diff --git a/npc/Makefile b/npc/Makefile
index 2fa4357..efeb871 100644
--- a/npc/Makefile
+++ b/npc/Makefile
@@ -2,6 +2,8 @@ VERILATOR := verilator
 
 VSRC := $(wildcard vsrc/*.v)
 CPPSRC := $(wildcard csrc/*.cpp)
+PREFIX ?= .
+OBJDIR := $(PREFIX)/obj
 
 all:
 	@echo "Write this Makefile by your self."
@@ -11,6 +13,6 @@ sim: obj_dir
 	@echo "Write this Makefile by your self."
 
 obj_dir: $(VSRC) $(CPPSRC)
-	$(VERILATOR) $(VSRC:%=--cc %) $(CPPSRC:%=--exe %) --Mdir $@
+	$(VERILATOR) $(VSRC:%=--cc %) $(CPPSRC:%=--exe %) --Mdir $(OBJDIR)
 
 include ../Makefile
diff --git a/npc/obj_dir/Vexample.cpp b/npc/obj/Vexample.cpp
similarity index 100%
rename from npc/obj_dir/Vexample.cpp
rename to npc/obj/Vexample.cpp
diff --git a/npc/obj_dir/Vexample.h b/npc/obj/Vexample.h
similarity index 100%
rename from npc/obj_dir/Vexample.h
rename to npc/obj/Vexample.h
diff --git a/npc/obj_dir/Vexample.mk b/npc/obj/Vexample.mk
similarity index 100%
rename from npc/obj_dir/Vexample.mk
rename to npc/obj/Vexample.mk
diff --git a/npc/obj_dir/Vexample__Syms.cpp b/npc/obj/Vexample__Syms.cpp
similarity index 100%
rename from npc/obj_dir/Vexample__Syms.cpp
rename to npc/obj/Vexample__Syms.cpp
diff --git a/npc/obj_dir/Vexample__Syms.h b/npc/obj/Vexample__Syms.h
similarity index 100%
rename from npc/obj_dir/Vexample__Syms.h
rename to npc/obj/Vexample__Syms.h
diff --git a/npc/obj_dir/Vexample___024root.h b/npc/obj/Vexample___024root.h
similarity index 100%
rename from npc/obj_dir/Vexample___024root.h
rename to npc/obj/Vexample___024root.h
diff --git a/npc/obj_dir/Vexample___024root__DepSet_h625e39dc__0.cpp b/npc/obj/Vexample___024root__DepSet_h625e39dc__0.cpp
similarity index 100%
rename from npc/obj_dir/Vexample___024root__DepSet_h625e39dc__0.cpp
rename to npc/obj/Vexample___024root__DepSet_h625e39dc__0.cpp
diff --git a/npc/obj_dir/Vexample___024root__DepSet_hcb5acca5__0.cpp b/npc/obj/Vexample___024root__DepSet_hcb5acca5__0.cpp
similarity index 100%
rename from npc/obj_dir/Vexample___024root__DepSet_hcb5acca5__0.cpp
rename to npc/obj/Vexample___024root__DepSet_hcb5acca5__0.cpp
diff --git a/npc/obj_dir/Vexample___024root__DepSet_hcb5acca5__0__Slow.cpp b/npc/obj/Vexample___024root__DepSet_hcb5acca5__0__Slow.cpp
similarity index 100%
rename from npc/obj_dir/Vexample___024root__DepSet_hcb5acca5__0__Slow.cpp
rename to npc/obj/Vexample___024root__DepSet_hcb5acca5__0__Slow.cpp
diff --git a/npc/obj_dir/Vexample___024root__Slow.cpp b/npc/obj/Vexample___024root__Slow.cpp
similarity index 100%
rename from npc/obj_dir/Vexample___024root__Slow.cpp
rename to npc/obj/Vexample___024root__Slow.cpp
diff --git a/npc/obj_dir/Vexample__pch.h b/npc/obj/Vexample__pch.h
similarity index 100%
rename from npc/obj_dir/Vexample__pch.h
rename to npc/obj/Vexample__pch.h
diff --git a/npc/obj_dir/Vexample_classes.mk b/npc/obj/Vexample_classes.mk
similarity index 100%
rename from npc/obj_dir/Vexample_classes.mk
rename to npc/obj/Vexample_classes.mk