diff --git a/fsim/Makefile b/fsim/Makefile
index 8b9a317c..ad8ab8e0 100644
--- a/fsim/Makefile
+++ b/fsim/Makefile
@@ -15,8 +15,8 @@ mem_gen = $(base_dir)/fsim/fpga_mem_gen
 sim_dir = .
 output_dir = $(sim_dir)/output
 
-BACKEND = fpga
-CONFIG = DefaultFPGAConfig
+BACKEND ?= fpga
+CONFIG ?= DefaultFPGAConfig
 
 include $(base_dir)/Makefrag
 include $(sim_dir)/Makefrag
diff --git a/vsim/Makefile b/vsim/Makefile
index 48ca070e..c15e704d 100644
--- a/vsim/Makefile
+++ b/vsim/Makefile
@@ -15,8 +15,8 @@ mem_gen = $(base_dir)/vsim/vlsi_mem_gen
 sim_dir = .
 output_dir = $(sim_dir)/output
 
-BACKEND = rocketchip.RocketChipBackend
-CONFIG = DefaultVLSIConfig
+BACKEND ?= rocketchip.RocketChipBackend
+CONFIG ?= DefaultVLSIConfig
 
 include $(base_dir)/Makefrag
 include $(sim_dir)/Makefrag