diff --git a/makefile b/makefile
index a02a110..c55bc00 100644
--- a/makefile
+++ b/makefile
@@ -2,23 +2,28 @@
 
 REV=${shell [ -d .hg ] && hg id -n || git rev-parse --short HEAD}
 
-.PHONY:	em emwarn debug trace fixed
+em_objs = em.o em
+em_deps = \
+  em.c regs.h emdev.h ea64v.h ea32i.h fp.h dispatch.h geom.h \
+  devpnc.h devamlc.h swap.h
 
-em:	# normal
-	rm -rf em.o
-	cc -DREV=\"${REV}\" -DNOTRACE -DFAST -O -Winline em.c -o em
+.PHONY:	emwarn debug trace fixed
 
-emwarn: # lots of compiler warnings
-	rm -rf em.o
-	cc -DREV=\"${REV}\" -DNOTRACE -DFAST -O -Wall -Wextra -pedantic -Wconversion em.c -o em
+# normal
+em: $(em_deps)
+	$(CC) -DREV=\"${REV}\" -DNOTRACE -DFAST -O -Winline em.c -o em
 
-debug:   # gdb
-	rm -rf em.o
-	cc -DREV=\"${REV}\" -DNOTRACE -DFAST -g -O0 em.c -o em
+# lots of compiler warnings
+emwarn: $(em_deps)
+	$(CC) -DREV=\"${REV}\" -DNOTRACE -DFAST -O -Wall -Wextra -pedantic -Wconversion em.c -o em
 
-trace:   # tracing
-	rm -rf em.o
-	cc -DREV=\"${REV}\" -DFAST -O em.c -o em
+# gdb
+debug: $(em_deps)
+	$(CC) -DREV=\"${REV}\" -DNOTRACE -DFAST -g -O0 em.c -o em
+
+# tracing
+trace: $(em_deps)
+	$(CC) -DREV=\"${REV}\" -DFAST -O em.c -o em
 
 # the fixed clock rate build is useful for making problems reproduceable.
 #
@@ -26,6 +31,9 @@ trace:   # tracing
 # PRIMOS.COMI to get a more consistent instruction count for the
 # failure, then enable tracing a little before that with -trace <IC - 100>
 
-fixed:  # fixed clock rate
-	rm -rf em.o
-	cc -DREV=\"${REV}\" -DFIXEDCLOCK -DNOIDLE -DFAST -O em.c -o em
+# fixed clock rate
+fixed: $(em_deps)
+	$(CC) -DREV=\"${REV}\" -DFIXEDCLOCK -DNOIDLE -DFAST -O em.c -o em
+
+clean:
+	rm -f $(em_objs)