diff --git a/abi/src/lib.rs b/abi/src/lib.rs
index 3d8b3a1..c72406a 100644
--- a/abi/src/lib.rs
+++ b/abi/src/lib.rs
@@ -2,7 +2,9 @@
 #![feature(asm)]
 #![no_std]
 
+/// A trait that allows setting a struct back to its default value.
 pub trait SetDefault {
+    /// Set this struct back to its default value.
     fn set_default(&mut self);
 }
 
@@ -59,6 +61,7 @@ pub enum SystemCall {
     },
 }
 
+/// Represents a task buffer used for system calls.
 #[derive(Debug)]
 pub struct TaskBuffer {
     pub call: Option<SystemCall>
diff --git a/kernel/src/cap/channel.rs b/kernel/src/cap/channel.rs
index 9185bc8..1d05025 100644
--- a/kernel/src/cap/channel.rs
+++ b/kernel/src/cap/channel.rs
@@ -5,14 +5,21 @@ use util::managed_arc::{ManagedArc, ManagedArcAny, ManagedWeakPool3Arc};
 
 use super::{UntypedDescriptor};
 
+/// Channel descriptor.
 #[derive(Debug)]
 pub struct ChannelDescriptor {
     value: Option<u64>,
     next: Option<ManagedArcAny>,
 }
+/// Channel capability. Reference-counted smart pointer to channel
+/// descriptor.
+///
+/// Channels are used for inter-process communication of different
+/// tasks.
 pub type ChannelCap = ManagedArc<RwLock<ChannelDescriptor>>;
 
 impl ChannelCap {
+    /// Create a channel capability from an untyped capability.
     pub fn retype_from(untyped: &mut UntypedDescriptor) -> Self {
         let mut arc: Option<Self> = None;
 
@@ -32,10 +39,13 @@ impl ChannelCap {
 }
 
 impl ChannelDescriptor {
+    /// Put a value to the channel.
     pub fn put(&mut self, value: u64) {
         self.value = Some(value);
     }
 
+    /// Take a value from the channel. If there's no value in the
+    /// channel, `None` is returned.
     pub fn take(&mut self) -> Option<u64> {
         self.value.take()
     }
diff --git a/kernel/src/cap/cpool.rs b/kernel/src/cap/cpool.rs
index 0465c90..7e6e09f 100644
--- a/kernel/src/cap/cpool.rs
+++ b/kernel/src/cap/cpool.rs
@@ -6,39 +6,57 @@ use util::managed_arc::{ManagedArc, ManagedArcAny, ManagedWeakPool256Arc};
 
 use super::{UntypedDescriptor};
 
+/// Capability pool descriptor.
 #[derive(Debug)]
 pub struct CPoolDescriptor {
     weak_pool: ManagedWeakPool256Arc,
     next: Option<ManagedArcAny>,
 }
+/// Capability pool capability. Reference-counted smart pointer to
+/// capability pool descriptor. Capability pool itself is a
+/// `ManagedWeakPool` with 256 entries.
+///
+/// Capability pool capability is used to hold multiple capabilities
+/// together so as to be addressable in user-space programs.
 pub type CPoolCap = ManagedArc<RwLock<CPoolDescriptor>>;
 
 impl CPoolDescriptor {
+    /// Create a new pointer to a capability descriptor using the
+    /// index. If nothing is in the entry, `None` is returned.
     pub fn upgrade_any(&self, index: usize) -> Option<ManagedArcAny> {
         unsafe { self.weak_pool.read().upgrade_any(index, |ptr, type_id| { super::upgrade_any(ptr, type_id) }) }
     }
 
+    /// Like `upgrade_any`, but returns a value with the specified
+    /// type.
     pub fn upgrade<T: Any>(&self, index: usize) -> Option<ManagedArc<T>>
         where ManagedArc<T>: Any {
         self.weak_pool.read().upgrade(index)
     }
 
+    /// Downgrade a capability into the capability pool (weak pool) at
+    /// a specified index.
     pub fn downgrade_at<T: Any>(&self, arc: &ManagedArc<T>, index: usize)
         where ManagedArc<T>: Any {
         self.weak_pool.read().downgrade_at(arc, index)
     }
 
+    /// Downgrade a capability into the capability pool (weak pool) at
+    /// a free index.
     pub fn downgrade_free<T: Any>(&self, arc: &ManagedArc<T>) -> Option<usize>
         where ManagedArc<T>: Any {
         self.weak_pool.read().downgrade_free(arc)
     }
 
