Chapter 5

Operating Systems Overview

• Definition: Software managing hardware resources.
• Key Characteristics:
  • Resource management.
  • Acts as an interface between applications and hardware.
  • Offers a library of functions for applications.

Embedded Operating Systems (OS)

• Fusion of application and OS:
  • Resides mostly in ROM.
• Characteristics:
  • Less overhead.
  • Configurable (only required components are included).
  • Device drivers often moved out of OS to tasks.

Desktop vs Embedded OS

Feature	Desktop OS	Embedded OS
Compilation	Separate	Together with application
Startup	Applications run first	Application starts RTOS

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Real-Time Systems (RTS)

Definition: Systems where correctness depends on both logical results and timing.

Characteristics

• Controlled by sensors and actuators.
• Strict timing constraints (deadlines):
  • Periodic Tasks: Time-driven (e.g., flashing LEDs).
  • Aperiodic Tasks: Event-driven (e.g., airbag deployment).

Deadline Types

• Soft: Minor delay acceptable.
• Firm: No delay allowed but no catastrophic outcome.
• Hard: Catastrophic outcome if missed.

Goals of RTOS

• Meet deadlines with deterministic scheduling.
• Efficient resource management with minimal overhead.

graph TD
    A[Real-Time Systems] --> B[Periodic Tasks]
    A --> C[Aperiodic Tasks]
    B --> D[Examples: Flash LEDs, Monitoring System]
    C --> E[Examples: Airbags, Button Press]

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RTOS Requirements

• Predictability:
  • Upper bound on execution time for services.
  • Deterministic scheduling policies.
• Precise Timing Services:
  • High resolution.
  • Short interrupt disabling periods.

Functionality of RTOS Kernel

• Processor management
• Memory management
• Timer management resource management
• Task management (resume, wait, etc.)
• Inter-task communication
• Task synchronization

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FreeRTOS Overview

• Purpose: Multitasking facilities with a simple scheduler.
• Features:
  • Fully preemptive with priority-based multitasking.
  • Supports communication (queues) and resource sharing (semaphores).
  • Minimal CPU and memory footprint.

Tasks in FreeRTOS

• Definition: Basic unit of scheduling (similar to threads).
• Structure:
  • Must return void and take a void pointer parameter.
  • Run in infinite loops and do not return.

void ATaskFunction(void *pvParameters) {
    for (;;) {
        // Task implementation
    }
}

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Task States in FreeRTOS

graph LR
    A[Not Running] --> B[Ready]
    B --> C[Running]
    A --> D[Blocked]
    A --> E[Suspended]
    D --> F[Event Triggered]
    E --> G[Resume Command]
    D --> H[Delay Expired]

Description of States

• Running: Actively executing (one at a time on single-core systems).
• Ready: Ready to execute but waiting for higher-priority tasks to finish.
• Blocked: Waiting for an event (e.g., timer, resource availability).
• Suspended: Explicitly halted and resumed via API calls.