feat(sched): implement preemptive time-slicing multitasking via PIT 8254 timer ISR (Vector 32)

kernel/asm/timer_isr.S

@@ -0,0 +1,49 @@
+/* opencoreC - Timer Preemptive Interrupt Trampoline (Vector 32 / 0x20) */
+.global timer_isr_entry
+.extern timer_interrupt_handler
+
+.section .text
+timer_isr_entry:
+    /* 1. Save all general-purpose registers to form a complete interrupt context frame */
+    push rax
+    push rbx
+    push rcx
+    push rdx
+    push rsi
+    push rdi
+    push rbp
+    push r8
+    push r9
+    push r10
+    push r11
+    push r12
+    push r13
+    push r14
+    push r15
+
+    /* 2. Pass current saved stack pointer in RDI */
+    mov rdi, rsp
+    call timer_interrupt_handler
+
+    /* 3. Switch stack pointer to returned value (RAX contains next thread's RSP) */
+    mov rsp, rax
+
+    /* 4. Restore all general-purpose registers */
+    pop r15
+    pop r14
+    pop r13
+    pop r12
+    pop r11
+    pop r10
+    pop r9
+    pop r8
+    pop rbp
+    pop rdi
+    pop rsi
+    pop rdx
+    pop rcx
+    pop rbx
+    pop rax
+
+    /* 5. Return from interrupt (IRETQ restores RIP, CS, RFLAGS, RSP, SS) */
+    iretq

kernel/src/arch/idt.rs

@@ -1,6 +1,8 @@
 //! Interrupt Descriptor Table (IDT) and Exception Handling
 
 use crate::kprintln;
+use crate::sched::SCHEDULER;
+use crate::drivers::timer::send_eoi;
 use core::mem::size_of;
 
 #[repr(C, packed)]
@@ -49,6 +51,7 @@ static mut IDT: [IdtEntry; 256] = [IdtEntry::missing(); 256];
 
 extern "C" {
     fn default_isr_stub();
+    fn timer_isr_entry();
 }
 
 // Global assembly ISR stub for default exception handling
@@ -85,6 +88,10 @@ pub unsafe fn init() {
         (*idt_ptr.add(i)).set_handler(stub_addr, 0, 0);
     }
 
+    // Set Timer Interrupt (Vector 32 / 0x20)
+    let timer_addr = timer_isr_entry as *const () as usize as u64;
+    (*idt_ptr.add(32)).set_handler(timer_addr, 0, 0);
+
     let descriptor = IdtDescriptor {
         limit: (size_of::<[IdtEntry; 256]>() - 1) as u16,
         base: idt_ptr as u64,
@@ -97,3 +104,15 @@ pub unsafe fn init() {
 pub extern "C" fn raw_exception_handler(stack_ptr: *const u64) {
     kprintln!("[CPU EXCEPTION] Trap triggered at stack frame: {:p}", stack_ptr);
 }
+
+#[no_mangle]
+pub extern "C" fn timer_interrupt_handler(saved_rsp: u64) -> u64 {
+    unsafe {
+        // Send End of Interrupt to controller
+        send_eoi();
+
+        // Perform preemptive scheduling quantum check
+        let sched = &mut *core::ptr::addr_of_mut!(SCHEDULER);
+        sched.preempt(saved_rsp)
+    }
+}

kernel/src/drivers/mod.rs

@@ -1 +1,2 @@
 pub mod serial;
+pub mod timer;