+    /// Size of the capability pool.
     pub fn size(&self) -> usize {
         256
     }
 }
 
 impl CPoolCap {
+    /// Create a capability pool capability from an untyped
+    /// capability.
     pub fn retype_from(untyped: &mut UntypedDescriptor) -> Self {
         let mut arc: Option<Self> = None;
 
diff --git a/kernel/src/cap/mod.rs b/kernel/src/cap/mod.rs
index deb6634..64b6ebf 100644
--- a/kernel/src/cap/mod.rs
+++ b/kernel/src/cap/mod.rs
@@ -1,6 +1,10 @@
+/// Untyped capability implementation.
 mod untyped;
+/// Capability pool capability implementation.
 mod cpool;
+/// Task capability implementation.
 mod task;
+/// Channel capability implementation.
 mod channel;
 
 pub use self::untyped::{UntypedDescriptor, UntypedCap};
@@ -15,8 +19,11 @@ use core::any::{TypeId};
 use util::managed_arc::{ManagedWeakPool256Arc, ManagedArcAny, ManagedArc};
 
 pub use abi::{SetDefault, TaskBuffer};
+/// Raw page struct representing a whole page.
 pub struct RawPage(pub [u8; PAGE_LENGTH]);
+/// Raw page capability. Represents a page with no other information.
 pub type RawPageCap = PageCap<RawPage>;
+/// Task buffer page capability. Represents a page of task buffer.
 pub type TaskBufferPageCap = PageCap<TaskBuffer>;
 
 impl SetDefault for RawPage {
@@ -27,6 +34,16 @@ impl SetDefault for RawPage {
     }
 }
 
+/// Create a managed Arc (capability) from an address of an kernel
+/// object (architecture-specific or general). The `type_id` should be
+/// a [TypeId](https://doc.rust-lang.org/std/any/struct.TypeId.html)
+/// of a capability. If the `type_id` is not recognized, `None` is
+/// returned.
+///
+/// # Safety
+///
+/// `ptr` must be a physical address pointing to a valid kernel object
+/// of type `type_id`.
 pub unsafe fn upgrade_any(ptr: PAddr, type_id: TypeId) -> Option<ManagedArcAny> {
     if type_id == TypeId::of::<CPoolCap>() {
         Some(unsafe { ManagedArc::from_ptr(ptr): CPoolCap }.into())
@@ -45,6 +62,9 @@ pub unsafe fn upgrade_any(ptr: PAddr, type_id: TypeId) -> Option<ManagedArcAny>
     }
 }
 
+/// Drop an architecture-specific `any` capability. `ManagedArcAny` is
+/// not itself droppable. It must be converted to its real type before
+/// dropping.
 pub fn drop_any(any: ManagedArcAny) {
     if any.is::<CPoolCap>() {
         any.into(): CPoolCap;
diff --git a/kernel/src/cap/task.rs b/kernel/src/cap/task.rs
index 4d2d5a0..937b240 100644
--- a/kernel/src/cap/task.rs
+++ b/kernel/src/cap/task.rs
@@ -7,6 +7,9 @@ use arch::{TaskRuntime, Exception};
 
 use super::{UntypedDescriptor, TopPageTableCap, CPoolCap, TaskBufferPageCap, ChannelCap};
 
+/// Switch to an idle task that runs in kernel-mode. This is used when
+/// no other tasks is runnable. Like normal context switching, this
+/// returns only when exceptions (interrupts) happen.
 pub fn idle() -> Exception {
     #[naked]
     unsafe fn idle_task() -> ! {
@@ -22,6 +25,7 @@ pub fn idle() -> Exception {
     }
 }
 
+/// Represent a task status.
 #[derive(Debug, Clone)]
 pub enum TaskStatus {
     Active,
@@ -29,6 +33,7 @@ pub enum TaskStatus {
     Inactive,
 }
 
+/// Task descriptor.
 #[derive(Debug)]
 pub struct TaskDescriptor {
     weak_pool: ManagedWeakPool3Arc,
@@ -37,9 +42,14 @@ pub struct TaskDescriptor {
     next_task: Option<TaskCap>,
     status: TaskStatus
 }
+/// Task capability. Reference-counted smart pointer to task
+/// descriptor.
+///
+/// Tasks represents isolated processes running.
 pub type TaskCap = ManagedArc<RwLock<TaskDescriptor>>;
 
 impl TaskCap {
+    /// Create a task capability from an untyped capability.
     pub fn retype_from(untyped: &mut UntypedDescriptor) -> Self {
         let mut arc: Option<Self> = None;
 
@@ -68,46 +78,58 @@ impl TaskCap {
 }
 
 impl TaskDescriptor {
+    /// Set the task's instruction pointer.
     pub fn set_instruction_pointer(&mut self, instruction_pointer: VAddr) {
         self.runtime.set_instruction_pointer(instruction_pointer)
     }
 
+    /// Set the task's stack pointer.
     pub fn set_stack_pointer(&mut self, stack_pointer: VAddr) {
         self.runtime.set_stack_pointer(stack_pointer)
     }
 
+    /// Set the task's root capability pool.
     pub fn downgrade_cpool(&self, cpool: &CPoolCap) {
         self.weak_pool.read().downgrade_at(cpool, 0)
     }
 
+    /// Read from the task's root capability pool.
     pub fn upgrade_cpool(&self) -> Option<CPoolCap> {
         self.weak_pool.read().upgrade(0)
     }
 
