LithOS Bare-Metal: an unusual path for an RTOS

Abstract

This paper presents LithOS Bare-Metal (LithOS-BM), a Real-Time Operating System (RTOS) partially compliant with the ARINC 653 APEX standard, designed for small embedded systems where partitioning or virtualization is unnecessary or infeasible, and rapid response times are critical.
Integrated Modular Avionics consolidates software functions into shared computing resources, reducing hardware, weight, and power consumption. ARINC 653 defines guidelines for Time and Space Partitioning (TSP), specifying a separation kernel
and an APEX API for application software.
Originally, LithOS was developed as a guest RTOS for the XtratuM Next Generation hypervisor, which enforces strict TSP within a virtualized environment. However, certain embedded applications do not require complex hypervisor-managed partitioning, particularly when strict timing constraints or hardware limitations prevent virtualization. 
LithOS-BM addresses this by replacing the hypervisor layer with a custom Board Support Package that interfaces directly with the hardware. This approach maintains ARINC 653 compliance, preemptive task scheduling, and temporal isolation while fully controlling the underlying hardware platform as a standalone RTOS.