kernel/src/drivers/timer.rs

@@ -0,0 +1,74 @@
+//! System Timer & PIC (Programmable Interval Timer 8254 on Vector 32)
+
+use crate::kprintln;
+
+const PIC1_COMMAND: u16 = 0x20;
+const PIC1_DATA: u16    = 0x21;
+const PIC2_COMMAND: u16 = 0xA0;
+const PIC2_DATA: u16    = 0xA1;
+
+const PIT_CHANNEL0_DATA: u16 = 0x40;
+const PIT_COMMAND_MODE: u16  = 0x43;
+
+const PIC_EOI: u8 = 0x20;
+
+#[inline]
+unsafe fn outb(port: u16, val: u8) {
+    core::arch::asm!("out dx, al", in("dx") port, in("al") val, options(nomem, nostack, preserves_flags));
+}
+
+#[inline]
+unsafe fn io_wait() {
+    outb(0x80, 0);
+}
+
+/// Send End of Interrupt to PIC
+pub unsafe fn send_eoi() {
+    outb(PIC1_COMMAND, PIC_EOI);
+}
+
+/// Remap PIC offsets to 32 (0x20) and 40 (0x28)
+unsafe fn remap_pic() {
+    // ICW1: Start initialization in cascade mode
+    outb(PIC1_COMMAND, 0x11);
+    io_wait();
+    outb(PIC2_COMMAND, 0x11);
+    io_wait();
+
+    // ICW2: Vector offsets (Master = 32, Slave = 40)
+    outb(PIC1_DATA, 0x20);
+    io_wait();
+    outb(PIC2_DATA, 0x28);
+    io_wait();
+
+    // ICW3: Cascade wiring
+    outb(PIC1_DATA, 0x04);
+    io_wait();
+    outb(PIC2_DATA, 0x02);
+    io_wait();
+
+    // ICW4: 8086 mode
+    outb(PIC1_DATA, 0x01);
+    io_wait();
+    outb(PIC2_DATA, 0x01);
+    io_wait();
+
+    // Mask all IRQs except IRQ 0 (Timer)
+    outb(PIC1_DATA, 0xFE); // Enable only IRQ0 (bit 0 = 0)
+    outb(PIC2_DATA, 0xFF); // Disable all on Slave
+}
+
+/// Initialize PIT to tick at target Hz (e.g. 100 Hz = 10 ms quantum)
+pub unsafe fn init(target_hz: u32) {
+    remap_pic();
+
+    let divisor = (1193182 / target_hz) as u16;
+
+    // Channel 0, lobyte/hibyte, Rate Generator (Mode 2)
+    outb(PIT_COMMAND_MODE, 0x36);
+    outb(PIT_CHANNEL0_DATA, (divisor & 0xFF) as u8);
+    outb(PIT_CHANNEL0_DATA, ((divisor >> 8) & 0xFF) as u8);
+
+    kprintln!("[TIMER] PIT 8254 initialized at {} Hz ({} ms time slice quantum).",
+        target_hz, 1000 / target_hz);
+}

kernel/src/main.rs

@@ -17,6 +17,7 @@ use sched::SCHEDULER;
 core::arch::global_asm!(include_str!("../asm/context.S"));
 core::arch::global_asm!(include_str!("../asm/syscall_entry.S"));
 core::arch::global_asm!(include_str!("../asm/ring3_enter.S"));
+core::arch::global_asm!(include_str!("../asm/timer_isr.S"));
 
 #[no_mangle]
 pub extern "C" fn _start() -> ! {
@@ -60,21 +61,18 @@ pub extern "C" fn _start() -> ! {
         mm::init(memmap_resp);
     }
 
-    // 6. Initialize IPC & Capability System
+    // 6. Initialize Preemptive Hardware Timer (PIT 8254 @ 100 Hz)
+    kprintln!("[BOOT] Configuring PIT 8254 timer for Preemptive Multi-Tasking...");
+    unsafe {
+        drivers::timer::init(100);
+    }
+
+    // 7. Initialize IPC & Capability System
     ipc::init();
 
-    // 7. Initialize Scheduler
+    // 8. Initialize Scheduler
     sched::init();
 
-    // 8. Self-Test Fast-Path IPC Ping
-    kprintln!("[TEST] Executing Kernel Fast-Path IPC Self-Test...");
-    let (status, resp_op, val) = ipc::fastpath::handle_fastpath_call(
-        ipc::fastpath::CAP_KERNEL_CONTROL,
-        ipc::fastpath::IPC_OP_PING,
-        42, 0, 0, 0
-    );
-    kprintln!("[TEST] IPC Fast-Path Ping Result: status={}, opcode={:#x}, val={}", status, resp_op, val);
-
     // 9. Inspect and Load Initial Userspace Processes
     unsafe {
         let mod_resp = MODULE_REQUEST.response;
@@ -102,7 +100,7 @@ pub extern "C" fn _start() -> ! {
                             proc.pml4_paddr,
                             proc.kernel_stack_top,
                         );
-                        kprintln!("[BOOT] Registered Thread '{}' with TID {:?}", name, tid);
+                        kprintln!("[BOOT] Registered Preemptible Thread '{}' with TID {:?}", name, tid);
                     }
                     Err(err) => {
                         kprintln!("[ERROR] Failed to load module [{}]: {}", i, err);
@@ -110,8 +108,10 @@ pub extern "C" fn _start() -> ! {
                 }
             }
 
