diff --git a/Makefile b/Makefile
index e5d07fd..ff8408b 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,9 @@
 
 TEST_OBJS = $(addsuffix .o,$(basename $(wildcard tests/*.S)))
 FIRMWARE_OBJS = firmware/start.o firmware/irq.o firmware/print.o firmware/sieve.o firmware/multest.o firmware/stats.o
-GCC_WARNS = -Wall -Wextra -Wshadow -Wundef
+GCC_WARNS  = -Wall -Wextra -Wshadow -Wundef -Wpointer-arith -Wcast-qual -Wcast-align -Wwrite-strings
+GCC_WARNS += -Wredundant-decls -Wstrict-prototypes -Wmissing-prototypes -pedantic # -Wconversion
+TOOLCHAIN_PREFIX = riscv64-unknown-elf-
 
 test: testbench.exe firmware/firmware.hex
 	vvp -N testbench.exe
@@ -44,23 +46,23 @@ firmware/firmware.hex: firmware/firmware.bin firmware/makehex.py
 	python3 firmware/makehex.py $< > $@
 
 firmware/firmware.bin: firmware/firmware.elf
-	riscv64-unknown-elf-objcopy -O binary $< $@
+	$(TOOLCHAIN_PREFIX)objcopy -O binary $< $@
 	chmod -x $@
 
 firmware/firmware.elf: $(FIRMWARE_OBJS) $(TEST_OBJS) firmware/sections.lds
-	riscv64-unknown-elf-gcc -Os -m32 -ffreestanding -nostdlib -o $@ \
+	$(TOOLCHAIN_PREFIX)gcc -Os -m32 -ffreestanding -nostdlib -o $@ \
 		-Wl,-Bstatic,-T,firmware/sections.lds,-Map,firmware/firmware.map,--strip-debug \
 		$(FIRMWARE_OBJS) $(TEST_OBJS) -lgcc
 	chmod -x $@
 
 firmware/start.o: firmware/start.S
-	riscv64-unknown-elf-gcc -c -m32 -o $@ $<
+	$(TOOLCHAIN_PREFIX)gcc -c -m32 -o $@ $<
 
 firmware/%.o: firmware/%.c
-	riscv64-unknown-elf-gcc -c -m32 -march=RV32I -Os $(GCC_WARNS) -ffreestanding -nostdlib -o $@ $<
+	$(TOOLCHAIN_PREFIX)gcc -c -m32 -march=RV32I -Os --std=c99 $(GCC_WARNS) -ffreestanding -nostdlib -o $@ $<
 
 tests/%.o: tests/%.S tests/riscv_test.h tests/test_macros.h
-	riscv64-unknown-elf-gcc -c -m32 -o $@ -DTEST_FUNC_NAME=$(notdir $(basename $<)) \
+	$(TOOLCHAIN_PREFIX)gcc -c -m32 -o $@ -DTEST_FUNC_NAME=$(notdir $(basename $<)) \
 		-DTEST_FUNC_TXT='"$(notdir $(basename $<))"' -DTEST_FUNC_RET=$(notdir $(basename $<))_ret $<
 
 toc:
diff --git a/firmware/firmware.h b/firmware/firmware.h
index f243f31..ce5e321 100644
--- a/firmware/firmware.h
+++ b/firmware/firmware.h
@@ -21,16 +21,16 @@ void print_dec(unsigned int val);
 void print_hex(unsigned int val);
 
 // sieve.c
-void sieve();
+void sieve(void);
 
 // multest.c
 uint32_t hard_mul(uint32_t a, uint32_t b);
 uint32_t hard_mulh(uint32_t a, uint32_t b);
 uint32_t hard_mulhsu(uint32_t a, uint32_t b);
 uint32_t hard_mulhu(uint32_t a, uint32_t b);
-void multest();
+void multest(void);
 
 // stats.c
-void stats();
+void stats(void);
 
 #endif
diff --git a/firmware/irq.c b/firmware/irq.c
index 2d76b1a..2dff1b0 100644
--- a/firmware/irq.c
+++ b/firmware/irq.c
@@ -109,7 +109,7 @@ uint32_t *irq(uint32_t *regs, uint32_t irqs)
 		print_dec(timer_irq_count);
 		print_str("\n");
 
-		__asm__("sbreak");
+		__asm__ volatile ("sbreak");
 	}
 
 	return regs;
diff --git a/firmware/multest.c b/firmware/multest.c
index 508ed36..c6323ec 100644
--- a/firmware/multest.c
+++ b/firmware/multest.c
@@ -7,7 +7,7 @@
 
 #include "firmware.h"
 
-uint32_t xorshift32() {
+static uint32_t xorshift32(void) {
 	static uint32_t x = 314159265;
 	x ^= x << 13;
 	x ^= x >> 17;
@@ -15,7 +15,7 @@ uint32_t xorshift32() {
 	return x;
 }
 
-void multest()
+void multest(void)
 {
 	int i;
 	for (i = 0; i < 10; i++)
@@ -76,7 +76,7 @@ void multest()
 
 		if (s_mul != h_mul || s_mulh != h_mulh || s_mulhsu != h_mulhsu || s_mulhu != h_mulhu) {
 			print_str("ERROR!\n");
-			asm volatile ("sbreak");
+			__asm__ volatile ("sbreak");
 			return;
 		}
 
diff --git a/firmware/sieve.c b/firmware/sieve.c
index 73550c8..20a45d8 100644
--- a/firmware/sieve.c
+++ b/firmware/sieve.c
@@ -52,7 +52,7 @@ static void print_prime(int idx, int val)
 	hash = mkhash(hash, val);
 }
 
-void sieve()
+void sieve(void)
 {
 	int i, j, k;
 	int idx = 1;
@@ -79,7 +79,7 @@ void sieve()
 		print_str(" OK\n");
 	} else {
 		print_str(" ERROR\n");
-		asm volatile ("sbreak");
+		__asm__ volatile ("sbreak");
 	}
 }
 
diff --git a/firmware/stats.c b/firmware/stats.c
index e0b6f6a..4a06751 100644
--- a/firmware/stats.c
+++ b/firmware/stats.c
@@ -25,10 +25,10 @@ static void stats_print_dec(unsigned int val, int digits, bool zero_pad)
 	}
 }
 
-void stats()
+void stats(void)
 {
 	unsigned int num_cycles, num_instr;
-	asm("rdcycle %0; rdinstret %1;" : "=r"(num_cycles), "=r"(num_instr));
+	__asm__("rdcycle %0; rdinstret %1;" : "=r"(num_cycles), "=r"(num_instr));
 	print_str("Cycle counter ........");
 	stats_print_dec(num_cycles, 8, false);
 	print_str("\nInstruction counter ..");