diff --git a/src/hulk/cpu.d b/src/hulk/cpu.d
index aa3af81..c46fae6 100644
--- a/src/hulk/cpu.d
+++ b/src/hulk/cpu.d
@@ -254,8 +254,7 @@ void cpuid1(uint * ebx, uint * ecx, uint * edx)
     __asm("cpuid", "=*{ebx},=*{ecx},=*{edx},{eax}", ebx, ecx, edx, 1u);
 }
 
-void swint(int swint_id)()
+void swint(ubyte swint_id)()
 {
-    static assert((0 <= swint_id) && (swint_id <= 255));
-    mixin(`__asm("int $$`, swint_id, `", "");`);
+    mixin(`__asm("int $$`, cast(int)cast(uint)swint_id, `", "");`);
 }
diff --git a/src/hulk/him.d b/src/hulk/him.d
index 23ad3d2..22affb5 100644
--- a/src/hulk/him.d
+++ b/src/hulk/him.d
@@ -236,6 +236,7 @@ struct Him
             mov %rsp, %rbp
             mov %rsp, %rsi
             and $$0xFFFFFFFFFFFFFFF0, %rsp
+            mov %rsp, %rdx
             mov %rax, %rdi
             movabs $$isr, %rax
             callq *%rax
@@ -273,10 +274,13 @@ struct Him
  * This function returns the stack pointer of the thread to execute when
  * returning from this function.
  */
-public extern(C) ulong * isr(ulong vector, ulong * stack_frame)
+public extern(C) ulong * isr(ulong vector, ulong * stack_frame, ulong * rsp)
 {
     interrupt_level++;
-    Thread.current_thread.stack_pointer = stack_frame;
+    if (interrupt_level == 1)
+    {
+        Thread.current_thread.stack_pointer = stack_frame;
+    }
     if ((vector >= INT_APIC_BASE) && (vector < (INT_APIC_BASE + INT_APIC_COUNT)))
     {
         Apic.isr(vector);
@@ -292,6 +296,10 @@ public extern(C) ulong * isr(ulong vector, ulong * stack_frame)
             Apic.isr(vector);
             break;
 
+        case INT_KERNEL_SWINT:
+            Thread.handle_swint(stack_frame);
+            break;
+
         default:
             Fb.rect(0, 0, Fb.width, Kfont.line_height, 0xCC0000u);
             Klog.writefln("\n **** Unhandled ISR %u ****", vector);
@@ -310,5 +318,12 @@ public extern(C) ulong * isr(ulong vector, ulong * stack_frame)
     }
     Thread.current_thread = Thread.current_thread.list_next;
     interrupt_level--;
-    return Thread.current_thread.stack_pointer;
+    if (interrupt_level == 0)
+    {
+        return Thread.current_thread.stack_pointer;
+    }
+    else
+    {
+        return rsp;
+    }
 }
diff --git a/src/hulk/hulk.d b/src/hulk/hulk.d
index 1df1528..a19d0c2 100644
--- a/src/hulk/hulk.d
+++ b/src/hulk/hulk.d
@@ -92,11 +92,22 @@ void hulk_start()
     /* Run kernel tests. */
     Test.run();
     Klog.writefln("\a5HULK Initialization Complete!");
-
+    Thread.start(&th);
     /* Idle loop. */
     Time.msleep(1);
+    Time.msleep(500);
+    Klog.writefln("Main thread!");
     for (;;)
     {
         hlt();
     }
 }
+
+void th()
+{
+    for (;;)
+    {
+        Klog.writefln("Hello from thread.");
+        Time.msleep(1000);
+    }
+}
diff --git a/src/hulk/thread.d b/src/hulk/thread.d
index 6019bc8..536a28c 100644
--- a/src/hulk/thread.d
+++ b/src/hulk/thread.d
@@ -10,12 +10,15 @@ import hulk.memory;
 import hulk.list;
 import hulk.cpu;
 import hulk.hurl;
+import ldc.llvmasm;
 
 struct Thread
 {
     /* TODO: currently kernel thread stack size is fixed. */
     enum STACK_SIZE = 16 * 1024;
 
+    enum ulong INT_TCTRL_YIELD = 0;
+
     /** Interrupt stack template. */
     static immutable __gshared ulong[ISF_COUNT] interrupt_stack_template = [
         0x16161616_16161616u, /* RBP */
@@ -41,8 +44,8 @@ struct Thread
         Gdt.SELECTOR_KERNEL_DATA, /* SS */
     ];
 
-    /** List of all kernel threads. */
-    private static __gshared Thread * threads;
+    /** List of all runnable kernel threads. */
+    private static __gshared Thread * runnable_threads;
 
     /** Currently executing kernel thread. */
     static __gshared Thread * current_thread;
@@ -62,11 +65,9 @@ struct Thread
     public void initialize(void function() fn)
     {
         list_init();
-        if (threads == null)
-        {
-            threads = &this;
-        }
+        suspend_interrupts();
         list_insert_after(current_thread);
+        resume_interrupts();
         stack_addr = A1.allocate(STACK_SIZE);
         ulong * stack_top = cast(ulong *)(stack_addr + STACK_SIZE);
         stack_pointer = stack_top - ISF_COUNT;
@@ -77,15 +78,12 @@ struct Thread
     }
 
     /**
-     * Initialize main kernel thread.
+     * Initialize first kernel thread.
      */
-    public void initialize_main_thread()
+    public void initialize_first_thread()
     {
         list_init();
-        if (threads == null)
-        {
-            threads = &this;
-        }
+        runnable_threads = &this;
         stack_addr = cast(void *)(Hurl.HULK_STACK_TOP - STACK_SIZE);
     }
 
@@ -95,7 +93,7 @@ struct Thread
     public static void initialize()
     {
         Thread * thread = A1.allocate!Thread();
-        thread.initialize_main_thread();
+        thread.initialize_first_thread();
         current_thread = thread;
     }
 
@@ -110,6 +108,36 @@ struct Thread
     {
         Thread * thread = A1.allocate!Thread();
         thread.initialize(fn);
+        swint(INT_TCTRL_YIELD);
         return thread;
     }
+
+    /**
+     * Trigger a thread management software interrupt.
+     */
+    public static void swint(T1)(T1 v1)
+    {
+        mixin(`__asm("int $$`, cast(int)INT_KERNEL_SWINT, `", "{rax}", v1);`);
+    }
+
+    /**
+     * Handle a thread management software interrupt.
+     *
+     * This function is called in interrupt context.
+     *
+     * @param stack_frame
+     *   Pointer to the stack frame.
+     */
+    public static void handle_swint(ulong * stack_frame)
+    {
+        switch (stack_frame[ISF_RAX])
+        {
+        case INT_TCTRL_YIELD:
+            Thread.current_thread = Thread.current_thread.list_next;
+            break;
+
+        default:
+            break;
+        }
+    }
 }