Building RTOS: A Beginner’s Guide to Embedded OS Development

This article will take you through the core concepts of Real-Time Operating Systems (RTOS) in a straightforward manner, gradually guiding you to create a small yet fully functional RTOS. We will cover the basic components of RTOS, task scheduling, portability, and task synchronization mechanisms.

1. Basic Components of RTOS

A minimal RTOS typically includes the following key components:

• • Scheduler: Responsible for scheduling the execution of tasks based on predefined policies (such as priority, round-robin, etc.). It determines which task runs at what time.
• • Task Control Block (TCB): This is a data structure that stores context information for each task, such as stack pointer, task state, priority, and waiting semaphore, etc. TCB is central to the scheduler’s management of tasks. A simple TCB structure example is as follows:

typedef struct tcb_s {
    unsigned int state; // Task state (e.g., running, ready, blocked)
    unsigned int priority; // Task priority
    unsigned int *stack_ptr; // Stack pointer
    void *(*task_func)(void *); // Task function pointer
    unsigned int sem; // Waiting semaphore
    // ... other fields ...
} tcb_t;

• • Portable Layer: This part of the code contains hardware-related operations, such as context switching, interrupt handling, and critical section protection. By separating hardware-related code from the core logic of RTOS, the portability of RTOS can be enhanced.
• • Synchronization: Used to coordinate the execution of multiple tasks, preventing issues such as data races and deadlocks. Common synchronization mechanisms include Semaphores and Mutexes.

2. Task Scheduling Algorithms

The scheduler is the core of RTOS, responsible for selecting the next task to run. A simple scheduling algorithm is priority-based preemptive scheduling:

1. 1. Find the highest priority ready task: The scheduler scans all tasks’ TCBs to find the one with the highest priority that is in the ready state.
2. 2. Context Switching: If a higher priority ready task is found, the scheduler saves the context of the currently running task into its TCB, then loads the context of the highest priority task to start running it. Context switching typically involves saving and restoring CPU registers.
3. 3. Task Execution: The selected task begins to run until one of the following occurs:

• • The task voluntarily yields the CPU (e.g., calls the yield() function).
• • A higher priority task becomes ready.
• • The task is blocked (e.g., waiting for a semaphore).

3. Implementation of the Portable Layer

The portable layer is the hardware abstraction layer of the RTOS, which masks the differences between different hardware platforms, allowing RTOS code to be easily ported to different target platforms. Key functions include:

• • Context Save and Restore (context_save, context_restore): This depends on the target CPU architecture and needs to be implemented based on the CPU register situation.
• • Interrupt Handling: Handling various interrupt requests, such as timer interrupts, external interrupts, etc. The interrupt handler may need to perform context switching to ensure real-time performance.
• • Critical Section Protection (enter_critical, exit_critical): Used to protect shared resources, preventing multiple tasks from accessing them simultaneously, leading to data inconsistency. This is typically achieved by disabling and enabling interrupts.

4. Task Synchronization and Mutual Exclusion

Semaphores are important mechanisms for implementing task synchronization and mutual exclusion. A semaphore can represent the availability of a resource or event.

• • wait(semaphore): The task calls the wait function to wait for a semaphore. If the semaphore is available, it continues running; otherwise, the task is blocked until the semaphore becomes available.
• • post(semaphore): When an event occurs or a resource becomes available, other tasks or interrupt handlers call the post function to release the semaphore, waking up the tasks waiting for that semaphore.

5. Building a Simple RTOS

Here is a simple RTOS architecture example (for reference only, actual implementation needs to consider more details and optimizations):

1. 1. Initialization: Initialize the TCB array, scheduler, semaphores, etc.
2. 2. Create Tasks: Add task functions and related information to the TCB array.
3. 3. Scheduling Loop: The scheduler enters a loop, continuously searching for the highest priority ready task and performing context switching.
4. 4. Task Execution: Each task executes within its allocated time slice.
5. 5. Interrupt Handling: The interrupt service routine handles hardware interrupts and may need to update task states or release semaphores.

Conclusion

Building an RTOS requires a deep understanding of operating system principles and the target hardware architecture. This article provides a simplified RTOS architecture to help you get started. The actual RTOS implementation is much more complex than described here, requiring consideration of more factors such as memory management, inter-task communication, error handling, etc. However, understanding the basic concepts presented in this article will lay a solid foundation for your RTOS development journey. Remember, practice is the best way to learn, and hands-on experience in building a simple RTOS will help you better understand these concepts.