diff --git a/accum.go b/accum.go
index a83fdd5..8db1235 100644
--- a/accum.go
+++ b/accum.go
@@ -1,5 +1,7 @@
 package z80
 
+// This file includes functions which related to "accumulator" or "ACL"
+
 import "math/bits"
 
 func (cpu *CPU) updateFlagArith8(r, a, b uint16, subtract bool) {
diff --git a/accum_test.go b/accum_test.go
index 0e18cf8..8fe1ce3 100644
--- a/accum_test.go
+++ b/accum_test.go
@@ -8,6 +8,121 @@ import (
 	"github.com/google/go-cmp/cmp"
 )
 
+const (
+	// C is an index for carry flag.
+	C = 0
+
+	// N is an index for add/subtract flag.
+	N = 1
+
+	// PV is an index for parity/overflow flag.
+	PV = 2
+
+	// X3 is reserved index for unused flag.
+	//X3 = 3
+
+	// H is an index for half carry flag.
+	H = 4
+
+	// X5 is reserved index for unused flag.
+	//X5 = 5
+
+	// Z is an index for zero flag.
+	Z = 6
+
+	// S is an index for sign flag.
+	S = 7
+)
+
+// flagOp provides flag operation.  At initial this will keep all bits.
+type flagOp struct {
+	Nand uint8
+	Or   uint8
+}
+
+// ApplyOn applies flag operation on uint8 in place.
+func (fo flagOp) ApplyOn(v *uint8) {
+	*v = *v&^fo.Nand | fo.Or
+}
+
+// Keep marks bit-n as keeping.
+func (fo flagOp) Keep(n int) flagOp {
+	var m uint8 = ^(0x01 << n)
+	fo.Nand &= m
+	fo.Or &= m
+	return fo
+}
+
+// Set marks bit-n as being 1.
+func (fo flagOp) Set(n int) flagOp {
+	var b uint8 = 0x01 << n
+	fo.Nand |= b
+	fo.Or |= b
+	return fo
+}
+
+// Reset marks bit-n as being 0.
+func (fo flagOp) Reset(n int) flagOp {
+	var b uint8 = 0x01 << n
+	fo.Nand |= b
+	fo.Or &= ^b
+	return fo
+}
+
+// Put modify bit-n with boolean value.
+func (fo flagOp) Put(n int, v bool) flagOp {
+	if v {
+		return fo.Set(n)
+	}
+	return fo.Reset(n)
+}
+
+func TestFlagOp_ApplyOn(t *testing.T) {
+	t.Parallel()
+	for _, c := range []struct {
+		op  flagOp
+		v   []uint8
+		exp []uint8
+	}{
+		{flagOp{0x00, 0x00}, []uint8{0x01, 0x00}, []uint8{0x01, 0x00}},
+		{flagOp{0x01, 0x00}, []uint8{0x01, 0x00}, []uint8{0x00, 0x00}},
+		{flagOp{0x01, 0x01}, []uint8{0x01, 0x00}, []uint8{0x01, 0x01}},
+		// undefined behavior
+		{flagOp{0x00, 0x01}, []uint8{0x01, 0x00}, []uint8{0x01, 0x01}},
+
+		{flagOp{0x00, 0x00}, []uint8{0x10, 0x00}, []uint8{0x10, 0x00}},
+		{flagOp{0x10, 0x00}, []uint8{0x10, 0x00}, []uint8{0x00, 0x00}},
+		{flagOp{0x10, 0x10}, []uint8{0x10, 0x00}, []uint8{0x10, 0x10}},
+		// undefined behavior
+		{flagOp{0x00, 0x10}, []uint8{0x10, 0x00}, []uint8{0x10, 0x10}},
+
+		{flagOp{0x00, 0x00}, []uint8{0x11}, []uint8{0x11}},
+		{
+			flagOp{0x11, 0x00},
+			[]uint8{0x11, 0x10, 0x01, 0x00},
+			[]uint8{0x00, 0x00, 0x00, 0x00},
+		},
+		{
+			flagOp{0x11, 0x11},
+			[]uint8{0x11, 0x10, 0x01, 0x00},
+			[]uint8{0x11, 0x11, 0x11, 0x11},
+		},
+		// undefined behavior
+		{
+			flagOp{0x00, 0x11},
+			[]uint8{0x11, 0x10, 0x01, 0x00},
+			[]uint8{0x11, 0x11, 0x11, 0x11},
+		},
+	} {
+		for i, v := range c.v {
+			c.op.ApplyOn(&v)
+			if v != c.exp[i] {
