XinuPi

XinuPi is the port of Embedded Xinu to the Raspberry Pi. XinuPi provides a simple, lightweight operating system for the Raspberry Pi that contains several orders of magnitude fewer lines of code than the Linux-based software stacks that normally run on the device. Its purpose is to provide an inexpensive, convenient platform for various areas of computer science education at a University level, including operating systems, embedded systems, networking, and programming languages. Another goal of XinuPi is to document some of the Raspberry Pi's hardware that has, until this point, been poorly documented or even undocumented. This includes the documentation on this Wiki as well as the XinuPi source code and the documentation generated from comments in it.

Acquiring hardware

See Raspberry Pi for information about the hardware itself.

Downloading and compiling

XinuPi is currently only available in source code form and only in the Embedded Xinu git repository. To obtain the code, install git and run:

git clone https://github.com/xinu-os/xinu
cd xinu

Note that this in fact the code for "Embedded Xinu" and not for "XinuPi" specifically. That is, the Raspberry Pi is one of several platforms that Embedded Xinu supports (and builds targeting it are referred to as "XinuPi").

To compile XinuPi, run:

make -C compile PLATFORM=arm-rpi

PLATFORM=arm-rpi is necessary to instruct the build system to target the Raspberry Pi. Additional arguments can be passed to make, such as the COMPILER_ROOT to specify the location of a GCC compiler targeting ARM (defaults to "arm-none-eabi-", meaning that no explicit setting is needed if "arm-none-eabi-gcc" and the corresponding binutils are already on the shell $PATH). See compile/README.compiling for more details; for example, details about cross compilers.

The compilation process produces a file "compile/xinu.boot", which can be copied to "kernel.img" on the SD card of a Raspberry Pi to run it (see Raspberry Pi#Booting).

XinuPi is licensed under a BSD-style license; see the copyright information in the source distribution for more details.

Features and implementation

• The core of XinuPi provides a preemptive multitasking operating system for the Raspberry Pi. See Preemptive multitasking (ARM) for more details about how Embedded Xinu implements preemptive multitasking on ARM-based platforms such as the Raspberry Pi; this includes information about thread creation and context switching. Also see BCM2835 System Timer for the timer on the Raspberry Pi that XinuPi uses to implement preemptive multitasking.
• Interrupt handling on the Raspberry Pi, required for the timer interrupt as well as many other devices, is described in Interrupt handling (Raspberry Pi).
• USB support was added to Embedded Xinu partly because of its important role in the Raspberry Pi, including to attach the Ethernet Controller on the Raspberry Pi Model B. See USB for general information about USB, or Synopsys DesignWare High-Speed USB 2.0 On-The-Go Controller for information specifically about the USB controller the Raspberry Pi provides.
• See SMSC LAN9512 for information about the built-in USB Ethernet Adapter on the Raspberry Pi Model B, and XinuPi's driver for it.

TODO: sound, graphics, keyboard support