PicoOS

This is a simple operating system for the Raspberry Pi Pico micro-controller. The code structure is strongly inspired by SerenityOS, however, no code is taken from that source directly.

My goal was to write a simple operating system because I was interested in operating systems. The code base is a huge mess and I later tried to rewrite everything, however, I never completed the rewrite.

Currently the system has the following capabilities:

• The system runs on a Raspberry Pi Pico microcontroller (actual hardware!). It uses a UART connection to communicate with a terminal window of the host machine.

  Instead of connecting the device directly, it is connected indirectly with another Raspberry Pi Pico which has the PicoProbe software instealled. This makes it possible to debug what is happening on the chip and the UART connection is exposed via USB.

• There is a very fragile file system implementation that supports most common operations. Some programs are embedded into the flash memory of the device and they are accessible in this file system.

• The system itself has an extremely simple shell program which is loaded on startup and which is accessible with the UART connection.

  There are some builtin shell commands, but it can also use posix_spawn to start a new process. This can be used to load any ELF file, but the system makes a ton of assumptions about the application.

The kernel has the following capabilities:

• There is a bare bone memory allocation algorithm.

• There is a scheduler that must run on a single core. It can switch between several threads that belong either to the kernel or to userland.

  I tried adding multi-core support later on, however, the debugging tools I was using were not sophisticated enough to debug what went wrong.

• There is some bare bone isolation between kernel and userland.

  Sadly, this microcontroller does not have a Memory Management Unit (MMU), therefore, proper isolation isn't possible. However, the Memory Protection Unit (MPU) is used to at least prevent accesses to kernel space.

• The following system calls are partially supported: read, write, open, close, fstat, wait, exit, chdir, get_working_directory, posix_spawn.

  Notice, that fork is not on that list since it requires an MMU, however, posix_spawn can do most of the things that fork can do.

Development Environment

1. Install required packages:

   pacman -S --needed python-invoke arm-none-eabi-gcc arm-none-eabi-gdb arm-none-eabi-newlib fmt
   

2. Install TIO from AUR:

   cdm ~/src/aur.archlinux.org
   git clone --depth 1 https://aur.archlinux.org/tio.git
   cd tio
   makepkg --install
   

3. Build openocd:

   cdm ~/src/github.com/raspberrypi
   git clone --branch picoprobe --depth 1 git@github.com:raspberrypi/openocd.git
   cd openocd
   ./bootstrap
   CFLAGS=-Wno-error ./configure --enable-picoprobe
   make -j24
   sudo make install
   

4. Build pico-sdk:

   cdm ~/dev
   git clone --branch tweaks git@github.com:asynts/pico-sdk.git
   

5. Build the project with:

   cdm Build
   cmake .. -GNinja -DPICO_SDK_PATH=~/dev/pico-sdk
   ninja
   

6. Connect Raspberry Pi Pico. The scripts expect two Raspberry devices where one is used for debugging and the other runs the operating system. There needs to be a UART connection from the debugee to the debugger.

   The debugger runs the picoprobe firmware.

7. Run inv probe to start up openocd.

8. Run inv tty, this will be the shell into the target system.

9. Run inv dbg, this will be used for debugging and to load the application.

Running the System

1. In the debugger terminal, run rebuild.

2. run will start the system. The shell is accessible in the inv tty terminal.