From bae43034a6ee8414d05f77e7e30b8ea09d105632 Mon Sep 17 00:00:00 2001
From: Aiken Harris <harraiken91@gmail.com>
Date: Thu, 18 Jun 2026 11:18:36 +0200
Subject: [PATCH] Implement thread preemption and context switching in dispatch
 interrupt handler

---
 xtoskrnl/includes/ke/schedule.hh |  3 ++
 xtoskrnl/ke/dispatch.cc          | 60 ++++++++++++++++++++++++++++++--
 xtoskrnl/ke/schedule.cc          | 35 +++++++++++++++++++
 3 files changed, 96 insertions(+), 2 deletions(-)

diff --git a/xtoskrnl/includes/ke/schedule.hh b/xtoskrnl/includes/ke/schedule.hh
index f71f63f..fbbdf8d 100644
--- a/xtoskrnl/includes/ke/schedule.hh
+++ b/xtoskrnl/includes/ke/schedule.hh
@@ -19,7 +19,10 @@ namespace KE
     {
         public:
             STATIC XTFASTCALL VOID InsertDeferredReadyList(IN PKTHREAD Thread);
+            STATIC XTAPI VOID ProcessQuantumEnd(VOID);
             STATIC XTAPI VOID ProcessReadyThread(IN PKTHREAD Thread);
+            STATIC XTFASTCALL VOID QueueReadyThread(IN PKTHREAD Thread,
+                                                    IN PKPROCESSOR_CONTROL_BLOCK Prcb);
             STATIC XTAPI VOID ReadyThread(IN PKTHREAD Thread);
     };
 }
diff --git a/xtoskrnl/ke/dispatch.cc b/xtoskrnl/ke/dispatch.cc
index 33ec4d2..b051b62 100644
--- a/xtoskrnl/ke/dispatch.cc
+++ b/xtoskrnl/ke/dispatch.cc
@@ -115,8 +115,64 @@ XTCDECL
 VOID
 KE::Dispatcher::HandleDispatchInterrupt(IN PKTRAP_FRAME TrapFrame)
 {
+    PKTHREAD CurrentThread, NextThread;
+    PKPROCESSOR_CONTROL_BLOCK Prcb;
+
+    /* Receive the Processor Control Block*/
+    Prcb = KE::Processor::GetCurrentProcessorControlBlock();
+
+    /* Raise runlevel to DISPATCH level */
+    KE::RaiseRunLevel RunLevel(DISPATCH_LEVEL);
+
     /* End the interrupt */
     HL::Pic::SendEoi();
+
+    /* Check if there is pending deferred work */
+    if(Prcb->DeferredReadyListHead.Next ||
+       Prcb->DpcData[0].DpcQueueDepth ||
+       Prcb->TimerRequest)
+    {
+        UNIMPLEMENTED;
+    }
+
+    /* Re-enable hardware interrupts */
+    AR::CpuFunctions::SetInterruptFlag();
+
+    /* Check if the current thread has exhausted its execution quantum */
+    if((Prcb->CurrentThread->Quantum <= 0) && (Prcb->CurrentThread != Prcb->IdleThread))
+    {
+        /* Trigger the scheduler to recalculate thread parameters */
+        KE::Scheduler::ProcessQuantumEnd();
+    }
+    else if(Prcb->NextThread)
+    {
+        /* Start a guarded code block */
+        {
+            /* Lock the processor control block */
+            KE::SpinLockGuard ControlBlockGuard(&Prcb->PrcbLock);
+
+            /* Capture the outgoing (preempted) and incoming threads */
+            CurrentThread = Prcb->CurrentThread;
+            NextThread = Prcb->NextThread;
+
+            /* Acknowledge the pending thread and swap the pointers */
+            Prcb->NextThread = NULLPTR;
+            Prcb->CurrentThread = NextThread;
+
+            /* Update scheduling states */
+            NextThread->State = Running;
+            CurrentThread->WaitReason = WrDispatchInt;
+
+            /* Re-queue the preempted thread back into the local run queue or defer it */
+            KE::Scheduler::QueueReadyThread(CurrentThread, Prcb);
+        }
+
+        /* Perform the context switch */
+        SwitchContext(CurrentThread, APC_LEVEL);
+    }
+
+    /* Disable hardware interrupts before returning from the handler */
+    AR::CpuFunctions::ClearInterruptFlag();
 }
 
 /**
@@ -198,7 +254,7 @@ KE::Dispatcher::UpdateRunTime(IN PKTRAP_FRAME TrapFrame,
         /* Reset the adjustment threshold counter */
         ControlBlock->AdjustDpcThreshold = DPC_ADJUST_THRESHOLD;
 
-        /* Request a DISPATCH_LEVEL software interrupt to process the pending DPCs */
+        /* Request a DISPATCH level software interrupt to process the pending DPCs */
         HL::Irq::SendSoftwareInterrupt(DISPATCH_LEVEL);
 
         /* Evaluate if the DPC request rate is below the ideal threshold */
@@ -231,7 +287,7 @@ KE::Dispatcher::UpdateRunTime(IN PKTRAP_FRAME TrapFrame,
     /* Check if the thread has exhausted its quantum, ignoring the idle thread */
     if((Thread->Quantum <= 0) && (Thread != ControlBlock->IdleThread))
     {
-        /* Request a DISPATCH_LEVEL software interrupt to preempt the thread */
+        /* Request a DISPATCH level software interrupt to preempt the thread */
         HL::Irq::SendSoftwareInterrupt(DISPATCH_LEVEL);
     }
 }
diff --git a/xtoskrnl/ke/schedule.cc b/xtoskrnl/ke/schedule.cc
index c9b5ae1..320acad 100644
--- a/xtoskrnl/ke/schedule.cc
+++ b/xtoskrnl/ke/schedule.cc
@@ -36,6 +36,20 @@ KE::Scheduler::InsertDeferredReadyList(IN PKTHREAD Thread)
     RTL::LifoQueue::PushEntryList(&Prcb->DeferredReadyListHead, &Thread->SwapListEntry);
 }
 
+/**
+ * Handles the expiration of the currently executing thread's time quantum.
+ *
+ * @return This routine does not return any value.
+ *
+ * @since XT 1.0
+ */
+XTAPI
+VOID
+KE::Scheduler::ProcessQuantumEnd(VOID)
+{
+    UNIMPLEMENTED;
+}
+
 /**
  * Transitions a thread to the ready state and queues it for execution within the dispatcher database.
  *
@@ -74,6 +88,27 @@ KE::Scheduler::ProcessReadyThread(IN PKTHREAD Thread)
     }
 }
 
+/**
+ * Enqueues a thread into the target processor's execution queue.
+ *
+ * @param Thread
+ *        Supplies a pointer to the thread that is ready for execution.
+ *
+ * @param Prcb
+ *        Supplies a pointer to the Processor Control Block of the target core.
+ *
+ * @return This routine does not return any value.
+ *
+ * @since XT 1.0
+ */
+XTFASTCALL
+VOID
+KE::Scheduler::QueueReadyThread(IN PKTHREAD Thread,
+                                IN PKPROCESSOR_CONTROL_BLOCK Prcb)
+{
+    UNIMPLEMENTED;
+}
+
 /**
  * Prepares a thread for execution by safely acquiring the dispatcher database lock.
  *