diff --git a/context/mod.rs b/context/mod.rs
index c7c02a88375d2220d2cd14c5d707a4f2cac8ee61..f563c051c50aa194a1825a905e37f0c768aefecb 100644
--- a/context/mod.rs
+++ b/context/mod.rs
@@ -26,6 +26,7 @@ pub struct ContextList {
 }
 
 impl ContextList {
+    /// Create a new context list.
     pub fn new() -> Self {
         ContextList {
             map: BTreeMap::new(),
@@ -33,14 +34,17 @@ impl ContextList {
         }
     }
 
+    /// Get the nth context.
     pub fn get(&self, id: usize) -> Option<&RwLock<Context>> {
         self.map.get(&id)
     }
 
+    /// Get the current context.
     pub fn current(&self) -> Option<&RwLock<Context>> {
         self.map.get(&CONTEXT_ID.load(Ordering::SeqCst))
     }
 
+    /// Create a new context.
     pub fn new_context(&mut self) -> Result<&RwLock<Context>> {
         if self.next_id >= CONTEXT_MAX_CONTEXTS {
             self.next_id = 1;
@@ -56,10 +60,13 @@ impl ContextList {
 
         let id = self.next_id;
         self.next_id += 1;
+
         assert!(self.map.insert(id, RwLock::new(Context::new(id))).is_none());
-        Ok(self.map.get(&id).expect("failed to insert new context"))
+
+        Ok(self.map.get(&id).expect("Failed to insert new context. ID is out of bounds."))
     }
 
+    /// Spawn a context from a function.
     pub fn spawn(&mut self, func: extern fn()) -> Result<&RwLock<Context>> {
         let context_lock = self.new_context()?;
         {
@@ -109,6 +116,9 @@ pub fn contexts_mut() -> RwLockWriteGuard<'static, ContextList> {
 }
 
 /// Switch to the next context
+///
+/// # Safety
+///
 /// Do not call this while holding locks!
 pub unsafe fn switch() {
     use core::ops::DerefMut;
diff --git a/scheme/debug.rs b/scheme/debug.rs
index 0555b091acf624f3d839a84967c1c2cfe27c5c5a..57675079700fbbb554e4b744eab10e8bd6f9bd43 100644
--- a/scheme/debug.rs
+++ b/scheme/debug.rs
@@ -1,7 +1,7 @@
 use core::str;
 
 use syscall::Result;
-use super::Scheme;
+use super::{Scheme, Fd};
 
 pub struct DebugScheme;
 
@@ -14,21 +14,21 @@ impl Scheme for DebugScheme {
     /// Read the file `number` into the `buffer`
     ///
     /// Returns the number of bytes read
-    fn read(&mut self, _file: usize, _buffer: &mut [u8]) -> Result<usize> {
+    fn read(&mut self, _file: Fd, _buffer: &mut [u8]) -> Result<usize> {
         Ok(0)
     }
 
     /// Write the `buffer` to the `file`
     ///
     /// Returns the number of bytes written
-    fn write(&mut self, _file: usize, buffer: &[u8]) -> Result<usize> {
+    fn write(&mut self, _file: Fd, buffer: &[u8]) -> Result<usize> {
         //TODO: Write bytes, do not convert to str
         print!("{}", unsafe { str::from_utf8_unchecked(buffer) });
         Ok(buffer.len())
     }
 
     /// Close the file `number`
-    fn close(&mut self, _file: usize) -> Result<()> {
+    fn close(&mut self, _file: Fd) -> Result<()> {
         Ok(())
     }
 }
diff --git a/scheme/mod.rs b/scheme/mod.rs
index d01931c461ff909351e7c39e43d8b47b6c94f767..2e1c01550d3123b3a421e7db70cdfb062d7b45a0 100644
--- a/scheme/mod.rs
+++ b/scheme/mod.rs
@@ -17,8 +17,11 @@ use syscall::Result;
 
 use self::debug::DebugScheme;
 
+pub use self::fd::Fd;
+
 /// Debug scheme
 pub mod debug;
+mod fd;
 
 /// Scheme list type
 pub type SchemeList = BTreeMap<Box<[u8]>, Arc<Mutex<Box<Scheme + Send>>>>;
@@ -50,16 +53,16 @@ pub trait Scheme {
     /// Returns a file descriptor or an error
     fn open(&mut self, path: &[u8], flags: usize) -> Result<usize>;
 
-    /// Read the file `number` into the `buffer`
+    /// Read from some file descriptor into the `buffer`
     ///
     /// Returns the number of bytes read
-    fn read(&mut self, file: usize, buffer: &mut [u8]) -> Result<usize>;
+    fn read(&mut self, fd: Fd, buffer: &mut [u8]) -> Result<usize>;
 
-    /// Write the `buffer` to the `file`
+    /// Write the `buffer` to the file descriptor
     ///
     /// Returns the number of bytes written
-    fn write(&mut self, file: usize, buffer: &[u8]) -> Result<usize>;
+    fn write(&mut self, fd: Fd, buffer: &[u8]) -> Result<usize>;
 
-    /// Close the file `number`
-    fn close(&mut self, file: usize) -> Result<()>;
+    /// Close the file descriptor
+    fn close(&mut self, fd: Fd) -> Result<()>;
 }