OCEOS/oceos introduction

Introduction and Main Concepts

OCEOS is a pre-emptive real time operating system (RTOS) with a small memory footprint intended for hard real time systems that use the GR716 micro-controller, SPARC LEON2/3/4 processors and ARM Cortex-M micro-processors.

It was developed by O.C.E. Technology with support from the European Space Agency (ESA) under project 4000127901/19/NL/AS and 4000135473/21/NL/GLC/js.

This document describes the features and use of OCEOS, and details its behavior and system calls.

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 start a task to complete the processing.

The RTOS starts tasks and schedules them for execution based on priority. In hard real time systems scheduling must ensure that each task completes no later than its deadline.

Being early can also be a problem. OCEOS provides a timed output service that allows an output be set for a precise system time independently of scheduling.

OCEOS supports up to 255 tasks, and up to 15 current start requests for each task. Each task has a fixed priority. There are 254 task priorities, 1 (highest task priority) to 254 (lowest task priority).

More than one task may have the same priority. Tasks of the same priority are FIFO scheduled. Time slicing between tasks does not occur in OCEOS.

In OCEOS each task has a pre-emption threshold. A task can only be pre-empted by a task with a 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 mutual exclusion semaphores (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 a user defined error handling function may be called and actions taken such as disabling a task or resetting the system.

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.

Purpose of the Software

OCEOS is an object library for target architecture which provides directives to schedule fixed priority tasks, and supporting mutexes, semaphores, data queues, timed outputs, and interrupt handling.

There are also directives for logging, error handling, GR716 memory protection and GR716 watchdog control.

OCEOS Main Features

• Fixed priority preemptive scheduling
• Based on Stack Resource Policy – unbounded priority inversion, chained blocking, and deadlocks cannot occur
• Deterministic processing
• Fast real-time performance
• Single stack rather than separate stack for each task
• Small code footprint ( <10 kBytes for scheduling and mutex)
• Inter-task communication (Counting Semaphores and Data Queues)
• Synchronisation (Mutexes)
• Task preemption level
• Interrupt support (most of the directives can be called from IRQ handlers)
• Very short time for disabling interrupts
• Very low interrupt latency
• Nested interrupts support
• High precision timed actions (Software Timers) for data output and task start at pricise time
• Supports SPARC V8 LEON2/3/4 and ARM Cortex-M single core targets
• Fault Detection, Isolation and Recovety
• GR716 Memory protection functionality
• Enhancements for Cobham-Gaisler GR716 applications
• Sample Applications for Quick start
• Online Documentation
• Tutorial applications to demonstrate main features of OCEOS
• DMON debug tool support (execution timeline, CPU usage and profiling, commands to display information for OCEOS modules)
• Support & ISVV services available from OCE Technology Ltd.
• MISRA C Compliant
• Developed to ECSS Category B and ISO 26262 standards
• Developed in cooperation with European Space Agency (ESA)