This document specifies both the interface and the behavior of the APEX services. Behavior is specified to the extent needed to describe behavior relevant to calling applications.

Where necessary, assumptions are made as to the support or behavior provided by the operating system and hardware. This should not be construed as a specification for the O/S or hardware. However, where the O/S or hardware does not coincide with the stated assumptions, the API behaviors specified herein may not match the actual behavior.

ARINC 653 is intended for use in a partitioned software environment. In order to assure a high degree of portability, aspects of the partitioned environment are discussed and assumed. However, this specification does not define the complete system, hardware or software requirements for partitioning, nor does it provide guidance on proper implementation of partitioning and in particular, robust partitioning. It must not be construed that compliance to this standard in any way assures robust partitioning.

The main objectives of this Subset are the following:

• Achieve an execution model and O/S abstractions for a simpler software design than with ARINC Specification 653, Part 1.

• Simplify dynamic behavior, make it easier to characterize and enable formal methods for Worst Case Execution Time (WCET) analysis.

• Maintain portability of applications with ARINC Specification 653, Part 1 (i.e., an application that is compliant to ARINC Specification 653, Part 4 should operate in ARINC Specification 653, Part 1 compliant environment).

• Downsize O/S code footprint, reduce O/S overhead, improve real-time performance.

• Scale down O/S complexity to enable formal proof analysis.

• Reuse configuration tools from ARINC Specification 653, Part 1 compliant environments.

• Reduce cost of certification.

Purpose

This document defines a Subset for the Application Programming Interface (API) specified by ARINC Specification 653, Part 1, Required Services. This document is based on Supplement 3 to Part 1. Based on a simpler execution model, with only one or two processes, and a reduced set of services, it defines "restrictions" to the ARINC 653 Part 1 API.

The primary objective of this Specification is to define a restricted interface between the Operating System (O/S) of an avionics computer resource and the application software. Included within this specification are the interface requirements between the application software and the O/S and the list of services which allow the application software to control the scheduling, communication, and status information of its internal processing elements.

This Specification defines the data exchanged statically (via configuration) or dynamically (via services) as well as the behavior of services provided by the O/S and used by the application. It is not the intent of this specification to dictate implementation requirements on either the hardware or software of the system, nor is it intended to drive certain system level requirements within the system which follows this standard.

The majority of this document describes the runtime environment for embedded avionics software. This list of services identifies the minimum functionality provided to the application software and is, therefore, the industry standard interface. It is intended for this interface to be as generic as possible, since an interface with too much complexity or too many system specific features is normally not accepted over a variety of systems. The software specifications of the APEX interface are High Order Language (HOL) independent, allowing systems using different compilers and languages to follow this interface.