diff --git a/src/hulk/hulk.d b/src/hulk/hulk.d
index 4322c50..d59a8db 100644
--- a/src/hulk/hulk.d
+++ b/src/hulk/hulk.d
@@ -26,6 +26,7 @@ import hulk.usb;
 import hulk.pit;
 import hulk.time;
 import hulk.test;
+import hulk.thread;
 
 extern extern(C) __gshared ubyte _hulk_bss_size;
 
@@ -33,7 +34,7 @@ extern extern(C) __gshared ubyte _hulk_bss_size;
 private __gshared HulkHeader hulk_header = {
     &_hulk_bss_size, /* hulk_bss_size */
     &hulk_start, /* entry */
-    16u * 1024u, /* stack_size */
+    Thread.STACK_SIZE, /* stack_size */
     Hurl.HULK_BASE, /* virt_base */
     Hurl.HULK_STACK_TOP, /* virt_stack_top */
     Hurl.HULK_FRAMEBUFFER, /* virt_fb_buffer */
@@ -88,6 +89,7 @@ void hulk_start()
     Pit.initialize();
     Pci.initialize();
     Usb.initialize();
+    Thread.initialize();
 
     resume_interrupts();
 
diff --git a/src/hulk/idt.d b/src/hulk/idt.d
index 80f1a86..ce2ee9e 100644
--- a/src/hulk/idt.d
+++ b/src/hulk/idt.d
@@ -11,11 +11,12 @@ import hulk.klog;
 import hulk.apic;
 import hulk.cpu;
 import hulk.kfont;
+import hulk.thread;
 
 struct Idt
 {
     /** Interrupt stack frame indices. */
-    private static enum
+    public static enum
     {
         ISF_RBP,
         ISF_R15,
@@ -147,7 +148,7 @@ struct Idt
                 mov $$", isr, ", %rdi
                 movabs $$isr, %rax
                 callq *%rax
-                mov %rbp, %rsp
+                mov %rax, %rsp
                 popq %rbp
                 popq %r15
                 popq %r14
@@ -184,8 +185,15 @@ struct Idt
     }
 }
 
-public extern(C) void isr(ulong vector, ulong * stack_frame)
+/**
+ * Interrupt service routine.
+ *
+ * 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)
 {
+    Thread.current_thread.stack_pointer = stack_frame;
     if ((vector >= Idt.INT_APIC_BASE) && (vector < (Idt.INT_APIC_BASE + Idt.INT_APIC_COUNT)))
     {
         Apic.isr(vector);
@@ -217,4 +225,5 @@ public extern(C) void isr(ulong vector, ulong * stack_frame)
             }
         }
     }
+    return Thread.current_thread.stack_pointer;
 }
diff --git a/src/hulk/thread.d b/src/hulk/thread.d
new file mode 100644
index 0000000..9ab703a
--- /dev/null
+++ b/src/hulk/thread.d
@@ -0,0 +1,102 @@
+/**
+ * Kernel thread functionality.
+ */
+module hulk.thread;
+
+import hulk.hurl.a1;
+import hulk.idt;
+import hulk.gdt;
+import hulk.memory;
+import hulk.list;
+import hulk.cpu;
+import hulk.hurl;
+
+struct Thread
+{
+    /* TODO: currently kernel thread stack size is fixed. */
+    enum STACK_SIZE = 16 * 1024;
+
+    /** List of all kernel threads. */
+    private static __gshared List!Thread threads;
+
+    /** Currently executing kernel thread. */
+    static __gshared Thread * current_thread;
+
+    /** Current kernel thread stack pointer. */
+    ulong * stack_pointer;
+    static assert(stack_pointer.offsetof == 0);
+
+    /** Base address of kernel thread stack. */
+    void * stack_addr;
+
+    /** Interrupt stack template. */
+    static immutable __gshared ulong[Idt.ISF_COUNT] interrupt_stack_template = [
+        0x16161616_16161616u, /* RBP */
+        0x15151515_15151515u, /* R15 */
+        0x14141414_14141414u, /* R14 */
+        0x13131313_13131313u, /* R13 */
+        0x12121212_12121212u, /* R12 */
+        0x11111111_11111111u, /* R11 */
+        0x10101010_10101010u, /* R10 */
+        0x09090909_09090909u, /* R9 */
+        0x08080808_08080808u, /* R8 */
+        0x06060606_06060606u, /* RDI */
+        0x05050505_05050505u, /* RSI */
+        0x04040404_04040404u, /* RDX */
+        0x03030303_03030303u, /* RCX */
+        0x02020202_02020202u, /* RBX */
+        0x01010101_01010101u, /* RAX */
+        0u, /* Error Code */
+        0u, /* RIP */
+        Gdt.SELECTOR_KERNEL_CODE, /* CS */
+        0u, /* RFLAGS */
+        0u, /* RSP */
+        Gdt.SELECTOR_KERNEL_DATA, /* SS */
+    ];
+
+    /**
+     * Initialize a kernel thread.
+     */
+    public void initialize(void function() fn)
+    {
+        stack_addr = A1.allocate(STACK_SIZE);
+        ulong * stack_top = cast(ulong *)(stack_addr + STACK_SIZE);
+        stack_pointer = stack_top - Idt.ISF_COUNT;
+        memcpy64(stack_pointer, &interrupt_stack_template[0], Idt.ISF_COUNT);
+        stack_pointer[Idt.ISF_RIP] = cast(ulong)fn;
+        stack_pointer[Idt.ISF_RFLAGS] = read_rflags();
+        stack_pointer[Idt.ISF_RSP] = cast(ulong)stack_top;
+    }
+
+    /**
+     * Initialize main kernel thread.
+     */
+    public void initialize_main_thread()
+    {
+        stack_addr = cast(void *)(Hurl.HULK_STACK_TOP - STACK_SIZE);
+    }
+
+    /**
+     * Initialize kernel threading.
+     */
+    public static void initialize()
+    {
+        Thread * thread = A1.allocate!Thread();
+        thread.initialize_main_thread();
+        current_thread = thread;
+    }
+
+    /**
+     * Create and start a kernel thread.
+     *
+     * TODO: fn must not return.
+     * TODO: threads are always runnable.
+     * TODO: threads cannot be destroyed.
+     */
+    public static Thread * start(void function() fn)
+    {
+        Thread * thread = A1.allocate!Thread();
+        thread.initialize(fn);
+        return thread;
+    }
+}