What is Prex?

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.

Prex Design 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.

Project Goals

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. ;-)
Royalty Free


Prex is royalty-free software released under Revised BSD License.


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

Hardware Requirements

H/W image Prex minimum hardware requirements are as follows:
  • Processor: 32-bit Processor (x86, ARM, PowerPC)
  • Memory: 256KB of RAM (Kernel < 25k bytes)
  • Timer: programmable interval timer

Note: RAM requirements may vary depending on the number of system servers being supported.