Raspberry Pi Operating System

Yet another unix-like toy operating system running on Raspberry Pi 3/4, which is built when I was preparing labs for operating system course at Fudan University, following the classic framework of xv6.

Tested on Raspberry Pi 3A+, 3B+, 4B.

Related works

• linux: real world operating system
• circle: contains lots of portable drivers
• s-matyukevich's
• bztsrc's

What's different?

• We use musl as user programs libc instead of reinventing one.
• The set of syscalls supported by our kernel is a subset of linux's.
• Compared to xv6, we use a queue-based scheduler and hash pid for process sleeping and waking.

Features

• AArch64 only
• Basic multi-core support
• Memory management
• Virtual memory without swapping
• Process management
• Disk driver(EMMC): ported from circle
• File system: ported from xv6
• C library: musl
• Shell: ported from xv6
  • Support argc, envp
  • Support pipe

Prerequisite

For Ubuntu 20.04 on x86, just run make init and skip this section.

GCC toolchain

If you are cross compiling on macOS,

1. run brew install zstd,
2. install aarch64-unknown-linux-gnu by macos-cross-toolchains,
3. change the prefix such as CROSS := aarch64-unknown-linux-gnu- in config.mk.

If you are native compiling, i.e., running natively on ARMv8, set CROSS := in config.mk to use local gcc.

If $(CROSS)gcc --version gives out version less than 9.3.0, remove -mno-outline-atomics from Makefile.

QEMU

You can install QEMU (>= 6.0) by package manager or compile it from source such as

git clone https://github.com/qemu/qemu.git
mkdir -p qemu/build
(cd qemu/build && ../configure --target-list=aarch64-softmmu && make -j8)

On some OS such as CentOS, you may also need to install the following dependencies

yum install ninja-build
yum install pixman-devel.aarch64

Then add the generated qemu-system-aarch64 to PATH or just modify the QEMU variable in config.mk.

Build musl

First, fetch musl by git submodule update --init --recursive.

Then if you are cross compiling, run (cd libc && export CROSS_COMPILE=X && ./configure --target=aarch64), where X is the CROSS variable in config.mk. Otherwise, run (cd libc && ./configure).

File system tools

mtools is used to build our fat32 file system image, which can be installed by package manager, e.g, sudo apt install mtools on Ubuntu and brew install mtools on macOS.

sfdisk is used to create MBR-partitioned disk images. It has already been installed on most Linux distributions. But for macOS,

1. run brew install util-linux to install sfdisk,
2. modify the PATH according to the instructions given by brew.

Development

• make qemu: Emulate the kernel at obj/kernel8.img.
• make: Create a bootable sd card image at obj/sd.img for Raspberry Pi 3, which can be burned to a tf card using Raspberry Pi Imager.
• make lint: Lint source code of kernel and user programs.

Raspberry Pi 4

It works on Pi 4 as well. Change RASPI := 3 to RASPI := 4 in Makefile, run make clean && make and have fun with your Pi 4.

Logging level

Logging level is controlled via compiler option -DLOG_XXX in Makefile, where XXX can be one of

• ERROR
• WARN
• INFO
• DEBUG
• TRACE

Defaults to -DLOG_INFO.

Debug mode

Enabling debug mode via compiler option -DDEBUG in Makefile will incorporate runtime assertions, testing and memory usage profiling(see below). Defaults to -DNOT_DEBUG.

Profile

We can inspect the information of processes and memory usage(shown when -DDEBUG) by Ctrl+P. This may output something like

1 sleep  init
2 run    idle
3 runble idle
4 run    idle
5 run    idle
6 sleep  sh fa: 1  

where each row contains the pid, state, name and father's pid of each process.

Project structure

.
├── Makefile
├── mksd.mk: Part of Makefile for generating bootable image.
|
├── boot: Official boot loader.
├── libc: C library musl.
|
├── inc: Kernel headers.
├── kern: Kernel source code.
└── usr: User programs.