diff --git a/core/definitions.mk b/core/definitions.mk
index 8aacf1d8b..7e53c0508 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -274,6 +274,19 @@ $(patsubst ./%,%, \
   )
 endef
 
+###########################################################
+## Find all of the S files under the named directories.
+## Meant to be used like:
+##    SRC_FILES := $(call all-c-files-under,src tests)
+###########################################################
+
+define all-S-files-under
+$(patsubst ./%,%, \
+  $(shell cd $(LOCAL_PATH) ; \
+          find -L $(1) -name "*.S" -and -not -name ".*") \
+ )
+endef
+
 ###########################################################
 ## Find all of the html files under the named directories.
 ## Meant to be used like: