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Plus Too Mac Replica, New Progress!

After a long hiatus, there’s new progress for the Plus Too FPGA-based Macintosh replica. Till Harbaum has ported my original, unfinished design to the MiST board, and has begun making fixes and improvements. The video shows Plus Too booting on the MiST board, and selecting a boot floppy disk image from the MiST’s overlay menu.

Plus Too is a reimplementation of a Macintosh Plus, synthesized in an FPGA. All of the original Mac’s chips are modeled in a hardware description language and implemented in the FPGA’s logic cells: the 68000 CPU, VIA, SCC, IWM, memory controller, video circuitry, etc. I did the original work four years ago using an Altera DE1 evaluation board, which pairs an FPGA with some SRAM, flash memory, and other components. It worked well enough to boot Mac System 6.0.8 from a replica floppy disk, but it suffered from stability problems, and had no support for keyboard, sound, SCSI, serial ports, or real-time clock.

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Till Harbaum designed the MiST board to implement classic 16-bit computers like the Amiga and Atari ST as a System-on-a-Chip using modern hardware, and it’s better suited to the task of replication than a generic board like the DE1. It combines a powerful Altera FPGA with a separate ARM-based CPU. The ARM CPU isn’t used directly in the replicated system, but performs helper functions like loading the selected FPGA core from the SD card, managing configuration overlay menus, and mimicking external peripherals like disks. MiST already has working cores for the Amiga and Atari ST, as well as a large number of 8-bit computers and game consoles. The classic Mac is the only computer from that era that’s still missing.

Till’s progress with Plus Too on MiST so far:

  • ported the original design to the MiST
  • adapted the memory controller to use SDRAM instead of SRAM
  • updated the 68000 CPU model
  • replaced the 6522 VIA implementation with one from the BBC Micro
  • implemented keyboard support
  • added an on-screen menu to select floppy disk images
  • fixed assorted timing bugs

Somewhere along the way, the stability problems I observed with the DE1 version also disappeared. This might be a hardware difference, or something related to the timing problems that were fixed.

Till is currently working on implementing on-the-fly GCR encoding of standard Macintosh disk images, similar to the way Floppy Emu works. The original Plus Too required disk images to be GCR encoded ahead of time, using a separate utility program.

His roadmap after that includes:

  • get both floppies to work nicely with .dsk images (read only)
  • make the video timing closer to a real Mac
  • add sound

Additional info on the MiST board is at http://code.google.com/p/mist-board/
Source code for Mac-MiST is at https://github.com/mist-devel/mist-board/tree/master/cores/plus_too
Binaries are at https://github.com/mist-devel/mist-binaries/tree/master/cores/plus_too
Some discussion about the Mac core for MiST is at http://atari-forum.com/viewtopic.php?f=101&t=28648
Additional German-language discussion is at http://mist-fpga.net/viewtopic.php?f=24&t=102

Read 9 comments and join the conversation 

9 Comments so far

  1. Nick October 22nd, 2015 10:14 am

    Hello, what a wonderful project! What is the status of the serial ports? I would love to use some vintage hardware I still have with the Plus Too MiST.

  2. Till October 22nd, 2015 11:46 am

    The MISTs firmware can deal with pl2303 based USB->RS232 cables. The Atari ST core can redirect its rs232 communication to this. It shouldn’t be too hard to allow the mac to do the same.

  3. Mark October 23rd, 2015 6:09 am

    I’d love if someone was assembling MiST boards for sale in North America (and ideally for less money). The cost of getting a board has been holding me back.

  4. Charles October 23rd, 2015 4:26 pm

    how much are they?

  5. Till October 24th, 2015 11:34 pm

    Most of my changes so far can probably easily be backported to the de1. You don’t need a mist board at all for this.

  6. Anonymous October 29th, 2015 4:35 pm

    It’s 220 USD + S&H I imagine…. http://lotharek.pl/product.php?pid=96

  7. Dylan November 4th, 2015 8:16 pm

    I think $US220 (or less than $200 if you can get together a few friends) is a very good deal. If you go to a DE1, you could maybe save $50, but wouldn’t be a part of this amazing community (C64, Nintendi, Amiga, Atari, BBC, Acorn, …).

  8. Till November 8th, 2015 10:00 am

    One major advantage of the mist is that it implements usb in its io controller and translates this to ps2 internally. So you can use the latest wireless usb input devices even though cores like the plus too internally only work with ps2 devices.

  9. Till November 8th, 2015 10:03 am

    Btw: Sound and on-the-fly floppy encoding is now implemented together with various other small improvements.

    Currrent work focusses on getting a mac plus compatible harddisk running from sd card.

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