This is a github repo, originally rebased from Prex+ site. Maintained and developed by me (champ.yen@gmail.com). Feel free to submit pull requests.

Since the original Prex+ 0.9.0 release, this project has been significantly enhanced and is now released as Prex+ (2604 release). Key developments include:

• Expanded ARM Support: Added support for Raspberry Pi Zero W (ARM1176JZF-S) and improved the Integrator/CP support.
• New Hardware Drivers: Implemented interrupt-driven BCM2835 SD Host (SDHC) and DMA drivers for the Raspberry Pi.
• Filesystem & Storage: Implemented a completely new FATFS file system from scratch, along with a new SDMMC stack driver with support for multi-sector reads.
• Build System Modernization: Unified configuration scripts (--enable-mmu) and target configurations. Fixed compilation against modern GCC (GCC 15), resolving stack boundary and strict PIC/PIE issues.
• x86-pc Boot Fixes: Resolved critical bootldr size limitations, fixed ELF relocation logic for driver modules (drv.ko), and fixed memory overlapping/page fault panics in the exec server's ELF loader.
• System Capabilities: Addressed capability denials (CAP_SYSFILES) ensuring seamless startup of the shell (cmdbox) and init scripts.
• Console Output: Ensured reliable diagnostic serial output across targets, especially for running without VGA in QEMU.

Prex+ is an open source, royalty-free, real-time operating system for embedded systems. It is designed and implemented for resource-constrained systems that require predictable timing behavior. The highly portable code of Prex+ is written in 100% ANSI C based on traditional microkernel architecture.

The Prex+ microkernel provides only fundamental features for task, thread, memory, IPC, exception, and synchronization. The other basic OS functions - process, file system, application loading, and networking, are provided by the user mode servers. In addition, Prex+ provides a POSIX emulation layer in order to utilize existing *NIX applications. This design allows the system to perform both of the native real-time task and the generic POSIX process simultaneously without degrading real-time performance. It also helps platform designers to construct OS by choosing suitable system servers for their target requisition. Learn more »

The project targets the following goals:

• To provide a small, portable, real-time, secure, and robust operating system.
• To provide simple and clean source codes for education and an experimental test-bed.
• To conform to open standards as much as possible.
• To enjoy our life with kernel hacking. ;-)

See current development status »

Prex+ is royalty-free software released under Revised BSD License. See License Information »

Prex+ has the following features:

• Task & Thread Control: preemptive priority scheduling with 256 priority levels
• Memory Management: memory protection, virtual address mapping, shared memory, MMU or MMU-less configuration
• IPC: object name space, synchronous message passing between threads
• Exception: fault trapping, framework for POSIX signal emulation
• Synchronization: semaphores, condition variables, and mutexes with priority inheritance
• Timers: sleep timers, one-shot or periodic timers
• Interrupt: nested interrupt service routines, and prioritized interrupt service threads
• Device I/O: minimum synchronous I/O interface, DPC (Deferred Procedure Call)
• Security: task capability, pathname-based access control, I/O access permission.
• Real-time: low interrupt latency, high resolution timers and scheduling priority control
• Power Management: power policy, idle thread, DVS (Dynamic Voltage Scaling)
• Debugging Facility: event logging, kernel dump, GDB remote debug
• File Systems: multi-threaded, VFS framework, buffer cache, ramfs, fatfs, arfs, etc.
• POSIX Emulation: pid, fork, exec, file I/O, signal, pipe, tty, pthread, etc.
• Libc: C library fully optimized to generate a small executable file
• CmdBox: a small binary that includes tiny versions of many UNIX utilities.
• Networking: (plan) TCP/IP stack, BSD socket interface

1. Develop BSP for QEMU armv7 virt platform (VirtIO serial, block, input, sound, and net devices).
2. Add new audio and net servers.
3. Introduce a USB stack and server.
4. Add RISC-V and Cortex-M support.
5. Add exFAT and SDXC support.