diff --git a/firmware/irq.c b/firmware/irq.c
index 2dff1b0..e67b16d 100644
--- a/firmware/irq.c
+++ b/firmware/irq.c
@@ -30,7 +30,6 @@ uint32_t *irq(uint32_t *regs, uint32_t irqs)
 
 	if ((irqs & 6) != 0)
 	{
-		int i, k;
 		uint32_t pc = regs[0] - 4;
 		uint32_t instr = *(uint32_t*)pc;
 
@@ -59,8 +58,8 @@ uint32_t *irq(uint32_t *regs, uint32_t irqs)
 			print_str("\n");
 		}
 
-		for (i = 0; i < 8; i++)
-		for (k = 0; k < 4; k++)
+		for (int i = 0; i < 8; i++)
+		for (int k = 0; k < 4; k++)
 		{
 			int r = i + k*8;
 
diff --git a/firmware/multest.c b/firmware/multest.c
index c6323ec..37ecf32 100644
--- a/firmware/multest.c
+++ b/firmware/multest.c
@@ -17,8 +17,7 @@ static uint32_t xorshift32(void) {
 
 void multest(void)
 {
-	int i;
-	for (i = 0; i < 10; i++)
+	for (int i = 0; i < 10; i++)
 	{
 		uint32_t a = xorshift32();
 		uint32_t b = xorshift32();
diff --git a/firmware/print.c b/firmware/print.c
index da0dd63..a3000f5 100644
--- a/firmware/print.c
+++ b/firmware/print.c
@@ -35,8 +35,7 @@ void print_dec(unsigned int val)
 
 void print_hex(unsigned int val)
 {
-	int i;
-	for (i = 32-4; i >= 0; i -= 4)
+	for (int i = 32-4; i >= 0; i -= 4)
 		*((volatile uint32_t*)OUTPORT) = "0123456789ABCDEF"[(val >> i) % 16];
 }
 
diff --git a/firmware/sieve.c b/firmware/sieve.c
index 20a45d8..f3350c6 100644
--- a/firmware/sieve.c
+++ b/firmware/sieve.c
@@ -54,18 +54,17 @@ static void print_prime(int idx, int val)
 
 void sieve(void)
 {
-	int i, j, k;
 	int idx = 1;
 	hash = 5381;
 	print_prime(idx++, 2);
-	for (i = 0; i < BITMAP_SIZE; i++) {
+	for (int i = 0; i < BITMAP_SIZE; i++) {
 		if (bitmap_get(i))
 			continue;
 		print_prime(idx++, 3+2*i);
-		for (j = 2*(3+2*i);; j += 3+2*i) {
+		for (int j = 2*(3+2*i);; j += 3+2*i) {
 			if (j%2 == 0)
 				continue;
-			k = (j-3)/2;
+			int k = (j-3)/2;
 			if (k >= BITMAP_SIZE)
 				break;
 			bitmap_set(k);