diff --git a/src/runtime/runtime_unix.go b/src/runtime/runtime_unix.go
index 464d8dfe..aa3515f1 100644
--- a/src/runtime/runtime_unix.go
+++ b/src/runtime/runtime_unix.go
@@ -16,8 +16,9 @@ func usleep(usec uint) int
 //export malloc
 func malloc(size uintptr) unsafe.Pointer
 
+// void *mmap(void *addr, size_t length, int prot, int flags, int fd, off_t offset);
 //export mmap
-func mmap(addr unsafe.Pointer, length, prot, flags, fd int, offset int) unsafe.Pointer
+func mmap(addr unsafe.Pointer, length uintptr, prot, flags, fd int, offset int) unsafe.Pointer
 
 //export abort
 func abort()
diff --git a/src/runtime/runtime_unix_heap.go b/src/runtime/runtime_unix_heap.go
index cee1f1e1..9224369a 100644
--- a/src/runtime/runtime_unix_heap.go
+++ b/src/runtime/runtime_unix_heap.go
@@ -5,8 +5,10 @@
 
 package runtime
 
-var heapSize uintptr = 128 * 1024          // small amount to start
-const heapMaxSize = 1 * 1024 * 1024 * 1024 // 1GB for the entire heap
+import "unsafe"
+
+var heapSize uintptr = 128 * 1024 // small amount to start
+var heapMaxSize uintptr
 
 var heapStart, heapEnd uintptr
 
@@ -14,9 +16,21 @@ func preinit() {
 	// Allocate a large chunk of virtual memory. Because it is virtual, it won't
 	// really be allocated in RAM. Memory will only be allocated when it is
 	// first touched.
-	addr := mmap(nil, heapMaxSize, flag_PROT_READ|flag_PROT_WRITE, flag_MAP_PRIVATE|flag_MAP_ANONYMOUS, -1, 0)
-	heapStart = uintptr(addr)
-	heapEnd = heapStart + heapSize
+	heapMaxSize = 1 * 1024 * 1024 * 1024 // 1GB for the entire heap
+	for {
+		addr := mmap(nil, heapMaxSize, flag_PROT_READ|flag_PROT_WRITE, flag_MAP_PRIVATE|flag_MAP_ANONYMOUS, -1, 0)
+		if addr == unsafe.Pointer(^uintptr(0)) {
+			// Heap was too big to be mapped by mmap. Reduce the maximum size.
+			// We might want to make this a bit smarter than simply halving the
+			// heap size.
+			// This can happen on 32-bit systems.
+			heapMaxSize /= 2
+			continue
+		}
+		heapStart = uintptr(addr)
+		heapEnd = heapStart + heapSize
+		break
+	}
 }
 
 // growHeap tries to grow the heap size. It returns true if it succeeds, false