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I Built an Atari 2600 Game From Scratch in 2026 — Here's What the TIA Taught Me

#atari #retrogaming #gamedev #6502

Last week I set out to build the simplest possible Atari 2600 game: a title screen, a blinking dot you can move with the joystick, and walls to keep it on screen. It took a full day of debugging. The Atari 2600's Television Interface Adapter (TIA) is unforgiving, and every bug taught me something about how this 1977 hardware actually works.

The result is FIREFLY — a minimal, clean 4K ROM that's now a public starting point for anyone who wants to build Atari 2600 homebrew. Source and ROM are on GitHub.

FIREFLY gameplay — yellow walls, blinking firefly

The goal

After abandoning a more ambitious port, I wanted the smallest possible proof of concept:

  1. Boot to a title screen with FIREFLY text and a blinking dot
  2. Press fire → game screen with a movable blinking firefly
  3. Yellow wall border the firefly can't leave

That's it. No score, no enemies, no sound. Just a working kernel.

Bug #1: The invisible sprite

The title screen worked immediately — block text via the playfield, blinking dot via the ball sprite. But pressing fire gave a solid blue screen (my diagnostic color) with no sprite at all.

I tried the player sprite (GRP0), then the ball (ENABL). Neither appeared. The kernel was clearly running (background color changed), but sprites refused to draw.

The breakthrough came from testing in an emulator with screenshots: drawing the ball on all 192 scanlines produced nothing, but drawing it for 16 lines (like the title screen's dot) worked perfectly.

I never fully root-caused why 192 consecutive ENABL writes fail while 16 work — but the lesson stuck: on the TIA, bound your sprite drawing to the smallest possible vertical window. Don't spray register writes across the whole frame.

Bug #2: The 256-line blank

With the firefly visible and movable, I added walls. Moving to the top or bottom of the screen made the bottom border disappear and the side walls stretch downward.

The culprit was my BlankLines helper:

BlankLines:
    sta WSYNC
    dex
    bne BlankLines
    rts
Enter fullscreen mode Exit fullscreen mode

When the firefly is at Y=0, I call this with X=0. dex turns 0 into $FF, bne loops 256 times instead of zero. The frame ballooned, pushing the bottom wall off-screen.

The fix:

BlankLines:
    cpx #0
    beq BLDone
BLLoop:
    sta WSYNC
    dex
    bne BLLoop
BLDone:
    rts
Enter fullscreen mode Exit fullscreen mode

Always guard zero-count loops on the 6502. dex/bne with X=0 is a classic trap.

Bug #3: Phantom dashes from TIA state

At one point the game screen showed three black dashes on the left edge — with no sprite code running at all. A bare-minimum blue-screen ROM rendered perfectly clean, so the dashes came from my code.

The title screen sets CTRLPF, positions the ball with HMBL, and executes HMOVE. That state leaked into the game frame. Adding HMCLR (clear horizontal motion) and resetting CTRLPF in the game VBLANK fixed it.

Lesson: the TIA has no per-frame reset. Every register you touch in one screen persists into the next. When switching game states, clear motion registers explicitly.

Bug #4: The asymmetric wall

Moving left, the firefly stopped flush against the wall. Moving right, it stopped 4px early. The playfield is 40 blocks wide; my right-edge clamp was just wrong. Bumping the max X from 144 to 150 closed the gap.

Not a deep lesson — just arithmetic. But it reminded me that the Atari's coordinate systems (player pixels vs. playfield blocks vs. color clocks) never quite line up, and you verify by looking, not by math.

What finally worked

The game kernel:

  • 192 scanlines: 8 top wall (full-width PF) + 176 middle (side walls only, ball at FireY) + 8 bottom wall
  • Ball sprite: 8px wide (CTRLPF=$30), positioned with the classic divide-by-15 routine + HMOVE
  • Blink: toggle ENABL based on frame counter — walls stay solid because they use the playfield, not the ball
  • Clamping: guard inc/dec against wraparound before moving, not after

Total: ~400 lines of 6502 assembly, 4K ROM.

Try it

If you've ever wanted to write Atari 2600 homebrew, this is a clean starting point — a working kernel with title screen, sprite, input, and collision-free walls, minus all the game logic you'll add yourself.

The TIA doesn't forgive sloppy timing. But once you respect its 262 scanlines, it does exactly what you tell it. Every time.

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