diff --git a/src/arm/Compare_test.go b/src/arm/Compare_test.go
index 16cecdd..4ed047c 100644
--- a/src/arm/Compare_test.go
+++ b/src/arm/Compare_test.go
@@ -27,3 +27,19 @@ func TestCompareRegisterNumber(t *testing.T) {
 		assert.DeepEqual(t, code, pattern.Code)
 	}
 }
+
+func TestCompareRegisterRegister(t *testing.T) {
+	usagePatterns := []struct {
+		Left  cpu.Register
+		Right cpu.Register
+		Code  uint32
+	}{
+		{arm.X0, arm.X1, 0xEB01001F},
+	}
+
+	for _, pattern := range usagePatterns {
+		t.Logf("cmp %s, %s", pattern.Left, pattern.Right)
+		code := arm.CompareRegisterRegister(pattern.Left, pattern.Right)
+		assert.DeepEqual(t, code, pattern.Code)
+	}
+}
diff --git a/src/arm/Load.go b/src/arm/Load.go
index afbd176..0b4f6de 100644
--- a/src/arm/Load.go
+++ b/src/arm/Load.go
@@ -13,9 +13,7 @@ func LoadRegister(destination cpu.Register, base cpu.Register, offset int, lengt
 		return 0b01111<<27 | common
 	case 4:
 		return 0b10111<<27 | common
-	case 8:
+	default:
 		return 0b11111<<27 | common
 	}
-
-	panic("invalid length")
 }
diff --git a/src/arm/Store.go b/src/arm/Store.go
index ceeb839..65c246b 100644
--- a/src/arm/Store.go
+++ b/src/arm/Store.go
@@ -15,9 +15,7 @@ func StoreRegister(source cpu.Register, base cpu.Register, offset int, length by
 		return 0b01111<<27 | common
 	case 4:
 		return 0b10111<<27 | common
-	case 8:
+	default:
 		return 0b11111<<27 | common
 	}
-
-	panic("invalid length")
 }
diff --git a/src/arm/arm_test.go b/src/arm/arm_test.go
index b81e610..9a6693f 100644
--- a/src/arm/arm_test.go
+++ b/src/arm/arm_test.go
@@ -27,4 +27,12 @@ func TestNotEncodable(t *testing.T) {
 	assert.False(t, encodable)
 	_, encodable = arm.XorRegisterNumber(arm.X0, arm.X0, -1)
 	assert.False(t, encodable)
+	_, encodable = arm.AddRegisterNumber(arm.X0, arm.X0, 0xFFFF)
+	assert.False(t, encodable)
+	_, encodable = arm.AddRegisterNumber(arm.X0, arm.X0, 0xF0000000)
+	assert.False(t, encodable)
+	_, encodable = arm.SubRegisterNumber(arm.X0, arm.X0, 0xFFFF)
+	assert.False(t, encodable)
+	_, encodable = arm.SubRegisterNumber(arm.X0, arm.X0, 0xF0000000)
+	assert.False(t, encodable)
 }