Summary

O.C.E. Technology's (OCE) OCEOS is a real-time, pre-emptive, fixed priority, operating system that can be used in applications that require European Cooperation for Space Standardization (ECSS) Category B or ISO 26262 standards. OCEOS was developed with support of the European Space Agency.

It has a small memory footprint (< 10 kBytes), requires only one system stack rather than a stack for each task, and provides support for precisely timed data outputs independent of task scheduling. It is available initially for applications running on SPARC-based hardware with ARM release early in 2021.

OCEOS provides the following facilities:

  • Fixed priority, pre-emptive scheduling
  • Based on Stack Resource Policy – unbounded priority inversion, chained blocking, and deadlocks cannot occur
  • Single stack rather than separate stack for each task
  • Small code footprint (< 10 kBytes for scheduling and mutex)
  • Mutex, Counting Semaphore, and Data Queue support
  • High precision timed actions (data output and task start)
  • Supports SPARC V8 LEON2/3/4 single core targets initially
  • Suitable for Cobham-Gaisler GR716 applications
  • DMON debug tool support (execution timeline, CPU usage)
  • Support & ISVV services available from OCE
  • Extensive customer support package
  • Developed to ECSS Category B and ISO 26262 standards
  • Developed in cooperation with European Space Agency (ESA)

OCEOS was developed for high reliability aerospace applications. Its small size and efficiency make it suited for use in embedded systems requiring compactness and high reliability. Real time software is often written as a set of trap/interrupt handlers and tasks managed by a RTOS. The trap/interrupt handlers start due to anomalous conditions or external happenings. They carry out the immediately necessary processing and may ask the RTOS to start a task to complete the processing. The RTOS then schedules the task for execution based on its priority.

In hard real time systems scheduling must ensure that each task completes no later than its deadline, but being early can also be a problem. OCEOS provides a timed output service that allows a data output be set for a precise time independently of scheduling. A task also can be scheduled to start at a precise time, but the actual start time may be later depending on task priority.

OCEOS supports up to 254 tasks with up to 15 current execution instances of each task, allowing one task service multiple units of the same type. Each task has a fixed priority and more than one task may have the same priority. Tasks of the same priority are FIFO scheduled, there is no time slicing between tasks in OCEOS. In OCEOS a pre-emption threshold higher than the task’s priority can be set for a task so that once it starts execution it can only be pre-empted by a task with higher priority than this threshold.

Pre-emptions and any traps/interrupts that occur will delay a task’s completion and potentially cause it to miss its deadline. Careful analysis is needed to ensure that task deadlines are always met. OCEOS supports this analysis and allows relatively simple determination of worst case behaviour. Problems such as unbounded priority inversion, chained blocking, and deadlocks cannot occur in OCEOS.

OCEOS provides mutexes to protect critical shared code or data, and inter-task communication using semaphores and queues. A system state variable provides a summary of the current state of the system. Error conditions such as missed deadlines are logged and the system state variable updated. If the system state is not normal actions such as disabling a task or resetting the system may be taken.

OCEOS does not allow dynamic creation of tasks at run time. Virtual memory is not supported. Task priorities are fixed. OCEOS is based on the Stack Resource Policy extension of the Priority Ceiling Protocol [Baker 1991]. OCEOS is provided as a library and is statically linked with an application. Services not needed by an application are omitted by the linker.

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Last updated: 2022-02-16

OCEOS Real-Time Operating System

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