diff --git a/nucleus/src/arch/aarch64/memory/features.rs b/nucleus/src/arch/aarch64/memory/features.rs
index fd93aa3..7dd4412 100644
--- a/nucleus/src/arch/aarch64/memory/features.rs
+++ b/nucleus/src/arch/aarch64/memory/features.rs
@@ -1,3 +1,5 @@
+//! Print MMU suported features for debugging.
+
 use {
     crate::println,
     cortex_a::regs::{RegisterReadOnly, RegisterReadWrite, ID_AA64MMFR0_EL1, SCTLR_EL1, TCR_EL1},
diff --git a/nucleus/src/arch/aarch64/memory/mmu.rs b/nucleus/src/arch/aarch64/memory/mmu.rs
index a63820d..ab310c6 100644
--- a/nucleus/src/arch/aarch64/memory/mmu.rs
+++ b/nucleus/src/arch/aarch64/memory/mmu.rs
@@ -282,6 +282,7 @@ pub trait HierarchicalLevel: TableLevel {
     // fn translate() -> Directory<NextLevel>;
 }
 
+/// Specify allowed page size for each level.
 pub trait HierarchicalPageLevel: TableLevel {
     /// Size of the page that can be contained in this table level.
     type PageLevel: PageSize;
@@ -479,9 +480,11 @@ where
 /// Errors from mapping layer
 #[derive(Debug, Snafu)]
 pub enum TranslationError {
+    /// No page found. @todo
     NoPage,
 }
 
+/// Virtual address space. @todo
 pub struct VirtSpace {
     l0: Unique<Directory<L0PageGlobalDirectory>>,
 }
diff --git a/nucleus/src/arch/aarch64/memory/mod.rs b/nucleus/src/arch/aarch64/memory/mod.rs
index 12dbef9..22cb456 100644
--- a/nucleus/src/arch/aarch64/memory/mod.rs
+++ b/nucleus/src/arch/aarch64/memory/mod.rs
@@ -29,9 +29,11 @@ pub use addr::VirtAddr;
 pub use page_size::PageSize;
 pub use phys_frame::PhysFrame;
 
-// @todo ??
+/// @todo ??
 pub trait FrameAllocator {
+    /// Allocate a physical memory frame.
     fn allocate_frame(&mut self) -> Option<PhysFrame>; // @todo Result<>
+    /// Deallocate a physical frame.
     fn deallocate_frame(&mut self, frame: PhysFrame);
 }