-            kprintln!("[BOOT] Starting Multi-Tasking & IPC Rendezvous...");
+            kprintln!("[BOOT] Enabling Interrupts & Starting Preemptive Scheduler...");
             kprintln!("-------------------------------------------------------");
+
+            // Pick first ready thread and switch to it with interrupts enabled
             (*sched_ptr).schedule();
         } else {
             kprintln!("[BOOT] No userspace modules detected. Staying in Ring 0 idle loop.");
@@ -122,7 +122,7 @@ pub extern "C" fn _start() -> ! {
 
     loop {
         unsafe {
-            core::arch::asm!("hlt");
+            core::arch::asm!("sti; hlt");
         }
     }
 }

kernel/src/sched/mod.rs

@@ -11,27 +11,34 @@ extern "C" {
     fn switch_to(prev_rsp: *mut u64, next_rsp: u64);
 }
 
+const DEFAULT_QUANTUM_TICKS: u32 = 5; // 5 ticks @ 100Hz = 50ms time slice
+
 pub struct Scheduler {
     pub threads: [Thread; MAX_THREADS],
     pub current_tid: usize,
     pub next_tid: u64,
+    pub quantum_remaining: u32,
+    pub ticks: u64,
 }
 
 pub static mut SCHEDULER: Scheduler = Scheduler {
     threads: [const { Thread::empty() }; MAX_THREADS],
     current_tid: 0,
     next_tid: 1,
+    quantum_remaining: DEFAULT_QUANTUM_TICKS,
+    ticks: 0,
 };
 
 impl Scheduler {
     pub fn init(&mut self) {
-        // Slot 0 is reserved for Kernel Idle/Bootstrap thread
         self.threads[0].id = 0;
         self.threads[0].name = "kernel_idle";
         self.threads[0].state = ThreadState::Running;
         self.current_tid = 0;
         self.next_tid = 1;
-        kprintln!("[SCHED] Scheduler initialized (Multi-Thread TCB table ready).");
+        self.quantum_remaining = DEFAULT_QUANTUM_TICKS;
+        self.ticks = 0;
+        kprintln!("[SCHED] Preemptive Multi-Tasking Scheduler initialized (Quantum: {} ticks).", DEFAULT_QUANTUM_TICKS);
     }
 
     pub fn create_user_thread(
@@ -55,23 +62,33 @@ impl Scheduler {
                 self.threads[i].user_rsp = user_rsp;
                 self.threads[i].user_rip = entry_rip;
 
-                // Setup initial kernel stack for IRETQ return
+                // Build initial interrupt frame for timer_isr_entry / iretq return:
+                // [kstack - 5]: SS (0x1B)
+                // [kstack - 4]: RSP (user_rsp)
+                // [kstack - 3]: RFLAGS (0x202)
+                // [kstack - 2]: CS (0x23)
+                // [kstack - 1]: RIP (entry_rip)
+                // [kstack - 20..-6]: 15 GPRs (rax, rbx, rcx, rdx, rsi, rdi, rbp, r8..r15)
                 unsafe {
                     let kstack = kernel_stack_top as *mut u64;
-                    let stack_ptr = kstack.sub(16);
+                    let frame_ptr = kstack.sub(20);
 
-                    *stack_ptr.add(0) = 0; // r15
+                    // GPRs (all 0)
-                    *stack_ptr.add(1) = 0; // r14
+                    for j in 0..15 {
-                    *stack_ptr.add(2) = 0; // r13
+                        *frame_ptr.add(j) = 0;
-                    *stack_ptr.add(3) = 0; // r12
-                    *stack_ptr.add(4) = 0; // rbx
-                    *stack_ptr.add(5) = 0; // rbp
-                    *stack_ptr.add(6) = initial_user_trampoline as *const () as usize as u64; // RIP for switch_to
-
-                    self.threads[i].context.rsp = stack_ptr as u64;
                     }
 
-                kprintln!("[SCHED] Created User Thread [TID {}] '{}' (RIP: {:#x}, RSP: {:#x})",
+                    // IRETQ Frame
+                    *frame_ptr.add(15) = entry_rip;  // RIP
+                    *frame_ptr.add(16) = 0x23;       // CS (User Code)
+                    *frame_ptr.add(17) = 0x0202;     // RFLAGS (IF enabled)
+                    *frame_ptr.add(18) = user_rsp;   // RSP (User Stack)
+                    *frame_ptr.add(19) = 0x1B;       // SS (User Data)
+
+                    self.threads[i].context.rsp = frame_ptr as u64;
+                }
+
+                kprintln!("[SCHED] Created Preemptible Thread [TID {}] '{}' (RIP: {:#x}, RSP: {:#x})",
                     tid, name, entry_rip, user_rsp);
                 return Some(tid);
             }
@@ -109,14 +126,72 @@ impl Scheduler {
         if self.threads[self.current_tid].state == ThreadState::Running {
             self.threads[self.current_tid].state = ThreadState::Ready;
         }
+        self.quantum_remaining = DEFAULT_QUANTUM_TICKS;
         self.schedule();
     }
 
