Oct 11, 2011 7:45 AM

Plus Too: a Homebrew Mac Plus

Steve Chamberlin over at Big Mess o’ Wires has been steadily working away at creating a Mac Plus clone using a Field Programmable Gate Array (FPGA), and he just hit a huge milestone by getting the system to boot and run! Big Mess o’ Wires chronicles the progress of the Plus Too project in detail, […]

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Steve Chamberlin over at Big Mess o' Wires has been steadily working away at creating a Mac Plus clone using a Field Programmable Gate Array (FPGA), and he just hit a huge milestone by getting the system to boot and run!

Big Mess o' Wires chronicles the progress of the Plus Too project in detail, covering topics such as the classic Mac's Crazy Disk Encoding Schemes, the 68000 CPU's interleaved memory design, various Mac Toolbox Mysteries, an SD Card Floppy Emulator, and more.

Steve describes his success over at his blog:

Plus Too is a home-made replica of a classic Macintosh computer using an FPGA. The project reached a major milestone yesterday, booting to the Finder for the first time, and running several programs. Since then I’ve been getting many inquiries, and because not everyone’s been following the project since its beginning, I’ve created a Plus Too project summary page to document the progress so far and my plans for the next version. If you’re new to Plus Too, please begin by reading the project summary page.
The screenshot above shows a Mac Write document opened with Plus Too. Because there’s no keyboard support yet, the document was created on another computer and added to the Plus Too boot disk image. Below the Macintosh screen region, hardware debugging information is displayed in green. This debug overlay is possible because a pixel-doubled 512 x 342 Mac image conveniently leaves some extra vertical space on a 1024 x 768 VGA display. From left to right, the debugging information shows the current state of the CPU address bus, data bus in, data bus out, address strobes, previous address, and breakpoint address. A poor-man’s breakpoint system is implemented by setting a breakpoint address with panel switches. When the address bus matches the breakpoint address, the CPU’s memory transfer acknowledge signal is withheld, effectively pausing the CPU.

It has been fascinating to follow the progress of the Plus Too project over the past months, and it's exciting to see it finally working. Sure, there are software emulators that will allow you to run the classic Mac OS in all its 68k glory, but a hardware re-implementation of an old-school Mac is way cooler.