This is a quick overview of using Pico-Mac.

If you're a certain age, this project will take you right back to the 11th grade computer lab, full of black & white Macintosh computers with floppy disks.

We took the excellent pico-mac emulator and fine tuned a few things for the Adafruit Fruit Jam:

• The RP2350's HSTX peripheral is used to create a digital video signal on the board's DVI-D video output (compatible with HDMI monitors)
• The on board USB hub is supported, so you can connect a keyboard & mouse to the two USB ports
• The SD card is supported, so you can load your favorite Macintosh software
• The PSRAM chip is supported and can emulate a 4MB Macintosh (otherwise, up to 400kB of RAM can be emulated)
• The I2S audio chip is supported, so you can hear the basic system beep (as well as digital audio from some games and applications) on the internal speaker or via the external speaker/headphone jack

Install the Emulator

There are multiple emulator images provided, each with different settings:

• "4096k" emulates a Mac with 4MB RAM, using the psram chip, while "400k" emulates a non-standard mac with 400kB RAM without using psram. For applications that do not need much RAM, the "400k" emulator may be faster. "-psram" accompanies "4096k" as a reminder that this uses the PSRAM chip.
• "640x480" emulates a Mac with a standard VGA resolution screen. "512x342" emulates the original Mac desktop resolution precisely.
• "-oc" overclocks the Fruit Jam to 264MHz for faster emulation. This overclocks the RP2350 chip that powers the Fruit Jam. Just like PC overclocking, there’s some risk of reduced component lifespan, though the extent (if any) can’t be precisely quantified and could vary from one chip to another. Proceed at your own discretion. (-oc -psram builds, if added, also overclock the PSRAM chip to 133MHz)

If you're not sure which image you want, choose 4096-512x342-psram.

Plug the Fruit Jam into your computer with a good USB C data cable. While holding down the UF2 button (Button #1), slide the power switch from OFF to ON. Or, if your Fruit Jam was already powered on, hold the UF2 button and press the reset button briefly.

Your Fruit Jam will connect in bootloader mode, adding a drive called "RP2350" to your computer. Simply drag or copy the umac uf2 file to this drive.

After the firmware loads, the device will reboot into the umac emulator.

Using the built-in image

The built in image is simply the System 6.0.8 Startup disk from archive.org. It's useful for checking out whether your keyboard & mouse work, but it doesn't have any useful applications on it.

We can relatively easily package any bootable disk image e.g., from archive.org instead, as long as it fits within about 5MB. Just update the URL in fetch-rom-dsk.sh and the checksums in roms.sha256sum.

Using an SD card image

umac uses "raw disc images". These images can be any size, from 400kB to 1.44MB and even hard drive size. These images will always begin with the two bytes "LK" and often have the ".img" suffix.

Copy your selected disc image to the root folder of the sd card and rename it to have exactly one of these two names:

• umac0.img, if the emulator is allowed to make changes to the disc image
• umac0ro.img, if the emulator must treat the image as read only

Some images that work well with the emulator

• glider (sounds don't work right)
• dark castle
• system 7 hypercard (sounds and some card transitions don't work right)

Limitations & problems

Due to a limitation of the HSTX peripheral in "1bpp" color mode, it's not possible to achieve "fully black" and "fully white" pixels. You can have white & grey, or black & grey. As a result, the PSRAM builds and 512x342 builds have their "black" pixels actually appear as grey; and the 640x480, 400kB build has its "white" pixels appear as grey.

In my opinion the "white & grey" mode is actually evoking the feel of a CRT with the brightness pushed up too far, it's kinda charming retro. But maybe I'm fooling myself.

Sometimes the keyboard or mouse misbehave. I've found that using the on/off switch to fully power cycle everything will usually fix it. Hot plugging keyboard & mouse should also be possible.

Other things a full guide might touch on ...

• How to use a PC or modern Mac to copy items to a floppy image