A from-scratch port of Atari Jaguar's Tempest 2000 (Llamasoft / Atari, 1994) to the Sega Mega Drive + Mega CD. Plays as a Mode 1 cartridge — code lives on a Mega Drive cart, but uses the Mega CD's Sub CPU, Word RAM, and RF5C164 PCM chip for audio and graphics work the standalone Mega Drive can't do alone.
No CD-ROM disc required. Just a Mega Drive + Mega CD attached.
▶ Watch the demo on YouTube — gameplay captured on real Mega Drive + Mega CD hardware.
Status: v0.2.1-beta (released 2026-06-01). Patch release on top of v0.2-beta with the real fix for the rave4 voice-sample echo on real hardware (the v0.2 attempt only handled half the failure mode). v0.2's headline content — enemy variants (fuse-tanker, pulsar-tanker, super-flipper) and a 10-entry Hall of Fame — is unchanged. Verified working on real Mega Drive + Mega CD hardware + ares; BlastEm has known Mode 1 quirks. See CHANGELOG for what shipped in each version and Known limitations below.
The Mega CD is best known for FMV games, but its real gift is hardware: a second 68000 CPU, 256 KB of shared "Word RAM", a dedicated PCM audio chip, and an ASIC graphics engine. Mode 1 lets you exploit all of that without the CD-ROM constraints — boot from a cartridge, but still call into the Mega CD silicon for anything you can offload.
For a port of Tempest 2000 — a 60 Hz vector-3D shooter with full MOD-tracker music — that turns out to be exactly the architecture you want.
┌─────────────────────────┐
┌────────────┐ │ MEGA DRIVE │
│ CART ROM │◄───────│ Main 68000 ─────► VDP│ (sprites, planes, web)
│ ~615 KB │ │ │ │
└────────────┘ └──────┼──────────────────┘
│ gate-array comm regs
▼
┌─────────────────────────┐
│ MEGA CD │
│ Sub 68000 ────► RF5C164│ (MOD playback + SFX)
│ │ │
│ ▼ │
│ PRG RAM (512 KB) │ MOD data lives here
│ │
│ Word RAM (256 KB) ◄───►│ shared with Main
│ │ │
│ ▼ │
│ ASIC stamp/map engine │ wave-transition zoom
└─────────────────────────┘
Main CPU (cart) runs the game loop:
- VDP sprite + tile rendering, plane priority, palette
- Player input, web shape geometry, entity pool, collision
- Wave system, scoring, lives, game state machine
- Drives the Sub CPU via gate-array communication registers
Sub CPU (Mega CD) is a dedicated audio + special-effects engine:
- Decodes a 4-channel MOD tracker file and streams samples to the RF5C164
- Plays one-shot PCM SFX (fire / hit / death) on a 5th channel
- Drives the ASIC stamp engine for the wave-transition zoom
Word RAM is the shared canvas:
- 16 pre-rasterised "lane variants" of the web (~12.8 KB each), one per player position around the rim. Per-vertex 1/Z perspective baked in.
- Per frame, the variant matching the player's current lane gets DMA'd to VRAM — gives true 3D web parallax at 60 Hz without per-frame software rasterising.
PRG RAM holds the active MOD file (one of rave4 / tune7 / tune5 / tune12 for gameplay, tune13 for the title screen — all five lifted from the Jag source's tune table).