+    /// Set the task's top page table.
     pub fn downgrade_top_page_table(&self, pml4: &TopPageTableCap) {
         self.weak_pool.read().downgrade_at(pml4, 1)
     }
 
+    /// Read from the task's top page table.
     pub fn upgrade_top_page_table(&self) -> Option<TopPageTableCap> {
         self.weak_pool.read().upgrade(1)
     }
 
+    /// Set the task's buffer.
     pub fn downgrade_buffer(&self, buffer: &TaskBufferPageCap) {
         self.weak_pool.read().downgrade_at(buffer, 2)
     }
 
+    /// Read from the task's buffer.
     pub fn upgrade_buffer(&self) -> Option<TaskBufferPageCap> {
         self.weak_pool.read().upgrade(2)
     }
 
+    /// Current task status.
     pub fn status(&self) -> TaskStatus {
         self.status.clone()
     }
 
+    /// Set the current task status.
     pub fn set_status(&mut self, status: TaskStatus) {
         self.status = status;
     }
 
+    /// Switch to the task. The function is returned when exception
+    /// happens.
     pub fn switch_to(&mut self) -> Exception {
         if let Some(pml4) = self.upgrade_top_page_table() {
             pml4.write().switch_to();
@@ -116,8 +138,12 @@ impl TaskDescriptor {
     }
 }
 
+/// The first task initialized by the kernel.
 static FIRST_TASK: Mutex<Option<TaskCap>> = Mutex::new(None);
 
+/// Register a new task. Using `FIRST_TASK` static, this forms a
+/// linked-list that allows an iterator to iterate over all created
+/// tasks.
 fn register_task(cap: TaskCap) {
     let mut first_task = FIRST_TASK.lock();
     if first_task.is_none() {
@@ -132,6 +158,7 @@ fn register_task(cap: TaskCap) {
     }
 }
 
+/// A task iterator.
 pub struct TaskIterator {
     next: Option<TaskCap>,
 }
@@ -152,6 +179,7 @@ impl Iterator for TaskIterator {
     }
 }
 
+/// Return a task iterator using `FIRST_TASK`.
 pub fn task_iter() -> TaskIterator {
     TaskIterator {
         next: FIRST_TASK.lock().clone(),
diff --git a/kernel/src/cap/untyped.rs b/kernel/src/cap/untyped.rs
index 11d9820..0cb421e 100644
--- a/kernel/src/cap/untyped.rs
+++ b/kernel/src/cap/untyped.rs
@@ -2,6 +2,7 @@ use common::*;
 use util::{RwLock, align_up};
 use util::managed_arc::{ManagedArc, ManagedArcAny};
 
+/// Untyped descriptor.
 #[derive(Debug)]
 pub struct UntypedDescriptor {
     start_paddr: PAddr,
@@ -9,9 +10,20 @@ pub struct UntypedDescriptor {
     watermark: PAddr,
     first_child: Option<ManagedArcAny>
 }
+/// Untyped capability. Reference-counted smart pointer to untyped
+/// descriptor.
+///
+/// Untyped capability represents free memory that can be retyped to
+/// different useful capabilities.
 pub type UntypedCap = ManagedArc<RwLock<UntypedDescriptor>>;
 
 impl UntypedCap {
+    /// Bootstrap an untyped capability using a memory region information.
+    ///
+    /// # Safety
+    ///
+    /// Can only be used for free memory regions returned from
+    /// `InitInfo`.
     pub unsafe fn bootstrap(start_paddr: PAddr, length: usize) -> Self {
         let des_paddr = align_up(start_paddr, UntypedCap::inner_alignment());
         assert!(des_paddr + UntypedCap::inner_length() <= start_paddr + length);
@@ -28,14 +40,19 @@ impl UntypedCap {
 }
 
 impl UntypedDescriptor {
+    /// Length of the untyped region.
     pub fn length(&self) -> usize {
         self.length
     }
 
+    /// Start physical address of the untyped region.
     pub fn start_paddr(&self) -> PAddr {
         self.start_paddr
     }
 
+    /// Allocate a memory region using the given length and
+    /// alignment. Shift the watermark of the current descriptor
+    /// passing over the allocated region.
     pub unsafe fn allocate(&mut self, length: usize, alignment: usize) -> PAddr {
         let paddr = align_up(self.watermark, alignment);
         assert!(paddr + length <= self.start_paddr + self.length);
@@ -44,6 +61,8 @@ impl UntypedDescriptor {
         paddr
     }
 
+    /// Derive and allocate a memory region to a capability that
+    /// requires memory region.
     pub unsafe fn derive<F>(&mut self, length: usize, alignment: usize, f: F) where F: FnOnce(PAddr, Option<ManagedArcAny>) -> ManagedArcAny {
         let paddr = self.allocate(length, alignment);
         self.first_child = Some(f(paddr, self.first_child.take()));