diff --git a/src/math/big/arith_arm64.s b/src/math/big/arith_arm64.s
index 18e513e..da6e408 100644
--- a/src/math/big/arith_arm64.s
+++ b/src/math/big/arith_arm64.s
@@ -109,13 +109,59 @@
 	MOVD	R0, c+72(FP)
 	RET
 
+#define vwOneOp(instr, op1)				\
+	MOVD.P	8(R1), R4;				\
+	instr	op1, R4;				\
+	MOVD.P	R4, 8(R3);
+
+// handle the first 1~4 elements before starting iteration in addVW/subVW
+#define vwPreIter(instr1, instr2, counter, target)	\
+	vwOneOp(instr1, R2);				\
+	SUB	$1, counter;				\
+	CBZ	counter, target;			\
+	vwOneOp(instr2, $0);				\
+	SUB	$1, counter;				\
+	CBZ	counter, target;			\
+	vwOneOp(instr2, $0);				\
+	SUB	$1, counter;				\
+	CBZ	counter, target;			\
+	vwOneOp(instr2, $0);
+
+// do one iteration of add or sub in addVW/subVW
+#define vwOneIter(instr, counter, exit)	\
+	CBZ	counter, exit;		\	// careful not to touch the carry flag
+	LDP.P	32(R1), (R4, R5);	\
+	LDP	-16(R1), (R6, R7);	\
+	instr	$0, R4, R8;		\
+	instr	$0, R5, R9;		\
+	instr	$0, R6, R10;		\
+	instr	$0, R7, R11;		\
+	STP.P	(R8, R9), 32(R3);	\
+	STP	(R10, R11), -16(R3);	\
+	SUB	$4, counter;
+
+// do one iteration of copy in addVW/subVW
+#define vwOneIterCopy(counter, exit)			\
+	CBZ	counter, exit;				\
+	LDP.P	32(R1), (R4, R5);			\
+	LDP	-16(R1), (R6, R7);			\
+	STP.P	(R4, R5), 32(R3);			\
+	STP	(R6, R7), -16(R3);			\
+	SUB	$4, counter;
 
 // func addVW(z, x []Word, y Word) (c Word)
+// The 'large' branch handles large 'z'. It checks the carry flag on every iteration
+// and switches to copy if we are done with carries. The copying is skipped as well
+// if 'x' and 'z' happen to share the same underlying storage.
+// The overhead of the checking and branching is visible when 'z' are small (~5%),
+// so set a threshold of 32, and remain the small-sized part entirely untouched.
 TEXT ·addVW(SB),NOSPLIT,$0
 	MOVD	z+0(FP), R3
 	MOVD	z_len+8(FP), R0
 	MOVD	x+24(FP), R1
 	MOVD	y+48(FP), R2
+	CMP	$32, R0
+	BGE	large		// large-sized 'z' and 'x'
 	CBZ	R0, len0	// the length of z is 0
 	MOVD.P	8(R1), R4
 	ADDS	R2, R4		// z[0] = x[0] + y, set carry
@@ -135,29 +181,46 @@
 	STP.P	(R8, R9), 16(R3)
 	SUB	$2, R0
 loop:				// do four times per round
-	CBZ	R0, len1	// careful not to touch the carry flag
-	LDP.P	32(R1), (R4, R5)
-	LDP	-16(R1), (R6, R7)
-	ADCS	$0, R4, R8
-	ADCS	$0, R5, R9
-	ADCS	$0, R6, R10
-	ADCS	$0, R7, R11
-	STP.P	(R8, R9), 32(R3)
-	STP	(R10, R11), -16(R3)
-	SUB	$4, R0
+	vwOneIter(ADCS, R0, len1)
 	B	loop
 len1:
 	CSET	HS, R2		// extract carry flag
 len0:
 	MOVD	R2, c+56(FP)
+done:
 	RET
+large:
+	AND	$0x3, R0, R10
+	AND	$~0x3, R0
+	// unrolling for the first 1~4 elements to avoid saving the carry
+	// flag in each step, adjust $R0 if we unrolled 4 elements
+	vwPreIter(ADDS, ADCS, R10, add4)
+	SUB	$4, R0
+add4:
+	BCC	copy
+	vwOneIter(ADCS, R0, len1)
+	B	add4
+copy:
+	MOVD	ZR, c+56(FP)
+	CMP	R1, R3
+	BEQ	done
+copy_4:				// no carry flag, copy the rest
+	vwOneIterCopy(R0, done)
+	B	copy_4
 