- Animated title screen — drifting stars, flashing START prompt, "MEGA TEMPEST" banner rendered + pulsed by the Mega CD ASIC every frame
- All 5 core enemies from the Jaguar original: Flipper, Tanker (splits into 2 flippers), Pulsar (pulse-gated kill), Fuseball (erratic lane-hopper), Spiker + Spike obstacles
- Per-wave progression — 16 waves, each with its own enemy quota and shape, then loops forever with escalating difficulty
- "WAVE N — GET READY" splash between waves, freezing the action for ~1.5 s
- Bonus life every 4 waves (capped at 9), celebrated with a "1UP!" on the splash
- Superzapper power-up — 1 charge per wave; clears every live enemy with a screen-flash + per-kill spark + the Jag's own Crackle PCM
- Six power-ups — dropped by killed tankers: LASER (rapid fire), JUMP (immune to rim deaths), extra life, extra superzapper charge, wave-skip, and an AI droid sidekick that walks the rim and fires at the nearest enemy
- Enemy variants — green fuse-tankers split into 2 fuseballs, cyan pulsar-tankers split into 2 pulsars, white super-flippers (≈1.5× faster) — introduced across waves 6-9 to ramp difficulty
- Hall of Fame — 10-entry hi-score table modelled on the Jag's, displayed on the title screen via an 8-second attract-loop swap with the banner; after game-over the player enters 3-letter initials if they qualify (session-only — persistence to Backup RAM is a v0.3 item)
- Web shape rotation — 8 shapes from the Jag (V, square, plus, triangle, pentagon, star, W, fan) extracted directly from
yak.s - 3-life player loop → game over → back to title; START pauses
- Iconic fly-down-tube wave transition rendered by the Mega CD ASIC
- MOD music — separate title theme + 4 gameplay tracks (
rave4,tune7,tune5,tune12) cycled per wave, matching the Jaguar's ownwebtunes[]set. Music keeps playing continuously through scene transitions. - 4 PCM SFX — FIRE, HIT, DEATH, ZAP — all extracted from the Jaguar sample bank
This list reflects the current v0.2.1-beta release. The core gameplay loop is complete and stable; the following are known and intentional gaps:
- Loops at wave 16. The Jag original has 99 levels with bonus stages and a warp mechanic between them. The port currently cycles the 16 wave definitions endlessly with escalating difficulty. Bonus stages + the level-99 victory + Beastly NG+ mode are scoped but not implemented.
- Hi-score table is session-only. Backup-RAM persistence between power cycles is a v0.3 item — the Mode 1 cart's bootstrap doesn't initialise the BIOS state that the BURAM calls need, so saves currently hang. Table + qualifying detection + initials entry all work in-session.
- No options screen or level select.
- Real-hardware behaviour. Verified booting + playing end-to-end on real Mega Drive + Mega CD as of 2026-05-30. One known hardware-only audio issue: a faint echo on some
rave4.modsamples that does not reproduce in ares — suspected PCM-RAM allocation overlap with SFX, pending fix. - Emulator support. Developed against ares, which models the Mega CD ASIC and Word RAM accurately. BlastEm has known Mode 1 sub-CPU / Word-RAM issues — graphics will glitch or hang.
- Tested with PAL TVs / 50 Hz timing: only US/JP 60 Hz mode has been driven through the wave cycle. PAL should boot but level pacing is untested.
Bug reports and feedback are very welcome.
The Jag rasterises the web with per-vertex 1/Z perspective every frame on its GPU. We can't do that on the Mega Drive — there's no FPU and tile-rendering 12.8 KB per frame would blow the budget. So instead we pre-bake every camera angle: at level install, software-render 16 web variants (one per player lane, each with that lane's perspective shift) into Word RAM. Per frame, DMA the variant matching the player's lane to VRAM. Result: per-vertex parallax at 60 Hz with ~7 ms per-frame DMA cost.
Between waves the web shrinks to a vanishing point — the iconic T2K transition. We can't scale plane B on the VDP, so we revive the otherwise-dormant Mega CD ASIC stamp engine. At transition start, the current web shape is baked into 5×5 ASIC stamps. The Sub CPU then renders the web into Word RAM via a trace vector with growing scale; the result DMAs to VRAM. About 28 frames of pure ASIC zoom, then the variant pipeline takes back over for the new wave.
Plane A holds a sparse pseudo-random scatter of single-pixel stars across the full screen. The 4 star tiles are re-DMA'd each frame with the bright pixel shifted by 1, so all stars drift diagonally over a ~1 s cycle. Plane B (web) and sprites both run at priority=1; plane A stars at priority=0 — so the web naturally "floats" in space without painting around the V's exact silhouette.