+				t.Fatalf("failed %#v.ApplyOn(0x%02x): expect=0x%02x actual=0x%02x", c.op, c.v[i], c.exp[i], v)
+			}
+		}
+	}
+}
+
 func carry2n(c bool) int {
 	if c {
 		return 1
diff --git a/flag_test.go b/flag_test.go
deleted file mode 100644
index f43f35f..0000000
--- a/flag_test.go
+++ /dev/null
@@ -1,118 +0,0 @@
-package z80
-
-import "testing"
-
-const (
-	// C is an index for carry flag.
-	C = 0
-
-	// N is an index for add/subtract flag.
-	N = 1
-
-	// PV is an index for parity/overflow flag.
-	PV = 2
-
-	// X3 is reserved index for unused flag.
-	//X3 = 3
-
-	// H is an index for half carry flag.
-	H = 4
-
-	// X5 is reserved index for unused flag.
-	//X5 = 5
-
-	// Z is an index for zero flag.
-	Z = 6
-
-	// S is an index for sign flag.
-	S = 7
-)
-
-// flagOp provides flag operation.  At initial this will keep all bits.
-type flagOp struct {
-	Nand uint8
-	Or   uint8
-}
-
-// ApplyOn applies flag operation on uint8 in place.
-func (fo flagOp) ApplyOn(v *uint8) {
-	*v = *v&^fo.Nand | fo.Or
-}
-
-// Keep marks bit-n as keeping.
-func (fo flagOp) Keep(n int) flagOp {
-	var m uint8 = ^(0x01 << n)
-	fo.Nand &= m
-	fo.Or &= m
-	return fo
-}
-
-// Set marks bit-n as being 1.
-func (fo flagOp) Set(n int) flagOp {
-	var b uint8 = 0x01 << n
-	fo.Nand |= b
-	fo.Or |= b
-	return fo
-}
-
-// Reset marks bit-n as being 0.
-func (fo flagOp) Reset(n int) flagOp {
-	var b uint8 = 0x01 << n
-	fo.Nand |= b
-	fo.Or &= ^b
-	return fo
-}
-
-// Put modify bit-n with boolean value.
-func (fo flagOp) Put(n int, v bool) flagOp {
-	if v {
-		return fo.Set(n)
-	}
-	return fo.Reset(n)
-}
-
-func TestFlagOp_ApplyOn(t *testing.T) {
-	t.Parallel()
-	for _, c := range []struct {
-		op  flagOp
-		v   []uint8
-		exp []uint8
-	}{
-		{flagOp{0x00, 0x00}, []uint8{0x01, 0x00}, []uint8{0x01, 0x00}},
-		{flagOp{0x01, 0x00}, []uint8{0x01, 0x00}, []uint8{0x00, 0x00}},
-		{flagOp{0x01, 0x01}, []uint8{0x01, 0x00}, []uint8{0x01, 0x01}},
-		// undefined behavior
-		{flagOp{0x00, 0x01}, []uint8{0x01, 0x00}, []uint8{0x01, 0x01}},
-
-		{flagOp{0x00, 0x00}, []uint8{0x10, 0x00}, []uint8{0x10, 0x00}},
-		{flagOp{0x10, 0x00}, []uint8{0x10, 0x00}, []uint8{0x00, 0x00}},
-		{flagOp{0x10, 0x10}, []uint8{0x10, 0x00}, []uint8{0x10, 0x10}},
-		// undefined behavior
-		{flagOp{0x00, 0x10}, []uint8{0x10, 0x00}, []uint8{0x10, 0x10}},
-
-		{flagOp{0x00, 0x00}, []uint8{0x11}, []uint8{0x11}},
-		{
-			flagOp{0x11, 0x00},
-			[]uint8{0x11, 0x10, 0x01, 0x00},
-			[]uint8{0x00, 0x00, 0x00, 0x00},
-		},
-		{
-			flagOp{0x11, 0x11},
-			[]uint8{0x11, 0x10, 0x01, 0x00},
-			[]uint8{0x11, 0x11, 0x11, 0x11},
-		},
-		// undefined behavior
-		{
-			flagOp{0x00, 0x11},
-			[]uint8{0x11, 0x10, 0x01, 0x00},
-			[]uint8{0x11, 0x11, 0x11, 0x11},
-		},
-	} {
-		for i, v := range c.v {
-			c.op.ApplyOn(&v)
-			if v != c.exp[i] {
-				t.Fatalf("failed %#v.ApplyOn(0x%02x): expect=0x%02x actual=0x%02x", c.op, c.v[i], c.exp[i], v)
-			}
-		}
-	}
-}