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Plus Too for the MiST board

This is a port of the Plus Too project to the MiST board.


  • Copy the core onto a sd card naming it core.rbf for auto boot
  • Copy the a Mac Plus rom onto the SD card under the name plus_too.rom
  • Copy disk images in dsk format (e.g. Disk605.dsk) onto the SD card

Boot the MIST. After a few seconds the floppy disk icon should appear. Open the on screen display using the F12 key and select the a disk image. The upload of the disk image will take a few seconds. Plus Too will then boot into the MacOS desktop.

The necessary rom images can be found in the original Plus Too project archive.

Floppy disk image format

Floppy disk images need to be in raw disk format. Double sided 800k disk images have to be exactly 819200 bytes in size. Single sided 400k disk images have to be exactly 409600 bytes in size.

Both the internal as well as the external floppy disk are supported. The first entry in the OSD refers to the internal floppy disk, the second one to the external floppy disk.

Currently floppy disk images cannot be uploaded while the Mac accesses a floppy disk. Thus it's recommended to wait for the desktop to appear until a second floppy can be inserted.

Before uploading a different disk image it's recommended to eject the previously inserted disk image from within MacOS.

Some system floppy disk images in matching dsk format can be found at here. Some nice applicatons on 400k dsk images can be found here. Official system disk images are available from apple e.g. via ftp. Under Linux these can be converted into the desired dsk format using Linux stuffit, unar and dc2dsk in that order.

Hard disk support

This Plus Too core implements the SCSI interface of the Macintosh Plus together with a 20MB harddisk. The core implements only a subset of the SCSI commands. This is currently sufficient to read and write the disk, to boot from it and to format it using the setup tools that come with MacOS 6.0.8.

The harddisk image to be used can be selected from the "Mount *.img" entry in the on-screen-display. If no image is selected the entire SD card is being used. Caution: This will allow you to format the entire SD card in Macintosh file system format which will prevent the MIST from booting from that card since the MIST cannot boot from a Macintosh HDD. Instead select a IMG file first so all HDD i/o is redirected into that file. The format of the disk image is the same as being used by the SCSI2SD project which is documented here.

Unlike the floppy the SCSI disk is writable and data can be written to the disk from within the core. Since the Mac has some quite short timeouts in the SCSI drivers at least the firmware version 151118 is needed when writing to SCSI disk.

It has been tested that OS 6.0.8 can format the SCSI disk as well as doing a full installation from floppy disk to the harddisk. But keep in mind that this is an early work in progress and expect data loss when working with HDD images.

A matching harddisk image file can be found here. This is a 20MB harddisk image with correct partitioning information and a basic SCSI driver installed. The data partition itself is empty and unformatted. After booting the Mac will thus ask whether the disk is to be initialized. Saying yes and giving the disk a name will result im a usable file system. You don't need to use the Setup tool to format this disk as it is already formatted. But you can format it if you want to. This is only been tested with OS 6.0.8.

CPU Speed

The CPU speed can be adjusted from "normal" which is roughly Mac Plus speed to "Fast" which is about 2.5 times faster. In fast mode some of the initial SCSI transfers time out and thus the core cannot boot from SCSI disk in this mode. But once it has booted to the desktop the turbo can be enabled and further SCSI disk transfers will work.