QP/C  7.2.1
Real-Time Embedded Framework
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Overview

QP Framework is a lightweight implementation of the Active Object model of computation specifically tailored for real-time embedded (RTE) systems. QP is both a software infrastructure for building applications consisting of Active Objects (actors) and a runtime environment for executing the Active Objects in a deterministic fashion. Additionally, QP Framework supports Hierarchical State Machines with which to specify the behavior of Active Objects [ROOM:94], [UML 2.5],[Sutter:10].

The main objectives of the QP Framework are:

  • to provide a modern, event-driven model of concurrency based on the best practices of concurrent programming [ROOM:94], [Cummings:10], [Sutter:10], collectively known as the Active Object (Actor) model of computation, which is inherently safer than the traditional "shared-state concurrency, mutual-exclusion, and blocking" approach based on a conventional Real-Time Operating System (RTOS);
  • to provide a higher-level of abstraction closer to the problem domain than the "naked" RTOS threads;
  • to provide the right abstractions for applying modern techniques like visual modeling, hierarchical state machines, and automatic code generation;
  • to bridge the semantic gap between the higher level modeling concepts (such as UML) and the traditional programming languages like C or C++.

Context and Functional Decomposition

The following block diagram shows the context of use and the high-level functional decomposition of a system built with the QP Framework.

Context and Functional Decomposition of a System Based on QP
  • At the top, you can see the QP Application, which consists of multiple event-driven Active Objects.
  • The QP Application is executed by the QP Framework (shown in yellow) that supports Active Objects, State Machines, and other services.
  • The QP Framework can run "standalone" utilizing one of the built-in Real-Time Kernels
  • Alternatively, QP Framework can run on top of a 3-rd party RTOS kernel or a General-Purpose OS (e.g., Linux/POSIX or Windows).
  • At the bottom and to the side are the Target System and Board Support Package (BSP), which consists of the hardware and basic software (e.g., device drivers).

Event-Driven Paradigm

The essential characteristic of the QP Framework is that it is event-driven. That means that any processing is determined in QP by events, which are handled as quickly as possible without blocking (or polling).

Inversion of Control

Like most event-driven systems, QP Framework is based on inversion of control, which means that the control of code execution resides in the QP Framework rather than the QP Application based on QP. Specifically, to handle events, QP Framework calls the QP Application, and not the other way around. Of course, QP Applications can also call services provided in QP Framework, but the main flow of control always begins with the QP Framework.

Framework, NOT a Library

That inversion of control gives the event-driven infrastructure (QP) all the defining characteristics of a framework rather than a library.

The main difference between a framework and a library is that when you use a library, such as a conventional RTOS, you write the main body of each thread, and you call the library code that you want to reuse (e.g., a semaphore). When you use a framework, such as QP, you reuse the main body and write the code it calls (inversion of control).

Active Objects

The QP Application consists of Active Objects (Actors), which are strictly encapsulated software objects conceptually running in their own threads of execution and communicating with each other asynchronously by exchanging events.

State Machines

Each Active Object in QP has a state machine that specifies the event-driven behavior of the Active Object. The main effort of QP application developers goes into elaborating the internal state machines of the Active Objects as well as other (passive) event-driven components.

Portability & Configurability of QP

A reusable architecture for embedded systems, like QP, needs to be adaptable to a wide range of application areas and target hardware selections. For that, QP needs to be highly configurable. Whenever this SRS document mentions "configurability", it explicitly specifies which of the following types of configurability is required:

Compile-Time Configurability

Compile-time configurability means that the specific configuration option (out of a given range of possibilities) is chosen at compile-time and cannot be changed later (e.g., at run-time). Compile-time configurability is typically used for:

  • Turning features on or off
  • Determining the size (and dynamic range) of various data types
  • Determining the maximum numbers of various objects in the framework or Application
  • Determining the presence (or absence) of class members
Remarks
In C or C++, compile-time configurability is typically implemented with the preprocessor macros.

Run-Time Configurability

Run-time configurability means that the specific configuration option is chosen at run-time. Unlike compile-time configurability, run-time configurability is finer granularity, usually on an object-by-object basis. It is typically used for:

  • Choosing a specific implementation of a class operation based on the object type (polymorphism)
  • Selecting features by calling or not calling the specific QP API
  • Selecting features by passing parameters to the specific QP API

Requirements (SRS)Events