Chasm
An original cavern flyer written to the Atari 2600's actual limits: a 160×192 field, the TIA's 128 colours, two sprites to a scanline, and the flicker you get when you want a third.
←→ steer · ↑↓
throttle · Space fire
Enter reset · Tab select
How to play
You are flying down a chasm that keeps getting narrower. Steer clear of the walls, shoot or dodge the mines, and fly through the fuel cells before the tank runs dry. Score is depth in metres, plus 25 a mine. Three ships.
Game variations
Press Game Select to cycle, then Game Reset to start — the way a cartridge with eight games on it worked.
| # | Game | What changes |
|---|
The two difficulty switches work as they did on the console: one per player, set before you start. A narrows the chasm and speeds the mines up; B is the novice setting.
What is actually being emulated
- The palette. All 128 TIA colours, generated the way the chip made them — a luminance and a colourburst phase, sixteen hues by eight lumas, taken round through YIQ. Nothing on screen is off-palette.
- The playfield. Forty blocks across, each four pixels wide, which is why the walls are so chunky and step so coarsely.
- Two sprites to a scanline. The TIA had two players, two missiles and a ball, and that was the lot. Objects that do not overlap vertically share one sprite as the beam moves down the screen; ones that do overlap have to take turns between frames. That is the flicker, and it is doing real work here rather than being an effect — turn it off above and the game quietly breaks the hardware's rules.
- The sound. Two channels, a five-bit frequency divider and the chip's actual 4-, 5- and 9-bit polynomial counters, which is why the noise has that particular grain and the tones land where they land instead of in tune.
- The switches. Six of them, including TV Type, which really did just drop the colour and leave you the luminance.
- Not the clock. The time above the title and under the gauges is the one thing on screen the hardware could not have done — the console had no clock chip and no battery, so it never knew what time it was. It is drawn in the same 4×5 font as everything else, twelve hours with the tens digit left blank below ten, and the colon carries the seconds.
The game itself is original. The console, its logos and its catalogue belong to their owners; what is borrowed here is the shape of the hardware, which is the interesting part.