diff --git a/src/lib/Makefile b/src/lib/Makefile
index 7b68551..a50bdf0 100644
--- a/src/lib/Makefile
+++ b/src/lib/Makefile
@@ -18,6 +18,7 @@
 	reflect\
 	regexp\
 	strconv\
+	sync\
 	tabwriter\
 	time\
 
@@ -95,4 +96,5 @@
 strconv.dirinstall: os.dirinstall utf8.install
 tabwriter.dirinstall: os.dirinstall io.dirinstall container/array.dirinstall
 time.dirinstall: once.install os.dirinstall
+sync.dirinstall:
 
diff --git a/src/lib/sync/Makefile b/src/lib/sync/Makefile
new file mode 100644
index 0000000..86aeeef
--- /dev/null
+++ b/src/lib/sync/Makefile
@@ -0,0 +1,56 @@
+# Copyright 2009 The Go Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+# DO NOT EDIT.  Automatically generated by gobuild.
+# gobuild -m >Makefile
+O=6
+GC=$(O)g
+CC=$(O)c -w
+AS=$(O)a
+AR=$(O)ar
+
+default: packages
+
+clean:
+	rm -f *.$O *.a $O.out
+
+test: packages
+	gotest
+
+coverage: packages
+	gotest
+	6cov -g `pwd` | grep -v '_test\.go:'
+
+%.$O: %.go
+	$(GC) $*.go
+
+%.$O: %.c
+	$(CC) $*.c
+
+%.$O: %.s
+	$(AS) $*.s
+
+O1=\
+	asm_$(GOARCH).$O\
+	mutex.$O\
+
+sync.a: a1
+
+a1:	$(O1)
+	$(AR) grc sync.a asm_$(GOARCH).$O mutex.$O
+	rm -f $(O1)
+
+newpkg: clean
+	$(AR) grc sync.a
+
+$(O1): newpkg
+
+nuke: clean
+	rm -f $(GOROOT)/pkg/sync.a
+
+packages: sync.a
+
+install: packages
+	cp sync.a $(GOROOT)/pkg/sync.a
+
diff --git a/src/lib/sync/asm_amd64.s b/src/lib/sync/asm_amd64.s
new file mode 100644
index 0000000..07389dd
--- /dev/null
+++ b/src/lib/sync/asm_amd64.s
@@ -0,0 +1,23 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// func cas(val *int32, old, new int32) bool
+// Atomically:
+//	if *val == old {
+//		*val = new;
+//		return true;
+//	}else
+//		return false;
+TEXT sync·cas(SB), 7, $0
+	MOVQ	8(SP), BX
+	MOVL	16(SP), AX
+	MOVL	20(SP), CX
+	LOCK
+	CMPXCHGL	CX, 0(BX)
+	JZ ok
+	MOVL	$0, 24(SP)
+	RET
+ok:
+	MOVL	$1, 24(SP)
+	RET
diff --git a/src/lib/sync/mutex.go b/src/lib/sync/mutex.go
new file mode 100644
index 0000000..accf55a
--- /dev/null
+++ b/src/lib/sync/mutex.go
@@ -0,0 +1,39 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sync
+
+package func cas(val *int32, old, new int32) bool
+
+export type Mutex struct {
+	key int32;
+	sema int32;
+}
+
+func xadd(val *int32, delta int32) (new int32) {
+	for {
+		v := *val;
+		if cas(val, v, v+delta) {
+			return v+delta;
+		}
+	}
+	panic("unreached")
+}
+
+func (m *Mutex) Lock() {
+	if xadd(&m.key, 1) == 1 {
+		// changed from 0 to 1; we hold lock
+		return;
+	}
+	sys.semacquire(&m.sema);
+}
+
+func (m *Mutex) Unlock() {
+	if xadd(&m.key, -1) == 0 {
+		// changed from 1 to 0; no contention
+		return;
+	}
+	sys.semrelease(&m.sema);
+}
+
diff --git a/src/lib/sync/mutex_test.go b/src/lib/sync/mutex_test.go
new file mode 100644
index 0000000..7a6dd18
--- /dev/null
+++ b/src/lib/sync/mutex_test.go
@@ -0,0 +1,53 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// GOMAXPROCS=10 gotest
+
+package sync
+
+import (
+	"sync";
+	"testing"
+)
+
+func HammerSemaphore(s *int32, cdone *chan bool) {
+	for i := 0; i < 1000; i++ {
+		sys.semacquire(s);
+		sys.semrelease(s);
+	}
+	cdone <- true;
+}
+
+export func TestSemaphore(t *testing.T) {
+	s := new(int32);
+	*s = 1;
+	c := new(chan bool);
+	for i := 0; i < 10; i++ {
+		go HammerSemaphore(s, c);
+	}
+	for i := 0; i < 10; i++ {
+		<-c;
+	}
+}
+
+
+func HammerMutex(m *Mutex, cdone *chan bool) {
+	for i := 0; i < 1000; i++ {
+		m.Lock();
+		m.Unlock();
+	}
+	cdone <- true;
+}
+
+export func TestMutex(t *testing.T) {
+	m := new(Mutex);
+	c := new(chan bool);
+	for i := 0; i < 10; i++ {
+		go HammerMutex(m, c);
+	}
+	for i := 0; i < 10; i++ {
+		<-c;
+	}
+}
+