diff --git a/am/src/platform/nemu/trm.c b/am/src/platform/nemu/trm.c
index f1802aa..d4a4370 100644
--- a/am/src/platform/nemu/trm.c
+++ b/am/src/platform/nemu/trm.c
@@ -5,10 +5,7 @@ extern char _heap_start;
 int main(const char *args);
 
 Area heap = RANGE(&_heap_start, PMEM_END);
-#ifndef MAINARGS
-#define MAINARGS ""
-#endif
-static const char mainargs[] = MAINARGS;
+static const char mainargs[MAINARGS_MAX_LEN] = MAINARGS_PLACEHOLDER; // defined in CFLAGS
 
 void putch(char ch) {
   outb(SERIAL_PORT, ch);
diff --git a/am/src/riscv/npc/trm.c b/am/src/riscv/npc/trm.c
index 0efe6e7..f805217 100644
--- a/am/src/riscv/npc/trm.c
+++ b/am/src/riscv/npc/trm.c
@@ -9,10 +9,7 @@ extern char _pmem_start;
 #define PMEM_END  ((uintptr_t)&_pmem_start + PMEM_SIZE)
 
 Area heap = RANGE(&_heap_start, PMEM_END);
-#ifndef MAINARGS
-#define MAINARGS ""
-#endif
-static const char mainargs[] = MAINARGS;
+static const char mainargs[MAINARGS_MAX_LEN] = MAINARGS_PLACEHOLDER; // defined in CFLAGS
 
 void putch(char ch) {
 }
diff --git a/scripts/platform/nemu.mk b/scripts/platform/nemu.mk
index 5536816..6f1a061 100644
--- a/scripts/platform/nemu.mk
+++ b/scripts/platform/nemu.mk
@@ -8,22 +8,28 @@ AM_SRCS := platform/nemu/trm.c \
            platform/nemu/mpe.c
 
 CFLAGS    += -fdata-sections -ffunction-sections
+CFLAGS    += -I$(AM_HOME)/am/src/platform/nemu/include
 LDFLAGS   += -T $(AM_HOME)/scripts/linker.ld \
              --defsym=_pmem_start=0x80000000 --defsym=_entry_offset=0x0
 LDFLAGS   += --gc-sections -e _start
 NEMUFLAGS += -l $(shell dirname $(IMAGE).elf)/nemu-log.txt
 
-CFLAGS += -DMAINARGS=\"$(mainargs)\"
-CFLAGS += -I$(AM_HOME)/am/src/platform/nemu/include
-.PHONY: $(AM_HOME)/am/src/platform/nemu/trm.c
+MAINARGS_MAX_LEN = 64
+MAINARGS_PLACEHOLDER = The insert-arg rule in Makefile will insert mainargs here.
+CFLAGS += -DMAINARGS_MAX_LEN=$(MAINARGS_MAX_LEN) -DMAINARGS_PLACEHOLDER=\""$(MAINARGS_PLACEHOLDER)"\"
+
+insert-arg: image
+	@python $(AM_HOME)/tools/insert-arg.py $(IMAGE).bin $(MAINARGS_MAX_LEN) "$(MAINARGS_PLACEHOLDER)" "$(mainargs)"
 
 image: image-dep
 	@$(OBJDUMP) -d $(IMAGE).elf > $(IMAGE).txt
 	@echo + OBJCOPY "->" $(IMAGE_REL).bin
 	@$(OBJCOPY) -S --set-section-flags .bss=alloc,contents -O binary $(IMAGE).elf $(IMAGE).bin
 
-run: image
+run: insert-arg
 	$(MAKE) -C $(NEMU_HOME) ISA=$(ISA) run ARGS="$(NEMUFLAGS)" IMG=$(IMAGE).bin
 
-gdb: image
+gdb: insert-arg
 	$(MAKE) -C $(NEMU_HOME) ISA=$(ISA) gdb ARGS="$(NEMUFLAGS)" IMG=$(IMAGE).bin
+
+.PHONY: insert-arg
diff --git a/scripts/platform/npc.mk b/scripts/platform/npc.mk
index 6cbc7f8..ea8a34d 100644
--- a/scripts/platform/npc.mk
+++ b/scripts/platform/npc.mk
@@ -12,10 +12,20 @@ CFLAGS    += -fdata-sections -ffunction-sections
 LDFLAGS   += -T $(AM_HOME)/scripts/linker.ld \
 						 --defsym=_pmem_start=0x80000000 --defsym=_entry_offset=0x0
 LDFLAGS   += --gc-sections -e _start
-CFLAGS += -DMAINARGS=\"$(mainargs)\"
-.PHONY: $(AM_HOME)/am/src/riscv/npc/trm.c
+
+MAINARGS_MAX_LEN = 64
+MAINARGS_PLACEHOLDER = The insert-arg rule in Makefile will insert mainargs here.
+CFLAGS += -DMAINARGS_MAX_LEN=$(MAINARGS_MAX_LEN) -DMAINARGS_PLACEHOLDER=\""$(MAINARGS_PLACEHOLDER)"\"
+
+insert-arg: image
+	@python $(AM_HOME)/tools/insert-arg.py $(IMAGE).bin $(MAINARGS_MAX_LEN) "$(MAINARGS_PLACEHOLDER)" "$(mainargs)"
 
 image: image-dep
 	@$(OBJDUMP) -d $(IMAGE).elf > $(IMAGE).txt
 	@echo + OBJCOPY "->" $(IMAGE_REL).bin
 	@$(OBJCOPY) -S --set-section-flags .bss=alloc,contents -O binary $(IMAGE).elf $(IMAGE).bin
+
+run: insert-arg
+	echo "TODO: add command here to run simulation"
+
+.PHONY: insert-arg
diff --git a/tools/insert-arg.py b/tools/insert-arg.py
new file mode 100644
index 0000000..6f39d76
--- /dev/null
+++ b/tools/insert-arg.py
@@ -0,0 +1,27 @@
+#!/usr/bin/env python3
+
+from sys import argv
+
+bin = argv[1]
+max_len = int(argv[2])
+placeholder = argv[3]
+mainargs = argv[4]
+
+if len(mainargs) >= max_len:
+    print("Error: mainargs should not be longer than {0} bytes\n".format( max_len))
+    exit(1)
+print("mainargs={0}".format(mainargs))
+
+fp = open(bin, 'r+b')
+data = fp.read()
+idx = data.find(str.encode(placeholder))
+if idx == -1:
+    print("Error: placeholder not found!\n")
+    exit(1)
+fp.seek(idx)
+mainargs_pad = str.encode(mainargs)+ ((max_len - len(mainargs)) * str.encode("\0"))
+if len(mainargs_pad) != max_len:
+    print("Error: len(mainargs_pad) != max_len\n")
+    exit(1)
+fp.write(mainargs_pad)
+fp.close()