 // func subVW(z, x []Word, y Word) (c Word)
+// The 'large' branch handles large 'z'. It checks the carry flag on every iteration
+// and switches to copy if we are done with carries. The copying is skipped as well
+// if 'x' and 'z' happen to share the same underlying storage.
+// The overhead of the checking and branching is visible when 'z' are small (~5%),
+// so set a threshold of 32, and remain the small-sized part entirely untouched.
 TEXT ·subVW(SB),NOSPLIT,$0
 	MOVD	z+0(FP), R3
 	MOVD	z_len+8(FP), R0
 	MOVD	x+24(FP), R1
 	MOVD	y+48(FP), R2
+	CMP	$32, R0
+	BGE	large		// large-sized 'z' and 'x'
 	CBZ	R0, len0	// the length of z is 0
 	MOVD.P	8(R1), R4
 	SUBS	R2, R4		// z[0] = x[0] - y, set carry
@@ -177,22 +240,32 @@
 	STP.P	(R8, R9), 16(R3)
 	SUB	$2, R0
 loop:				// do four times per round
-	CBZ	R0, len1	// careful not to touch the carry flag
-	LDP.P	32(R1), (R4, R5)
-	LDP	-16(R1), (R6, R7)
-	SBCS	$0, R4, R8
-	SBCS	$0, R5, R9
-	SBCS	$0, R6, R10
-	SBCS	$0, R7, R11
-	STP.P	(R8, R9), 32(R3)
-	STP	(R10, R11), -16(R3)
-	SUB	$4, R0
+	vwOneIter(SBCS, R0, len1)
 	B	loop
 len1:
 	CSET	LO, R2		// extract carry flag
 len0:
 	MOVD	R2, c+56(FP)
+done:
 	RET
+large:
+	AND	$0x3, R0, R10
+	AND	$~0x3, R0
+	// unrolling for the first 1~4 elements to avoid saving the carry
+	// flag in each step, adjust $R0 if we unrolled 4 elements
+	vwPreIter(SUBS, SBCS, R10, sub4)
+	SUB	$4, R0
+sub4:
+	BCS	copy
+	vwOneIter(SBCS, R0, len1)
+	B	sub4
+copy:
+	MOVD	ZR, c+56(FP)
+	CMP	R1, R3
+	BEQ	done
+copy_4:				// no carry flag, copy the rest
+	vwOneIterCopy(R0, done)
+	B	copy_4
 
 // func shlVU(z, x []Word, s uint) (c Word)
 // This implementation handles the shift operation from the high word to the low word,
diff --git a/src/math/big/arith_test.go b/src/math/big/arith_test.go
index e2b982c..fc20593 100644
--- a/src/math/big/arith_test.go
+++ b/src/math/big/arith_test.go
@@ -179,6 +179,23 @@
 	}
 }
 
+func testFunVWext(t *testing.T, msg string, f funVW, f_g funVW, a argVW) {
+	// using the result of addVW_g/subVW_g as golden
+	z_g := make(nat, len(a.z))
+	c_g := f_g(z_g, a.x, a.y)
+	c := f(a.z, a.x, a.y)
+
+	for i, zi := range a.z {
+		if zi != z_g[i] {
+			t.Errorf("%s\n\tgot z[%d] = %#x; want %#x", msg, i, zi, z_g[i])
+			break
+		}
+	}
+	if c != c_g {
+		t.Errorf("%s\n\tgot c = %#x; want %#x", msg, c, c_g)
+	}
+}
+
 func makeFunVW(f func(z, x []Word, s uint) (c Word)) funVW {
 	return func(z, x []Word, s Word) (c Word) {
 		return f(z, x, uint(s))
@@ -213,6 +230,49 @@
 	}
 }
 
