FreeRTOS 4-Level Priority Task Scheduler Simulation

Led a 3-person team in the design and implementation of this project.

This project was developed as part of the 2025-2026 Fall Semester Operating Systems Course, with the goal of simulating the task scheduling logic of the FreeRTOS kernel in a PC (Linux/POSIX) environment.

The project implements a 4-level priority queue structure and a Multi-level Feedback Queue (MLFQ) scheduling algorithm for managing both Real-Time and User tasks.

Features & Algorithm

The simulation is built around a structure called the "Dispatcher", which operates according to the following rules:

1. Real-Time Tasks (Priority 0):

   • Operates on FCFS (First Come First Served) logic.
   • Has the highest priority.
   • Executed without interruption (non-preemptive) until completion.

2. User Tasks (Priority 1, 2, 3):

   • Operates in a three-level Multi-level Feedback Queue structure.
   • Time Quantum (q): 1 second.
   • If a task runs for 1 second without completing, its priority is decreased (e.g. P1 -> P2).
   • If all tasks drop to the lowest level (Priority 3), Round Robin scheduling takes over.

Development Environment & Compiler Versions

This project has been tested on both Linux and Windows platforms and was developed using the following compiler versions:

1. Linux Environment (WSL)

Both the Linux binary and the Windows .exe file (via cross-compilation) were produced in this environment.

• Linux Compiler (GCC): gcc (GCC) 15.2.1 20251211
• Windows Cross Compiler (MinGW-w64): x86_64-w64-mingw32-gcc (GCC) 15.2.1 20250808

2. Windows Environment (Native)

Environment used for native compilation on Windows as an alternative:

• Windows Compiler (MSYS2): gcc (Rev2, Built by MSYS2 project) 14.2.0

Directory Structure

The project directory structure is as follows:

FreeRTOS_PC_Scheduler/
 bin/                 # Compiled output files
 FreeRTOS/            # FreeRTOS kernel source files
 include/             # Project header (.h) files
    common.h         
    FreeRTOSConfig.h 
    job_loader.h     
    job_queue.h      
    job_worker.h     
    scheduler.h      
    xtask.h          
 lib/                 # External libraries
 src/                 # Source (.c) files
    job_loader.c     
    job_queue.c      
    job_worker.c     
    main.c           
    scheduler.c      
    xtask.c          
 giris.txt            # Simulation input data
 Makefile             # Build configuration
 README.md            # Project documentation

Installation & Build

This project was prepared using POSIX standards. The following steps can be used to compile the project:

Build Steps

Open a terminal in the project directory and run the following command:

make            # Linux/WSL Environment
mingw32-make    # Windows Environment

This will produce an executable named freertos_sim.

To clean previous builds:

mingw32-make clean  # Windows
make clean          # Linux

Running

To start the simulation, pass the compiled program and the task list text file as an argument:

# If you are using Windows:
.\bin\windows\freertos_sim.exe .\giris.txt

# If you are using Linux/WSL:
./bin/linux/freertos_sim giris.txt

Input File Format (giris.txt)

Each line in the file represents one task, in the following format: <arrival_time>, <priority>, <duration>

Example:

12, 0, 1  (arrives at second 12, Priority 0 (RT), duration 1s)
12, 1, 2  (arrives at second 12, Priority 1 (User), duration 2s)
13, 3, 6  (arrives at second 13, Priority 3 (User), duration 6s)

Sample Output

When the program runs, you will see the task states and color-coded output in the terminal. Below is the output produced by the provided giris.txt file: