ARM_OS

ARM_OS is a small educational operating system for ARM, currently targeting the QEMU vexpress-a9 machine with a Cortex-A9 CPU.

The project is built from scratch and includes a custom boot path, high-half kernel, basic memory management, user-space process support, syscalls, a framebuffer-backed TTY, an initramfs, and an early VFS/filesystem layer.

Current Features

• ARMv7 Cortex-A9 target
• QEMU vexpress-a9 support
• Custom bootloader
• High-half kernel mapping
• MMU setup
• Physical and virtual memory management
• Kernel heap
• User task loading and scheduling
• Basic syscall layer
• User-space programs
• Initramfs support
• Early VFS layer
• Dummy filesystem and dummy disk image
• Console backend abstraction
• Framebuffer console using PL111
• TTY-backed standard input/output/error
• UART RX interrupt-driven input
• Kernel log output through UART
• Kernel log ring buffer exposed through /dev/klog
• User-space dmesg-style log reader
• Timer-based scheduling
• Sleep syscalls
• Basic kernel watchdog
• Minimal libc-like user library

Repository Layout

bootloader/        Early boot code
kernel/arch/arm/   ARM-specific startup, vectors, IRQ, MMU code
kernel/core/       Kernel entry, scheduler, panic, watchdog, boot info
kernel/mm/         Physical memory, virtual memory, heap, user access
kernel/proc/       User task and program loader support
kernel/sys/        Syscall dispatch and syscall implementations
kernel/fs/         VFS, initramfs, dummy filesystem, dummy disk
kernel/drivers/    UART, framebuffer and low-level device drivers
kernel/console/    Kernel/user-visible console support
kernel/time/       Kernel time and sleep support
userlib/           Minimal libc-like user-space library
user/programs/     User-space programs
tools/             Host-side image generation tools

Requirements

You need an ARM bare-metal toolchain and QEMU:

arm-none-eabi-gcc
arm-none-eabi-as
arm-none-eabi-ld
arm-none-eabi-objcopy
qemu-system-arm
python3
make

On macOS, these are typically available through Homebrew.

Build

make

This builds:

• the bootloader
• the kernel
• user-space programs
• the initramfs image
• the final boot image

Run

Serial-only mode:

make run

Framebuffer mode:

make run-fb

On macOS, run-fb uses QEMU Cocoa fullscreen mode because the PL111 window may otherwise open too small.

Debug

Start QEMU paused with a GDB server:

make debug

Then connect with an ARM-capable GDB:

arm-none-eabi-gdb build_kernel/kernel.elf
target remote :1234

User-Space

The system currently builds a small set of user-space programs into the initramfs:

• /init
• /bin/console
• /bin/cp
• /bin/echo
• /bin/mv
• /bin/print
• /bin/dmesg

User-space is linked against userlib, a small libc-like library adapted for this OS.

Roadmap

Planned work includes:

• Cleaner kernel subsystem boundaries
• Better boot diagnostics and hardware reporting
• Device abstraction for character and block devices
• Block device abstraction
• FAT12 and FAT16 filesystem support
• exFAT support
• Network device abstraction
• Ethernet, ARP, IPv4 and ICMP
• UDP support
• TCP/IP stack
• More complete userland

Status

This is an experimental operating system under active development. Interfaces and internal structure are expected to change frequently.