diff --git a/src/pkg/runtime/Makefile b/src/pkg/runtime/Makefile
index f9f40baddb..662c9407b3 100644
--- a/src/pkg/runtime/Makefile
+++ b/src/pkg/runtime/Makefile
@@ -138,7 +138,7 @@ runtime.acid: runtime.h proc.c
 chan.acid: runtime.h chan.c
 	$(CC) -a chan.c >chan.acid
 
-$(GOARCH)/asm.h: runtime.acid mkasmh
-	./mkasmh >$@.x
+$(GOARCH)/asm.h: runtime.acid mkasmh.sh
+	./mkasmh.sh >$@.x
 	mv -f $@.x $@
 
diff --git a/src/pkg/runtime/mkasmh b/src/pkg/runtime/mkasmh.sh
similarity index 93%
rename from src/pkg/runtime/mkasmh
rename to src/pkg/runtime/mkasmh.sh
index 5316eada30..6e6ebfca98 100755
--- a/src/pkg/runtime/mkasmh
+++ b/src/pkg/runtime/mkasmh.sh
@@ -7,7 +7,7 @@ set -e
 
 cat <<'EOF'
 // Assembly constants.
-// AUTOMATICALLY GENERATED BY mkasmh DURING BUILD
+// AUTOMATICALLY GENERATED BY mkasmh.sh DURING BUILD
 
 EOF