A Real-Time Operating System (RTOS) is an operating system designed for embedded systems that need to run applications in real-time. Unlike general-purpose operating systems, an RTOS provides a predictable and deterministic response to system events within strict timing constraints. In other words, an RTOS guarantees that a task will be executed within a specified amount of time, allowing embedded developers to build applications with hard real-time requirements.
In this document, we will cover the basics of RTOS in C++, including the following topics:
- RTOS fundamentals
- Task management
- Scheduling algorithms
- Inter-task communication
- Memory management
- Interrupt handling
An RTOS is designed to provide deterministic behavior by allowing an application to be split into multiple tasks, each with its own priority and execution time. Tasks are the basic building blocks of an RTOS application and can be thought of as independent programs running concurrently within a single system.
The RTOS scheduler is responsible for managing the execution of these tasks based on their priorities and execution time requirements. The scheduler is typically implemented as a timer interrupt that triggers at a fixed interval and selects the highest priority task that is ready to run.
In an RTOS, tasks are managed by the scheduler and can be created, deleted, and suspended dynamically. Task creation involves allocating memory for the task stack, setting the task priority, and specifying the entry point function.
The scheduler uses various scheduling algorithms to select the next task to run. The most common algorithms are:
- Priority-based scheduling
- Round-robin scheduling
- Earliest Deadline First (EDF) scheduling
Each algorithm has its own advantages and disadvantages and is suitable for different types of applications.
In an RTOS application, tasks often need to communicate with each other to share data or coordinate their activities. There are several mechanisms for inter-task communication, including:
- Message queues
- Semaphores
- Mutexes
- Event flags
Memory management is a critical aspect of an RTOS application, as tasks must be able to allocate and release memory dynamically. An RTOS typically provides several memory management schemes, including:
- Static memory allocation
- Dynamic memory allocation
- Memory pooling
Interrupts are a crucial part of embedded systems, and an RTOS must provide a mechanism for handling interrupts in a timely and deterministic manner. In an RTOS, interrupt handling is typically done by using interrupt service routines (ISRs) that communicate with tasks using message queues or other inter-task communication mechanisms.