diff --git a/include/mcl/op.hpp b/include/mcl/op.hpp
index ef240b4..a1aa4ad 100644
--- a/include/mcl/op.hpp
+++ b/include/mcl/op.hpp
@@ -51,6 +51,11 @@ enum Mode {
 	FP_XBYAK
 };
 
+enum PrimeMode {
+	PM_GENERIC = 0,
+	PM_NICT_P192
+};
+
 static inline const char *ModeToStr(Mode mode)
 {
 	switch (mode) {
@@ -99,7 +104,7 @@ struct Op {
 
 	bool isFullBit; // true if bitSize % uniSize == 0
 	bool isMont; // true if use Montgomery
-	bool isNIST_P192; // true if p is NIST_P192
+	PrimeMode primeMode;
 	bool isFastMod; // true if modulo is fast
 	/*
 		same fp_add, fp_sub if isFullBit
@@ -160,9 +165,9 @@ struct Op {
 		, fp_mulIPre(0)
 		, fpN1_modP(0)
 		, fp_mulI(0)
-		, isFullBit(true)
+		, isFullBit(false)
 		, isMont(false)
-		, isNIST_P192(false)
+		, primeMode(PM_GENERIC)
 		, isFastMod(false)
 		, fp_addNC(0), fp_subNC(0)
 		, fp_preInv(0)
diff --git a/src/fp.cpp b/src/fp.cpp
index 854069c..cf25956 100644
--- a/src/fp.cpp
+++ b/src/fp.cpp
@@ -410,10 +410,13 @@ void Op::init(const std::string& mstr, int base, size_t maxBitSize, Mode mode)
 	isFullBit = (bitSize % UnitBitSize) == 0;
 
 	const size_t roundBit = (bitSize + UnitBitSize - 1) & ~(UnitBitSize - 1);
+	primeMode = PM_GENERIC;
 #if defined(MCL_USE_LLVM) || defined(MCL_USE_XBYAK)
-	isNIST_P192 = (mode == FP_AUTO || mode == FP_LLVM || mode == FP_XBYAK)
-		&& mp == mpz_class("0xfffffffffffffffffffffffffffffffeffffffffffffffff");
-	if (isNIST_P192) {
+	if ((mode == FP_AUTO || mode == FP_LLVM || mode == FP_XBYAK)
+		&& mp == mpz_class("0xfffffffffffffffffffffffffffffffeffffffffffffffff")) {
+		primeMode = PM_NICT_P192;
+	}
+	if (primeMode == PM_NICT_P192) {
 		isMont = false;
 		isFastMod = true;
 	}
@@ -444,7 +447,7 @@ void Op::init(const std::string& mstr, int base, size_t maxBitSize, Mode mode)
 		throw cybozu::Exception("Op::init:not:support") << mstr;
 	}
 #ifdef MCL_USE_LLVM
-	if (isNIST_P192) {
+	if (primeMode == PM_NICT_P192) {
 		fp_mul = &mcl_fp_mul_NIST_P192;
 		fp_sqr = &mcl_fp_sqr_NIST_P192;
 		fpDbl_mod = &mcl_fpDbl_mod_NIST_P192;
diff --git a/src/fp_generator.hpp b/src/fp_generator.hpp
index 0364082..60dc014 100644
--- a/src/fp_generator.hpp
+++ b/src/fp_generator.hpp
@@ -227,7 +227,7 @@ struct FpGenerator : Xbyak::CodeGenerator {
 		align(16);
 		shr1_ = getCurr<void2op>();
 		gen_shr1();
-		if (!op.isNIST_P192 && op.N <= 4) { // support general op.N but not fast for op.N > 4
+		if (op.primeMode != PM_NICT_P192 && op.N <= 4) { // support general op.N but not fast for op.N > 4
 			align(16);
 			op.fp_preInv = getCurr<int2u>();
 			gen_preInv();
@@ -600,7 +600,7 @@ struct FpGenerator : Xbyak::CodeGenerator {
 	}
 	void gen_mul()
 	{
-		if (op_->isNIST_P192) {
+		if (op_->primeMode == PM_NICT_P192) {
 			StackFrame sf(this, 3, 10 | UseRDX, 8 * 6);
 			mulPre3(rsp, sf.p[1], sf.p[2], sf.t);
 			fpDbl_mod_NIST_P192(sf.p[0], rsp, sf.t);
@@ -888,7 +888,7 @@ struct FpGenerator : Xbyak::CodeGenerator {
 	}
 	void gen_fpDbl_mod(const mcl::fp::Op& op)
 	{
-		if (op.isNIST_P192) {
+		if (op.primeMode == PM_NICT_P192) {
 			StackFrame sf(this, 2, 6 | UseRDX);
 			fpDbl_mod_NIST_P192(sf.p[0], sf.p[1], sf.t);
 			return;
@@ -909,7 +909,7 @@ struct FpGenerator : Xbyak::CodeGenerator {
 	}
 	void gen_sqr()
 	{
-		if (op_->isNIST_P192) {
+		if (op_->primeMode == PM_NICT_P192) {
 			StackFrame sf(this, 2, 10 | UseRDX | UseRCX, 8 * 6);
 			sqrPre3(rsp, sf.p[1], sf.t);
 			fpDbl_mod_NIST_P192(sf.p[0], rsp, sf.t);