Tasking

Define the kernel task structure and task states. Why it matters: Every schedulable unit needs a descriptor holding its saved registers, state, stack pointer, and metadata. Focus on `Task` and `TaskState` in `kernel/src/task/mod.rs` while you trace the snippet.

pub struct Task {
    /// Unique task identifier — not yet read outside this module.
    #[allow(dead_code)]
    pub id: TaskId,
    /// Human-readable name — not yet read outside this module.
    #[allow(dead_code)]
    pub name: &'static str,
    pub state: TaskState,
    /// RSP saved by `switch_context` when this task is not running.
    pub saved_rsp: u64,
    /// Owns the allocated kernel stack — dropped when the `Task` is dropped.
    _stack: Box<[u8]>,
}

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Add a round-robin scheduler and ready queue. Why it matters: The scheduler determines which task runs next and drives all cooperative and preemptive multitasking. Focus on `Scheduler` and `Ready` in `kernel/src/task/scheduler.rs` while you trace the snippet.

struct Scheduler {
    tasks: Vec<Task>,
    /// Index of the last non-idle task that was dispatched (for round-robin).
    last_run: usize,
    /// Index of the task currently holding the CPU, or `None` while the
    /// scheduler loop itself is running.
    current: Option<usize>,
    /// Index of the dedicated idle task in `tasks`, registered via
    /// [`spawn_idle`].  The idle task is excluded from the normal round-robin
    /// rotation and is selected only when no non-idle task is `Ready`.
    idle_task: Option<usize>,
}

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Trigger scheduling from the timer interrupt. Why it matters: Timer-driven preemption ensures no single task monopolizes the CPU, which is the foundation of a multitasking OS. Focus on `timer_handler` and `signal_reschedule` in `kernel/src/arch/x86_64/interrupts.rs` while you trace the snippet.

extern "x86-interrupt" fn timer_handler(_stack_frame: InterruptStackFrame) {
    TICK_COUNT.fetch_add(1, Ordering::Relaxed);
    crate::task::signal_reschedule();
    unsafe {
        PICS.lock()
            .notify_end_of_interrupt(InterruptIndex::Timer as u8);
    }
}

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