diff --git a/asm/runtime.scm b/asm/runtime.scm
index 820d945..3bb4726 100644
--- a/asm/runtime.scm
+++ b/asm/runtime.scm
@@ -31,7 +31,7 @@ alloc-do-gc
 (drop)
 (drop)
 ;; Run garbage collection
-(push gc-start)
+(push gc-run)
 (call)
 ;; Tail-call allocation
 (push alloc)
@@ -123,6 +123,19 @@ reg-relocated
 (drop)
 (return)
 
+gc-loop
+(ref free)
+(ref scan)
+(<)
+(branch gc-loop-done)
+(ref scan)
+(relocate-reg)
+(push gc-loop)
+(jmp)
+gc-loop-done
+(return)
+
+
 main
 ;; These need to be initialized with the runtime
 (push memory)
diff --git a/scmvm/assembler.scm b/scmvm/assembler.scm
index f2f8b31..ad527a1 100644
--- a/scmvm/assembler.scm
+++ b/scmvm/assembler.scm
@@ -76,7 +76,10 @@
       (when (eq? (car inst) 'push)
         (if (number? (cadr inst))
             (write-word (cadr inst))
-            (write-word (assq-ref labels (cadr inst)))))
+            (let ([address (assq-ref labels (cadr inst))])
+              (if address
+                  (write-word (assq-ref labels (cadr inst)))
+                  (error (format #f "Could not find label ~a" (cadr inst)))))))
       (assembly-pass (cdr seq) labels))]))
 
 (define (assemble instructions port)