[fix] preemptive scheduler not interleaving tasks

kernel/main.cpp

@@ -55,6 +55,10 @@ static void irq_handler(trap_frame* frame) {
     UINT8 vector = (UINT8)frame->vector;
     UINT8 irq = vector - PIC_IRQ_BASE;
 
+    // Send EOI BEFORE handling, so PIC can deliver new interrupts
+    // immediately after a context switch inside the handler.
+    pic_send_eoi(irq);
+
     switch (irq) {
         case 0:  // PIT timer
             pit_irq_handler();
@@ -62,9 +66,6 @@ static void irq_handler(trap_frame* frame) {
         default:
             break;
     }
-
-    // Send EOI to PIC
-    pic_send_eoi(irq);
 }
 
 extern "C" void kernel_main() {
@@ -156,10 +157,11 @@ extern "C" void kernel_main() {
 
     // Task A: prints messages — preemption handles time slicing
     task_create("taskA", []() {
-        for (int i = 0; i < 3; i++) {
+        for (int i = 0; i < 10; i++) {
             serial_write("[taskA] running iteration ");
             serial_write_hex(i);
             serial_write("\n");
+            for (volatile int j = 0; j < 50000000; j++) {}
         }
         serial_write("[taskA] done\n");
     });
@@ -170,6 +172,7 @@ extern "C" void kernel_main() {
             serial_write("[taskB] hello from taskB #");
             serial_write_hex(i);
             serial_write("\n");
+            for (volatile int j = 0; j < 50000000; j++) {}
         }
         serial_write("[taskB] done\n");
     });
@@ -177,6 +180,7 @@ extern "C" void kernel_main() {
     // Task C: short task
     task_create("taskC", []() {
         serial_write("[taskC] quick task\n");
+        for (volatile int j = 0; j < 50000000; j++) {}
         serial_write("[taskC] finished\n");
     });