift creatures aboard with a tractor beam. It’s a fine game. But it is remembered for four instructions.

Cosmic Ark opens on the most famous starfield on the platform — crisp, even, drifting stars, far more points of light than the TIA has objects to draw. The book’s Hardware Quirks chapter tells the story of why it works (an HMOVE glitch Fulop found by accident and never fully explained, living in the analog seams of the chip). Here we get to do the thing that page can’t: read the actual code that pulls it off.

The shape of the program#

An ordinary frame, with one extraordinary pass — the stars get smeared on before the foreground:

The starfield, in four instructions#

The whole galaxy is one missile. It’s enabled once, as an ordinary object:

       LDA    #$02
       STA    ENAM0      ; turn missile 0 on — this is "the stars"

A missile can be drawn in exactly one place per scanline. To get a whole row of stars out of it, Cosmic Ark abuses horizontal positioning. Normally you set a motion value in HMM0 and then strobe HMOVE, and the object slides over once during the next blanking interval. Fulop does it backwards and badly on purpose — strobe HMOVE first, then write HMM0 while the TIA is still in the middle of applying the motion, mid-way through the HMOVE comb the strobe sets off:

       STA    WSYNC
       STA    HMOVE      ; (1) start the horizontal-move "comb"
       JSR    L1BDC      ; (2) wait an exact number of cycles…
       STA    HMM0       ; (3) …then poke the motion register MID-COMB

That middle step is the trick, and it’s the reason there’s a subroutine call in the hottest part of the kernel. L1BDC isn’t doing arithmetic — it’s a cycle-precise delay that also hands back the motion value in one move:

L1BDC: NOP              ; burn 2
       NOP              ; burn 2  — land the next STA exactly inside the comb
       LDA    #$60      ; the motion value to drop into HMM0
       RTS

HMOVE works by sending a burst of “move” pulses to each object during the extended blank after the strobe. Change HMM0 while that burst is still being delivered and the missile’s position logic comes unglued — instead of moving once, it gets retriggered again and again across the line, painting a string of evenly-spaced dots. Do it every scanline, nudge the value frame to frame, and the dots become a smoothly drifting starfield. (The deep electrical why — two clocks beating against each other, why it differs between consoles — is the quirk page’s story; the four instructions above are how.)

It’s the mirror image of Space Invaders. There, the TIA’s repeat feature (NUSIZ copies) was used exactly as documented to clone an object on purpose. Here, the undocumented repeat — a positioning glitch the manual never mentions — is used to clone an object the chip never agreed to clone. Same instinct (one object, many copies), opposite side of the spec.

Two phases sharing one ark#

Behind the stars, a bit flag ($B8) splits the game into its two acts, and the same kernel and objects serve both:

• Defense. The Ark holds the center of the screen while meteors streak in; you swivel and fire to clear them. The starfield scrolls behind the whole fight.
• Rescue. The Ark descends and tracks across a planet; you drop a tractor beam to scoop up pairs of animals and haul them back up, dodging the hazards that rise to meet you.

The Ark itself is a player sprite, the laser and tractor beam lean on the ball and the other missile, and the planet surface and HUD come from the playfield — leaving missile 0 free to spend its entire life being the heavens. That division of labor is the quiet design decision that makes the loud trick possible: the stars get a dedicated object precisely because everything else is covered.

Why read it#

Read Cosmic Ark for the rare chance to put a fingertip on the single most famous “bug as feature” in the VCS canon and find it’s only four instructions — a strobe, a counted delay, and a mistimed register write. The Hardware Quirks page explains why it shimmers; this one shows you the few bytes that do it. It’s the best illustration in the book of a truth the 2600 keeps teaching: on hardware this bare, the line between defect and effect is wherever a clever programmer decides to draw it.

Read the source. This teardown is drawn from a DiStella disassembly of the Cosmic Ark cartridge (Rob Fulop’s game, © 1982 Imagic). For the history and the hardware explanation of the starfield, see Cosmic Ark’s Star Field and the AtariAge deep-dive.