Atari TOS comes to Sega Mega CD through EmuTOS

The Sega Mega CD was built to give the Mega Drive more storage, extra processing power and better audio. Running an Atari-style desktop on it wasn’t part of Sega’s plan. That’s exactly what this EmuTOS port does. Instead of emulating a complete Atari ST, the project adapts EmuTOS directly to the Mega CD hardware. Boot it from a CD-R or cartridge and the Sega system starts behaving much more like a small 68000-based computer, complete with a GEM desktop, disk drives, keyboard support, printing and sound.

How EmuTOS works on the Mega CD

The clever part is how the two processors divide the work. A Mega Drive fitted with a Mega CD has two Motorola 68000 CPUs. EmuTOS itself runs on the Mega CD’s 68000 processor, along with GEMDOS, AES and VDI. The Mega Drive’s own 68000 handles much of the hardware interface.

That matters because the Atari ST and Mega Drive may share the same processor family, but almost everything around those CPUs works differently. Atari-style planar graphics have to be converted into a form Sega’s VDP can display. Controller input needs to be presented in a way the operating system understands. The same applies to sound, keyboard input and printer access.

The main Mega Drive CPU takes care of that hardware-facing work, while the Mega CD processor runs the operating system and desktop environment. It’s a neat split that makes good use of the hardware already inside the combined system.

This also means the project isn’t simply running an Atari ROM on Sega hardware. EmuTOS is an open-source operating system designed to provide TOS-compatible functionality on 68000-based machines, which makes it much easier to adapt to hardware that Atari never intended to support.

What TOS actually was

TOS was the operating system used by the Atari ST family from the mid-1980s onward. If you’ve used an ST, the layout is immediately familiar: windows, icons, menus, drive letters and mouse control through the GEM desktop. Underneath that desktop sat several separate components.

GEMDOS handled files and basic operating-system services. AES managed applications, windows and interface elements. VDI handled graphics output. Together, they formed the environment most users simply recognised as the Atari ST desktop. TOS was also compact. On later ST models it lived in ROM, so the machine could reach the desktop without loading a complete operating system from floppy disk first.

That relatively lean design helps explain why EmuTOS can be adapted to unusual 68000-based machines. The Mega Drive and Mega CD are very different from an Atari ST, but they still provide a familiar CPU architecture to build around.

Sega storage becomes Atari-style drives

Once EmuTOS starts, the Sega hardware gets mapped into familiar drive letters. The C: drive is a RAM disk and acts as the system drive. When booting from disc, the CD appears as D:. The Mega CD’s 8 KB internal backup RAM becomes I:, while cartridge save RAM or a CD Backup RAM Cart can appear as S:. It’s a straightforward approach, and it makes the system behave more like a desktop computer than a games console.

Desktop settings can also be stored in the Mega CD’s internal backup RAM, so the system doesn’t have to start from scratch every time. The supplied disc image includes several basic utilities, including a text editor, picture viewer, formatting tools and a backup program. Printer support is included as well. Output can be routed through the Mega Drive’s EXT port using suitable interface hardware.

Booting from CD or cartridge

There are two main ways to run the system on original hardware. You can burn the supplied CD image and start it through the Mega CD, or flash the cartridge version and boot from that instead. The CD build follows the normal region restrictions of Mega CD software, while the cartridge build doesn’t.

There is one requirement with the cartridge version: the CD tray has to be empty. The build system can also incorporate Sonic graphics and movement data taken from a user’s own game dumps. Those assets aren’t included with the release, but the option shows how Sega-specific material can be integrated into the GEM desktop environment.

An Atari style desktop on Sega hardware

The result isn’t an attempt to turn the Mega CD into a perfect Atari ST clone. Instead, EmuTOS uses the Mega CD’s 68000 for the operating system while the Mega Drive’s processor handles the Sega-specific hardware work. Graphics, input, storage and peripherals are adapted where needed, while the familiar GEM desktop remains largely intact.

That makes the project less about full-system emulation and more about adapting one 68000-based software environment to another 68000-based platform. The hardware is still very much Sega. The desktop just happens to look and behave like it belongs on an Atari.

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