MIT-Curricular/OS/C/Week12/rtos.c

#include <stdio.h>

// Define Task structure (simplified for memorization)
typedef struct {
    int id;                 // Task ID
    int period;             // Period (also deadline for simplicity)
    int execution_time;     // Worst-case execution time (WCET)
    // --- Simulation State ---
    int remaining_execution; // Remaining execution time for current instance
    int absolute_deadline;   // Absolute deadline for current instance
    int time_to_arrival;     // Time until the next instance arrives/is released
} Task;

// --- Global Variables ---
// Define the tasks for the simulation (Example Set)
// Format: {id, Period, ExecutionTime, 0, 0, 0} <-- Initial state values
Task tasks[] = {
    {1, 5, 2, 0, 0, 0},  // Task 1: Period=5, Exec Time=2
    {2, 8, 3, 0, 0, 0}   // Task 2: Period=8, Exec Time=3
    // Add more tasks here if needed
};
// Calculate number of tasks automatically
int num_tasks = sizeof(tasks) / sizeof(Task);
// Set simulation duration (e.g., Hyperperiod or a fixed time)
// LCM(5, 8) = 40
int simulation_time = 40;

// --- Rate Monotonic (RM) Simulation ---
void simulate_rm() {
    printf("--- Rate Monotonic Scheduling ---\n");
    // Reset task states for the simulation run
    for (int i = 0; i < num_tasks; i++) {
        tasks[i].remaining_execution = 0;
        tasks[i].absolute_deadline = 0;
        tasks[i].time_to_arrival = 0; // All tasks start at time 0
    }

    // Main simulation loop
    for (int time = 0; time < simulation_time; time++) {
        // 1. Check for task arrivals (release time)
        for (int i = 0; i < num_tasks; i++) {
            if (tasks[i].time_to_arrival == 0) {
                // Check if the previous instance of this task missed its deadline
                if (tasks[i].remaining_execution > 0) {
                    printf("!!! Time %d: Task %d MISSED DEADLINE !!!\n", time, tasks[i].id);
                    // Simple handling: Continue with the new instance, old one is lost
                }
                // Release new instance of the task
                tasks[i].remaining_execution = tasks[i].execution_time;
                tasks[i].absolute_deadline = time + tasks[i].period; // Deadline = Period
                tasks[i].time_to_arrival = tasks[i].period; // Set timer for the *next* arrival
            }
            tasks[i].time_to_arrival--; // Decrement time until the next arrival for all tasks
        }

        // 2. Select highest priority task to run (RM: Shortest Period has highest priority)
        int task_to_run = -1; // -1 indicates CPU Idle
        int highest_priority = 10000; // Initialize with a low priority (large period)

        for (int i = 0; i < num_tasks; i++) {
            // Check if task is ready (has arrived and needs execution)
            if (tasks[i].remaining_execution > 0) {
                // RM priority check: Lower period value means higher priority
                if (tasks[i].period < highest_priority) {
                    highest_priority = tasks[i].period;
                    task_to_run = i; // Select this task
                }
            }
        }

        // 3. Execute the selected task (or remain idle)
        if (task_to_run != -1) {
            // Task selected to run
            printf("Time %d: Task %d running\n", time, tasks[task_to_run].id);
            tasks[task_to_run].remaining_execution--; // Execute for one time unit

            // Optional: Check if task just finished
            // if (tasks[task_to_run].remaining_execution == 0) {
            //     printf("Time %d: Task %d finished\n", time + 1, tasks[task_to_run].id);
            // }
        } else {
            // No task ready to run
            printf("Time %d: CPU Idle\n", time);
        }
    }
    printf("--- RM Simulation Complete ---\n");
}

// --- Earliest Deadline First (EDF) Simulation ---
void simulate_edf() {
    printf("\n--- Earliest Deadline First Scheduling ---\n");
    // Reset task states
    for (int i = 0; i < num_tasks; i++) {
        tasks[i].remaining_execution = 0;
        tasks[i].absolute_deadline = 0;
        tasks[i].time_to_arrival = 0;
    }

    // Main simulation loop
    for (int time = 0; time < simulation_time; time++) {
        // 1. Check for task arrivals (same as RM)
        for (int i = 0; i < num_tasks; i++) {
            if (tasks[i].time_to_arrival == 0) {
                if (tasks[i].remaining_execution > 0) {
                    printf("!!! Time %d: Task %d MISSED DEADLINE !!!\n", time, tasks[i].id);
                }
                tasks[i].remaining_execution = tasks[i].execution_time;
                tasks[i].absolute_deadline = time + tasks[i].period;
                tasks[i].time_to_arrival = tasks[i].period;
            }
            tasks[i].time_to_arrival--;
        }

        // 2. Select highest priority task to run (EDF: Earliest Absolute Deadline has highest priority)
        int task_to_run = -1;
        int earliest_deadline = 10000; // Initialize with a late deadline

        for (int i = 0; i < num_tasks; i++) {
            // Check if task is ready
            if (tasks[i].remaining_execution > 0) {
                // EDF priority check: Lower deadline value means higher priority (earlier deadline)
                if (tasks[i].absolute_deadline < earliest_deadline) {
                    earliest_deadline = tasks[i].absolute_deadline;
                    task_to_run = i; // Select this task
                }
            }
        }

        // 3. Execute the selected task (same as RM)
        if (task_to_run != -1) {
            printf("Time %d: Task %d running\n", time, tasks[task_to_run].id);
            tasks[task_to_run].remaining_execution--;
            // Optional: Check finish
            // if (tasks[task_to_run].remaining_execution == 0) {
            //     printf("Time %d: Task %d finished\n", time + 1, tasks[task_to_run].id);
            // }
        } else {
            printf("Time %d: CPU Idle\n", time);
        }
    }
    printf("--- EDF Simulation Complete ---\n");
}

// --- Main Function ---
int main() {
    // Run Rate Monotonic simulation
    simulate_rm();

    // Run Earliest Deadline First simulation
    simulate_edf();

    return 0; // Indicate successful execution
}