Every game object — flipper, tanker, pulsar, fuseball, spiker, debris particle, superzapper spark — is one entry in a fixed 32-slot pool. Each new entity type overloads existing struct fields with per-type semantics rather than widening the struct. This is a hard-won rule: an earlier session lost ~10 hours to a struct-widening bug that turned out to be a memory-corruption-class issue we couldn't isolate, and never reproduced after reverting.
The megadev linker script anchors .data at 0xFF0000, which makes objcopy pad the output binary to that offset. We trim back to _rom_end (read from the sym file) after the build — cart binary down from a default 16 MB to actually-used ~615 KB (most of which is the 5 MOD files).
The web uses an 8-step pure-blue gradient across CRAM slots 5-8 + 12-15 (every valid xBGR blue intensity from 0x2 to 0xE), with the outline at 0x0E80 to match the Jag's electric-blue rim. The player claw still uses yellow on slot 11.
On-screen text uses a hand-designed 6×7 chunky-italic pixel font. Glyphs live inline in tools/extract_font.py, which emits the 4bpp tile blob DMA'd to VRAM at boot.
./build.sh # outputs ../mega-tempest-<version>.binbuild.sh is self-bootstrapping — run it from a fresh checkout on any
machine. On the first run it will:
- check for the three tools it can't install for you:
git,docker,python3; - fetch the Tempest 2000 game assets and convert them (see Assets below);
- clone the megadev framework next to this repo;
- build the megadev m68k toolchain Docker image (one-time, ~1–2 min);
- build the Sub CPU module (
sub/spx.smd), then the cart, and copy the result to../mega-tempest-<version>.bin.
Network access is needed on the first run (the clones + the Docker image
build). Subsequent runs reuse everything and just rebuild. The version is
pinned at the top of build.sh — bump it on each tagged release.
The music (*.mod), font, sprite shapes, and sound effects are fetched from
the mwenge/tempest2k source tree and
converted at build time by fetch_assets.sh (which calls the
tools/extract_*.py scripts). Both upstreams are pinned to specific commits
so the build is reproducible. To regenerate the assets without building:
./fetch_assets.shGrab mega-tempest-v0.2.1-beta.bin from the releases page (or build from source) and load it in an emulator that supports Mega CD Mode 1 cart booting. ares is recommended — accurate Sub CPU + Word RAM + ASIC. BlastEm has known quirks with Mode 1 Sub-CPU WR ownership.
ares mega-tempest-v0.2.1-beta.bin
On real hardware: flash to a flashcart that supports Mode 1, attach to a Mega CD, boot.
| Button | Title | Playfield | Game over |
|---|---|---|---|
| D-pad ◀ ▶ | — | walk lanes | — |
| A | — | fire | — |
| B | — | superzapper (1 charge per wave) | — |
| START | begin play | pause / resume | back to title |
Killed tankers drop a glyph that drifts to the rim — grab it by being on the same lane:
| Glyph | Effect |
|---|---|
| L | LASER — rapid fire for 5 s |
| J | JUMP — claw lifts above the lane, immune to rim deaths for 1 s |
| 1 | extra life (+1, capped at 9) |
| Z | extra superzapper charge (+1, capped at 5) |
| S | force the wave-end transition (instant skip to next wave) |
| D | AI droid sidekick — walks the rim and fires at the nearest enemy for ~20 s |
- Original game: Tempest 2000 — Llamasoft / Atari, Jaguar (1994), by Jeff Minter and team.
- Reverse-engineered Jag source: mwenge/tempest2k — invaluable reference for enemy AI, wave structure, polygon data, and palette decisions.
- Mega CD development framework: megadev — provided the Mode 1 boot setup, Sub CPU build pipeline, and VDP helpers.
- MOD player on RF5C164: based on the Chilly Willy lineage via matteusbeus/ModPlayer.