diff --git a/nemu/scripts/build.mk b/nemu/scripts/build.mk
index 9873326..ff7bae5 100644
--- a/nemu/scripts/build.mk
+++ b/nemu/scripts/build.mk
@@ -22,7 +22,7 @@ CXX := g++
 endif
 LD := $(CXX)
 INCLUDES = $(addprefix -I, $(INC_PATH))
-DEFINE ?=
+DEFINE = _
 CFLAGS  := -O2 -MMD -Wall -Werror $(INCLUDES) $(CFLAGS) -D$(DEFINE)
 LDFLAGS := -O2 $(LDFLAGS)