+    /// Preemptive tick called directly from timer ISR handler
+    pub fn preempt(&mut self, saved_rsp: u64) -> u64 {
+        self.ticks += 1;
+
+        if self.quantum_remaining > 0 {
+            self.quantum_remaining -= 1;
+            if self.quantum_remaining > 0 {
+                return saved_rsp; // Keep running current thread
+            }
+        }
+
+        // Time slice quantum expired -> Preempt current thread!
+        self.quantum_remaining = DEFAULT_QUANTUM_TICKS;
+
+        let prev_idx = self.current_tid;
+        self.threads[prev_idx].context.rsp = saved_rsp;
+        if self.threads[prev_idx].state == ThreadState::Running {
+            self.threads[prev_idx].state = ThreadState::Ready;
+        }
+
+        // Find next Ready thread (Round-Robin)
+        let mut next_idx = (prev_idx + 1) % MAX_THREADS;
+        let mut found = false;
+        for _ in 0..MAX_THREADS {
+            if self.threads[next_idx].state == ThreadState::Ready {
+                found = true;
+                break;
+            }
+            next_idx = (next_idx + 1) % MAX_THREADS;
+        }
+
+        if !found {
+            // Restore previous thread if it's still runnable
+            if self.threads[prev_idx].state == ThreadState::Ready {
+                self.threads[prev_idx].state = ThreadState::Running;
+            }
+            return saved_rsp;
+        }
+
+        self.current_tid = next_idx;
+        self.threads[next_idx].state = ThreadState::Running;
+
+        let next_pml4 = self.threads[next_idx].pml4_paddr;
+        let next_kstack = self.threads[next_idx].kernel_stack_top;
+        let next_rsp = self.threads[next_idx].context.rsp;
+
+        unsafe {
+            if next_pml4 != 0 {
+                (*core::ptr::addr_of_mut!(VMM)).load_cr3(next_pml4);
+            }
+            if next_kstack != 0 {
+                set_kernel_stack(next_kstack);
+            }
+        }
+
+        next_rsp
+    }
+
     pub fn schedule(&mut self) {
         let prev_idx = self.current_tid;
         let mut next_idx = (prev_idx + 1) % MAX_THREADS;
 
-        // Find next Ready thread (Round-Robin)
         let mut found = false;
         for _ in 0..MAX_THREADS {
             if self.threads[next_idx].state == ThreadState::Ready {
@@ -130,7 +205,7 @@ impl Scheduler {
             if self.threads[prev_idx].state == ThreadState::Running {
                 return;
             }
-            next_idx = 0; // idle thread
+            next_idx = 0;
         }
 
         if prev_idx == next_idx && self.threads[prev_idx].state == ThreadState::Running {
@@ -166,47 +241,3 @@ pub fn init() {
         (*core::ptr::addr_of_mut!(SCHEDULER)).init();
     }
 }
-
-#[no_mangle]
-pub extern "C" fn initial_user_trampoline() {
-    let (entry_rip, user_rsp) = unsafe {
-        let sched = &mut *core::ptr::addr_of_mut!(SCHEDULER);
-        let curr = sched.get_current_mut();
-        (curr.user_rip, curr.user_rsp)
-    };
-
-    unsafe {
-        // IRETQ to Ring 3
-        core::arch::asm!(
-            "mov ax, 0x1B",
-            "mov ds, ax",
-            "mov es, ax",
-            "mov fs, ax",
-            "mov gs, ax",
-            "push 0x1B",          // User SS
-            "push {user_rsp}",    // User RSP
-            "push 0x0202",        // RFLAGS (IF enabled)
-            "push 0x23",          // User CS
-            "push {entry_rip}",   // User RIP
-            "xor rax, rax",
-            "xor rbx, rbx",
-            "xor rcx, rcx",
-            "xor rdx, rdx",
-            "xor rsi, rsi",
-            "xor rdi, rdi",
-            "xor rbp, rbp",
-            "xor r8, r8",
-            "xor r9, r9",
-            "xor r10, r10",
-            "xor r11, r11",
-            "xor r12, r12",
-            "xor r13, r13",
-            "xor r14, r14",
-            "xor r15, r15",
-            "iretq",
-            user_rsp = in(reg) user_rsp,
-            entry_rip = in(reg) entry_rip,
-            options(noreturn)
-        );
-    }
-}