diff --git a/asm/Makefile b/asm/Makefile
index 5d4b046..e288671 100644
--- a/asm/Makefile
+++ b/asm/Makefile
@@ -2,10 +2,17 @@
 all:
 	false
 
+DEPSDIR := .d
+$(shell mkdir -p $(DEPSDIR))
+
 x230.G2HT35WW.img: ../x230.G2HT35WW.img
 	cp $< $@
 
-ec.img: ec.asm x230.G2HT35WW.img
+-include $(DEPSDIR)/ec.img.deps
+$(DEPSDIR)/ec.img.deps: Makefile
+	nasm -MT ec.img -M ec.asm >$@
+
+%.img: %.asm
 	nasm -f bin -o $@ $<
 
 diff: ec.img x230.G2HT35WW.img