diff --git a/Examples/Smoke/Src/Main.rux b/Examples/Smoke/Src/Main.rux
index af6bd62..d170f6f 100644
--- a/Examples/Smoke/Src/Main.rux
+++ b/Examples/Smoke/Src/Main.rux
@@ -4,7 +4,7 @@
     Licensed under the MIT License
 */
 
-import MacOS::{ GetStdHandle, StdHandle, WriteFile, ReadFile, GetProcessHeap, HeapAlloc, HeapFree };
+import MacOS::{ GetStdHandle, StdHandle, WriteFile, ReadFile, GetProcessHeap, HeapAlloc, HeapFree, Munmap };
 
 // Exercises the macOS bindings end-to-end: a non-zero exit code identifies the
 // step that failed; 0 means every binding behaved as expected.
@@ -22,7 +22,7 @@ func Main() -> int32 {
 
     // heap: allocate, write, read back, free
     let heap = GetProcessHeap();
-    let buf = HeapAlloc(heap, 0, 32u) as *char8;
+    let buf = HeapAlloc(heap, 0u32, 32u) as *char8;
     if buf == null {
         return 3i32;
     }
@@ -31,7 +31,7 @@ func Main() -> int32 {
     if *buf != c8'R' || *(buf + 1u) != c8'X' {
         return 4i32;
     }
-    if HeapFree(heap, 0, buf as *opaque) == 0 {
+    if HeapFree(heap, 0u32, buf as *opaque) == 0 {
         return 5i32;
     }
 
@@ -44,5 +44,18 @@ func Main() -> int32 {
         return 6i32;
     }
 
+    // mmap/munmap roundtrip: allocate a page, write/read, then unmap it.
+    let page = HeapAlloc(heap, 0u32, 4096u) as *char8;
+    if page == null {
+        return 7i32;
+    }
+    *page = c8'M';
+    if *page != c8'M' {
+        return 8i32;
+    }
+    if Munmap(page as *opaque, 4096u) != 0i64 {
+        return 9i32;
+    }
+
     return 0i32;
 }
diff --git a/Src/Platform.rux b/Src/Platform.rux
index 5b3098a..b40b08d 100644
--- a/Src/Platform.rux
+++ b/Src/Platform.rux
@@ -57,5 +57,10 @@ module MacOS {
         /// Frees a memory block. Currently a no-op on macOS (the backing pages
         /// are not unmapped), so it always reports success.
         func HeapFree(heap: *opaque, flags: uint32, mem: *opaque) -> bool32;
+
+        /// Unmaps a memory region. Backed by SYS_munmap; addr must be the base
+        /// of a prior HeapAlloc mapping and length its full size. Returns 0 on
+        /// success.
+        func Munmap(addr: *opaque, length: uint) -> int64;
     }
 }