Make something that runs on real hardware.
This store lists microfirmwares, the apps that run on a SidecarTridge Multi-device board plugged into a vintage computer. This guide is about writing one yourself, starting from a template rather than a blank page.
Step 0: pick your platform
Every SidecarTridge board does the same fundamental thing, but the computer on the other end of the cartridge port changes almost everything practical about the work. It decides which toolchain you install, which assembler the target half is written in, which template you start from, how you test it. So the guide splits into one track per platform, and you choose yours before you install anything.
It is the only platform with a published catalogue in this store, so it is the only one with a guide. When another platform ships, it gets its own track here. The shared ideas below will carry over, the commands will not.
What is the same on every platform
The per-platform tracks differ in their commands, but they are all built on the same four ideas. Reading these first will make your track make sense.
1. Your microfirmware has two halves
One half is C, compiled for the Raspberry Pi Pico W microcontroller on the board. The other half is a small program in the host computer's native code, living in the address space the board pretends to be. The two talk to each other, and most of your logic belongs in the C half, where you have room to work.
2. The board pretends to be a ROM cartridge
The host computer thinks it is reading a memory chip. The microcontroller answers those reads fast enough that the illusion holds. You never write that emulation yourself. It is already done, and it is the part you must be careful not to disturb.
3. Booster runs your app
You do not ship a whole firmware image. A bootloader called Booster owns the device: Wi-Fi, the web interface, the microSD card, installing and launching apps. Your microfirmware is one app it can load, which is why it is a microfirmware. Setting it up on a device is covered by the getting started guide .
4. It reaches users through this store
Booster fetches a catalogue of available apps over the network. That catalogue is what this site publishes, and it is also written up upstream as the apps catalogue . Getting listed is the last step of every track and works the same way for everyone: one pull request adding a pointer to files you host yourself. The Atari ST track covers it in step 11.