diff --git a/base64_arm64.go b/base64_arm64.go
index fd8cc70..3ccc13e 100644
--- a/base64_arm64.go
+++ b/base64_arm64.go
@@ -30,6 +30,11 @@ func encodeAsm(dst, src []byte, lut *[64]byte) int
 //go:noescape
 func decodeAsm(dst, src []byte, lut *[128]byte) int
 
+// If cond is 0, sets res = b, otherwise sets res = a.
+//
+//go:noescape
+func moveCond(res, a, b *byte, cond int)
+
 func encode(enc *Encoding, dst, src []byte) {
 	if len(src) >= 48 {
 		encoded := encodeAsm(dst, src, &enc.encode)
diff --git a/base64_arm64.s b/base64_arm64.s
index f76a6e9..92e346f 100644
--- a/base64_arm64.s
+++ b/base64_arm64.s
@@ -131,3 +131,23 @@ loop:
 done:
 	MOVD R2, ret+56(FP)
 	RET
+
+// func moveCond(res, a, b *byte, cond int)
+// If cond == 0 res=b, else res=a
+TEXT ·moveCond(SB),NOSPLIT,$0
+	MOVD	res+0(FP), R0
+	MOVD	a+8(FP), R1
+	MOVD	b+16(FP), R2
+	MOVD	cond+24(FP), R3
+
+	VEOR V0.B16, V0.B16, V0.B16
+	VMOV R3, V1.S4
+	VCMEQ V0.S4, V1.S4, V2.S4
+
+	VLD1 (R1), [V3.B16]
+	VLD1 (R2), [V4.B16]
+
+	VBSL V4.B16, V3.B16, V2.B16
+
+	VST1 [V2.B16], (R0)
+	RET
diff --git a/base64_arm64_test.go b/base64_arm64_test.go
index 2294f4b..f28d91e 100644
--- a/base64_arm64_test.go
+++ b/base64_arm64_test.go
@@ -90,3 +90,17 @@ func TestUrlDecodeSIMD(t *testing.T) {
 		}
 	}
 }
+
+func TestMoveCond(t *testing.T) {
+	a := []byte{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}
+	b := []byte{15,14,13,12,11,10,9,8,7,6,5,4,3,2,1,0}
+	c := make([]byte, 16)
+	moveCond(&c[0], &a[0], &b[0], 0)
+	if (!bytes.Equal(c, b)) {
+		t.Errorf("expected %x, got %x", b, c)
+	}
+	moveCond(&c[0], &a[0], &b[0], 1)
+	if (!bytes.Equal(c, a)) {
+		t.Errorf("expected %x, got %x", a, c)
+	}
+}