A community-driven hardware/software project to build a Genesis cartridge-style addon that transforms your Genesis (Model 1/2/3/Nomad) into a working Macintosh 128K/512K/Plus clone — while still allowing standard Genesis gameplay. Compatibility with Sega CDX and JVC X'Eye is a design priority, ensuring that their CD drive lids can still open with the MacintoshX cartridge attached.
- Use a single 68010 CPU (mounted on the cartridge) shared between Genesis and Macintosh environments.
- Provide memory isolation using a small FPGA/CPLD MMU so neither system interferes with the other.
- Enable clean, instruction-bound context switching triggered during idle cycles—no flags, no mid-instruction jumps.
- Offer pixel-perfect VGA output (640×480) from a 512×342 1‑bit framebuffer, avoiding composite fuzz.
- Include a standard DB‑19 floppy interface for use with real Macintosh floppy drives or BMOW Floppy Emu.
- Include a non-optional cartridge passthrough so original Genesis games can still be played.
- Power via a required PicoPSU module for full voltage support, including negative rails.
NOTE: The 68010 CPU is installed on the MacintoshX cartridge itself rather than inside the console. The onboard Genesis 68000 must be electrically disabled, likely via the cartridge port’s access to
/HALTand/RESET, to avoid bus contention. This design ensures reversibility for unmodified consoles and facilitates CPU arbitration.
- 68010 CPU — mounted on the cartridge; Genesis CPU is halted.
- FPGA/CPLD — handles MMU, bus arbitration, memory map switching.
- Separate ROM (128–256 KB) and SRAM (512 KB) mapped as Mac memory.
- VGA pixel-doubler logic (via FPGA or MCU like RP2040) for crisp display.
- PS/2 → Mac DE‑9 I/O converter (keyboard/mouse).
- DB‑19 Floppy port for Macintosh drive connectivity.
- Genesis passthrough cartridge connector for running games or accessories.
- PicoPSU + 7905 module to generate +12V and –5V rails required by Macintosh floppy drives.
- Logic simplification opportunities using insights from open Mac Plus implementations like the Plus Too project.
- Low-profile cartridge shell option to maintain compatibility with top-loading CD units like the Sega CDX and JVC X'Eye, ensuring their CD drive lids are not obstructed.
Using the 68010’s bus signals (/AS, /DTACK, /BR, /BG) and a hardware timer:
-
Monitor for a safe idle period (no bus activity).
-
Assert Bus Request (
/BR) at any time. -
The CPU will assert Bus Grant (
/BG) only after it finishes its current instruction and the bus is idle. -
Upon receiving
/BG, the MMU:- Swaps memory maps (Genesis ↔ Mac).
- Temporarily takes control of address/data bus.
-
Releases
/BR, CPU resumes execution in the alternate environment.
This ensures mid-instruction safety and a clean context swap without needing mode flags or software interrupts.
To allow the cartridge-mounted 68010 to take over, the onboard Genesis 68000 must be electrically disabled:
- Assert the /HALT line (active low) through the cartridge interface. This tri-states the onboard CPU's bus lines.
- Optionally assert /RESET for extra safety on startup.
- Once halted, the 68010 on the cartridge drives all address/data/control lines.
This method avoids any need to physically remove the Genesis CPU and maintains full reversibility.
- ROM + RAM: ~100 mA
- FPGA/MMU: ~75 mA
- VGA/MCU logic: ~150 mA
- Floppy drive: requires +12V and –5V, powered by required PicoPSU + 7905 module Total ≈330–450 mA @5 V (not including 12V/–5V floppy demand)
Reference Genesis & 32X operate on shared supplies; 32X uses an external adaptor due to higher power demands.
docs/— design docs, pin maps, memory layoutsfpga/— HDL source & simulation testbenchesboard/— PCB/FPC layout filesmcus/— PS/2→DE‑9 and VGA pixel‑doubler firmwaresw/— prototype code for context switching, floppy routines
We need help from retro-hardware enthusiasts, FPGA devs and 68K gurus!
- Schematics for MMU logic & bus monitoring
- Verilog/VHDL drivers for VGA and I/O conversion
- Prototype board layouts sized to fit within Genesis/Nomad casing
- Proof of concept firmware for floppy interface and context switch routines
- 3D printed case designs matching the 32X aesthetic
- Explore logic reduction by adapting techniques from Plus Too or similar projects
- Design low-profile enclosures that work with CDX and X'Eye systems
Jump into the discussion on our Discord: discord.gg/VEw2tt5EMz
Use the dedicated #macintoshx-genesis-discussion channel to brainstorm, share progress, or contribute ideas.
- Genesis and 32X power ratings (Wikipedia, SegaRetro)
- Open-source cartridge projects (EverDrive, MegaSD, etc.)
- Mac Plus FPGA clone inspiration (Plus Too project) — valuable for minimizing component count and adapting Mac logic to FPGA/CPLD efficiently
Two systems. One CPU. Game like it’s 1991. Compute like it’s 1986.