diff --git a/src/sync/mod.rs b/src/sync/mod.rs
index f9e469a..f9a22b4 100644
--- a/src/sync/mod.rs
+++ b/src/sync/mod.rs
@@ -1,6 +1,6 @@
 mod mutex;
 mod condvar;
-//mod semaphore;
+mod semaphore;
 pub mod test;
 
 pub use self::mutex::*;
diff --git a/src/sync/semaphore.rs b/src/sync/semaphore.rs
index 6ee6be7..bbe8ef3 100644
--- a/src/sync/semaphore.rs
+++ b/src/sync/semaphore.rs
@@ -1,2 +1,73 @@
-use super::mutex;
+//! A counting, blocking, semaphore.
+//!
+//! Borrowed from std at rust 1.7.0
 
+use super::SpinNoIrqLock as Mutex;
+use super::Condvar;
+
+/// A counting, blocking, semaphore.
+pub struct Semaphore {
+    lock: Mutex<isize>,
+    cvar: Condvar,
+}
+
+/// An RAII guard which will release a resource acquired from a semaphore when
+/// dropped.
+pub struct SemaphoreGuard<'a> {
+    sem: &'a Semaphore,
+}
+
+impl Semaphore {
+    /// Creates a new semaphore with the initial count specified.
+    ///
+    /// The count specified can be thought of as a number of resources, and a
+    /// call to `acquire` or `access` will block until at least one resource is
+    /// available. It is valid to initialize a semaphore with a negative count.
+    pub fn new(count: isize) -> Semaphore {
+        Semaphore {
+            lock: Mutex::new(count),
+            cvar: Condvar::new(),
+        }
+    }
+
+    /// Acquires a resource of this semaphore, blocking the current thread until
+    /// it can do so.
+    ///
+    /// This method will block until the internal count of the semaphore is at
+    /// least 1.
+    pub fn acquire(&self) {
+        let mut count = self.lock.lock();
+        while *count <= 0 {
+            // TODO: -> cvar.wait(count)
+            drop(count);
+            self.cvar.wait();
+            count = self.lock.lock();
+        }
+        *count -= 1;
+    }
+
+    /// Release a resource from this semaphore.
+    ///
+    /// This will increment the number of resources in this semaphore by 1 and
+    /// will notify any pending waiters in `acquire` or `access` if necessary.
+    pub fn release(&self) {
+        *self.lock.lock() += 1;
+        self.cvar.notify_one();
+    }
+
+    /// Acquires a resource of this semaphore, returning an RAII guard to
+    /// release the semaphore when dropped.
+    ///
+    /// This function is semantically equivalent to an `acquire` followed by a
+    /// `release` when the guard returned is dropped.
+    pub fn access(&self) -> SemaphoreGuard {
+        self.acquire();
+        SemaphoreGuard { sem: self }
+    }
+}
+
+impl<'a> Drop for SemaphoreGuard<'a> {
+    fn drop(&mut self) {
+        self.sem.release();
+    }
+}
\ No newline at end of file