+// Construct a vector comprising the same word, usually '0' or 'maximum uint'
+func makeWordVec(e Word, n int) []Word {
+	v := make([]Word, n)
+	for i := range v {
+		v[i] = e
+	}
+	return v
+}
+
+// Extended testing to addVW and subVW using various kinds of input data.
+// We utilize the results of addVW_g and subVW_g as golden reference to check
+// correctness.
+func TestFunVWExt(t *testing.T) {
+	// 32 is the current threshold that triggers an optimized version of
+	// calculation for large-sized vector, ensure we have sizes around it tested.
+	var vwSizes = []int{0, 1, 3, 4, 5, 8, 9, 23, 31, 32, 33, 34, 35, 36, 50, 120}
+	for _, n := range vwSizes {
+		// vector of random numbers, using the result of addVW_g/subVW_g as golden
+		x := rndV(n)
+		y := rndW()
+		z := make(nat, n)
+		arg := argVW{z, x, y, 0}
+		testFunVWext(t, "addVW, random inputs", addVW, addVW_g, arg)
+		testFunVWext(t, "subVW, random inputs", subVW, subVW_g, arg)
+
+		// vector of random numbers, but make 'x' and 'z' share storage
+		arg = argVW{x, x, y, 0}
+		testFunVWext(t, "addVW, random inputs, sharing storage", addVW, addVW_g, arg)
+		testFunVWext(t, "subVW, random inputs, sharing storage", subVW, subVW_g, arg)
+
+		// vector of maximum uint, to force carry flag set in each 'add'
+		y = ^Word(0)
+		x = makeWordVec(y, n)
+		arg = argVW{z, x, y, 0}
+		testFunVWext(t, "addVW, vector of max uint", addVW, addVW_g, arg)
+
+		// vector of '0', to force carry flag set in each 'sub'
+		x = makeWordVec(0, n)
+		arg = argVW{z, x, 1, 0}
+		testFunVWext(t, "subVW, vector of zero", subVW, subVW_g, arg)
+	}
+}
+
 type argVU struct {
 	d  []Word // d is a Word slice, the input parameters x and z come from this array.
 	l  uint   // l is the length of the input parameters x and z.
@@ -299,6 +359,24 @@
 	}
 }
 
+// Benchmarking addVW using vector of maximum uint to force carry flag set
+func BenchmarkAddVWext(b *testing.B) {
+	for _, n := range benchSizes {
+		if isRaceBuilder && n > 1e3 {
+			continue
+		}
+		y := ^Word(0)
+		x := makeWordVec(y, n)
+		z := make([]Word, n)
+		b.Run(fmt.Sprint(n), func(b *testing.B) {
+			b.SetBytes(int64(n * _S))
+			for i := 0; i < b.N; i++ {
+				addVW(z, x, y)
+			}
+		})
+	}
+}
+
 func BenchmarkSubVW(b *testing.B) {
 	for _, n := range benchSizes {
 		if isRaceBuilder && n > 1e3 {
@@ -316,6 +394,24 @@
 	}
 }
 
+// Benchmarking subVW using vector of zero to force carry flag set
+func BenchmarkSubVWext(b *testing.B) {
+	for _, n := range benchSizes {
+		if isRaceBuilder && n > 1e3 {
+			continue
+		}
+		x := makeWordVec(0, n)
+		y := Word(1)
+		z := make([]Word, n)
+		b.Run(fmt.Sprint(n), func(b *testing.B) {
+			b.SetBytes(int64(n * _S))
+			for i := 0; i < b.N; i++ {
+				subVW(z, x, y)
+			}
+		})
+	}
+}
+
 type funVWW func(z, x []Word, y, r Word) (c Word)
 type argVWW struct {
 	z, x nat