Design and architecture of real time operating system
Modern airliners such as Airbus A320, Boeing 787, and Russian MS 21 use so called Integrated Modular Avionics (IMA) architecture for airborne systems. This architecture is based on interconnection of devices and on board computers by means of uniform real time network. It allows significant reduction of cable usage, thus leading to reducing of takeoff weight of and airplane. IMA separates functions of collecting information (sensors) action (actuators) and avionics logic implemented by applied avionics software in on board computers. International standard ARINC 653 defines constraints on the underlying real time operation system and programming interfaces between operating system and associated applications. The standard regulates space and time partitioning of applied IMA related tasks. Most existing operating systems with ARINC 653 support are commercial and proprietary software. In this paper, we present JetOS, an open source real time operating system with complete support of ARINC 653 part 1 rev 3. JetOS originates from the open source project POK, created by French researchers. At that time POK was the only one open source OS with at least partial support for ARINC 653. Despite this, POK was not feasible for practical usage: POK failed to meet a number of fundamental requirements and was executable in emulator only. During JetOS development POK code was significantly redesigned. The paper discusses disadvantages of POK and shows how we solved those problems and what changes we have made in POK kernel and individual subsystems. In particular we fully rewrote real time scheduler, network stack and memory management. Also we have added some new features to the OS. One of the most important features is system partitions. System partition is a specialized application with extended capabilities, such as access to hardware (network card, PCI controller etc. Introduction of system partitions allowed us moving large subsystems out of the kernel and limiting the kernel to the minimal functionality: context switching, scheduling and message pass. In particular, we have moved network subsystem to system partition. This moving reduces kernel size and potentially reduces probability on having bug in kernel and simplifies verification process.