ABYSS — a DOOM-flavoured shooter for the Atari 800XL

PLAY

This is the real game — byte-for-byte the same 29,609-byte executable that runs on the hardware, here running in an Atari 800XL emulator compiled to WebAssembly. PAL, with sound: an audio engine that for most of this game's life was verified only as POKEY register traces, never actually heard.

CLICK TO DESCENDboots a PAL 800XL · enables sound

←↑↓→ or WASD move SPACE/X fire · fire starts, and fires — that's the whole scheme

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FIRE

HOW TO PLAY

You are somewhere under the world with a shotgun and no map. Four floors lie below you, each with one way down, and the way down is the only way on. There is no strafe, no run key, no automap and no use key — doors open because you walked into them. Everything you need is here; the full manual has the rest.

THE CONTROLS

Walk forward / back	`↑` `↓` or `W` `S`
Turn	`←` `→` or `A` `D`
Fire	`SPACE` or `X`
On a real Atari	joystick, port 1

Fire is also the carry-on button — it starts the game, clears every banner and restarts you when you die. Whenever the game is waiting, it is waiting for fire, and it says so on screen.

THE SHOTGUN — GET CLOSE

Two cells or less	60 damage
Three to five cells	28 damage
Further than that	12 — a peppering

The same shell is five times deadlier at point-blank. A husk takes one blast in the face or five across a room; the hulk takes two, or ten. Backing away and plinking is the most expensive way to kill anything here. And every shot is loud: it wakes whatever is within three cells.

DOORS, KEYS AND THE WAY OUT

Doors open when you walk into them — there is a deliberate beat before you step through. Locked doors are coloured, and answer with a latch click rather than the sound of a door. That click means you need a key, not this is a wall — and the key is always somewhere on the floor you are already standing on.

The exit is a doorway that breathes. Nothing else in this world moves by itself, so it is unmistakable once it is in front of you. Walk onto it, the status line changes to FIRE TO DESCEND, and the trigger drops you a floor.

DYING, SUPPLIES AND PAR

Medkit	+25 health, max 100
Shells	+15, max 99
Par for the whole descent	11:00

Dying puts you back at the start of the floor you died on — doors shut, items back, empty hands, full health. The asymmetry that decides runs: health follows you down, ammunition does not. Every floor hands you a fresh 50 shells whatever you had left, so hoarding them is throwing them away — but nothing heals you between floors, and a medkit you walked past on the first floor is health you do not have on the fourth.

THE DESCENT

Four hand-authored floors, five monsters apiece, every one placed and typed by the level files. THE VESTIBULE is husks only — one point-blank blast each. Gunners, frailer but quicker on the trigger, start on the second floor. The single hulk waits at the bottom: two shells up close, ten from across a room, and a third taller than anything above it. A shotgun with range-banded damage, a muzzle flash that lights the room rather than replacing it, and a real exit on every floor, proven reachable by a test that walks there.

The Red Cistern — **THE RED CISTERN** — a columned hall in magenta

Silent Colonnade — **SILENT COLONNADE** — grey stone under a blue vault

The Maw — **THE MAW** — violet stone, burning red sky

A hulk on THE MAW — **THE HULK** — the one at the bottom, three cells away on THE MAW. Two shells at this range, ten from across the room; it stays where it dies.

Muzzle flash — **THE FLASH** — each band keeps its hue; the thing you shot at stays visible

HOW IT WORKS

An Atari 800XL has a 1.77 MHz 6502 and 64 KB of RAM. A full-screen repaint costs more than a frame. Every part of this is a small measured decision:

29 KBGAME, LEVELS, ART AND SOUND

5CYCLES A PIXEL — THE 6502 FLOOR

~10.2 fpsWORLD RENDER

50 HzINPUT, WHATEVER THE RENDERER IS DOING

72/72CHECKS, ALL ON PIXELS OR STATE

1NIGHT TO BUILD

DRAWING A COLUMN IN ONE JUMP

Columns are painted by JSR-ing into unrolled store chains at a computed offset. The entry point is the run length: no loop counter, no compare per pixel, no overdraw. Five cycles a pixel is the floor for a 6502, and this sits on it.

WHY 10 FPS FEELS FINE

Two clocks, not one. The world redraws at ~10.2 fps, but input, the weapon, the kick, the bob, audio and the HUD all run in the vertical-blank interrupt at 50 Hz and never wait. Pull the trigger and the gun fires on the next video frame. The parts you feel are never on the slow path.

A FULL-SCREEN FLASH THE MODE CANNOT DO

GTIA mode 9 ORs each pixel into the background colour's low nibble — which every reference says to keep at zero. Writing a single bit there instead lifts the whole picture: a full-screen muzzle flash, four register writes, in a mode with no business having one.

MEASURED, NOT CLAIMED

Every screenshot on this page is generated by a tool that asserts each frame against the game state it claims to show. The acceptance sweep path-finds to every exit, plays the whole game through, and soaks it for 60,000 frames. The DEVLOG records every wrong turn.

WHAT REAL HARDWARE TAUGHT US

Built overnight, 31 July 2026, by Claude and Tony Gillett — an experiment in how close an 8-bit machine can get to the feel of DOOM, with an emulator harness measuring every step. Then it met real hardware.

ATTEMPT 1 — IT LOADS, AND THE PICTURE IS GARBAGE

Three modules lived at $A000–$BFFF: the BASIC ROM's address space on a real XL. The emulator boots with BASIC off; the hardware boots with it on. 130 green test runs never stood a chance of seeing it. Fixed with an eight-byte stub that banks the ROM out before anything loads.

ATTEMPT 2 — THE TITLE IS PERFECT, AND THE PICTURE ROLLS

The display list asked ANTIC for 248 scanlines against a hard limit of 240 — playfield DMA running where the vertical sync should be. The emulator renders a fixed window and simply clipped it; a CRT rolls. Two of the four status rows were blank anyway. Now 232.

ATTEMPT 3 — IT BOOTS, LOCKS, AND PLAYS

1 August 2026: the fixed build boots, locks and plays on the real 800XL, off an SD card, on a CRT. Both fixes are now assertions in the test sweep, and the question they left behind is the project's best souvenir: not does it pass? but what can this harness not see, even in principle?

AND EVERYTHING BUILT SINCE

The bestiary, the knockback, the lava, the redrawn fireballs and masonry that finally recedes with distance — all confirmed on the same CRT, including a renderer change and a memory reshuffle, the riskiest edits of the lot.

INCLUDING THE RISKY PARTS

The mortar courses now follow the perspective of the wall rather than running parallel to the horizontal, so they converge toward the vanishing point down a corridor. And the hue seam follows the walls: mode 9 allows only one hue per scanline, but the seams no longer sit at fixed rows, so 99.5% of a near wall carries the wall's own colour instead of 56%. Both were measured before they were believed, and both are now confirmed on the CRT. Nothing on this page is emulator-only.