diff --git a/bitcask-intro.pdf b/bitcask-intro.pdf
new file mode 100644
index 0000000..43e1284
Binary files /dev/null and b/bitcask-intro.pdf differ
diff --git a/entity/entry_test.go b/entity/entry_test.go
new file mode 100644
index 0000000..bf35d25
--- /dev/null
+++ b/entity/entry_test.go
@@ -0,0 +1,66 @@
+package entity
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestVerifyRecordCRC(t *testing.T) {
+	tests := []struct {
+		name    string
+		mutate  func([]byte)
+		wantOK  bool
+	}{
+		{
+			name:   "valid_roundtrip",
+			mutate: func([]byte) {},
+			wantOK: true,
+		},
+		{
+			name: "flip_payload_byte",
+			mutate: func(b []byte) {
+				if len(b) > MetaSize {
+					b[len(b)-1] ^= 0xFF
+				}
+			},
+			wantOK: false,
+		},
+		{
+			name: "flip_crc_byte",
+			mutate: func(b []byte) {
+				b[0] ^= 0xFF
+			},
+			wantOK: false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			e := NewEntryWithData([]byte("k"), []byte("v"))
+			buf := e.Encode()
+			tt.mutate(buf)
+			assert.Equal(t, tt.wantOK, VerifyRecordCRC(buf))
+		})
+	}
+}
+
+func TestNewTombstoneEntry_RoundTrip(t *testing.T) {
+	tests := []struct {
+		name string
+		key  []byte
+	}{
+		{name: "short", key: []byte("k")},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			e := NewTombstoneEntry(tt.key)
+			require.Equal(t, uint8(DeleteFlag), e.Meta.Flag)
+			require.Equal(t, uint32(len(tt.key)), e.Meta.KeySize)
+			require.Equal(t, uint32(0), e.Meta.ValueSize)
+			buf := e.Encode()
+			require.True(t, VerifyRecordCRC(buf))
+			require.Equal(t, int64(len(buf)), e.Size())
+		})
+	}
+}
diff --git a/lock_other.go b/lock_other.go
new file mode 100644
index 0000000..a2db75e
--- /dev/null
+++ b/lock_other.go
@@ -0,0 +1,10 @@
+//go:build !unix
+
+package tiny_bitcask
+
+import "os"
+
+// acquireDBLock is a no-op on non-Unix platforms; ExclusiveLock does not enforce single-writer access.
+func acquireDBLock(_ string, _, _ bool) (*os.File, error) {
+	return nil, nil
+}
diff --git a/lock_unix.go b/lock_unix.go
new file mode 100644
index 0000000..ad4599d
--- /dev/null
+++ b/lock_unix.go
@@ -0,0 +1,30 @@
+//go:build unix
+
+package tiny_bitcask
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+func acquireDBLock(dir string, readOnly, exclusive bool) (*os.File, error) {
+	if !exclusive {
+		return nil, nil
+	}
+	path := filepath.Join(dir, ".tiny-bitcask.lock")
+	f, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		return nil, err
+	}
+	how := syscall.LOCK_EX
+	if readOnly {
+		how = syscall.LOCK_SH
+	}
+	if err := syscall.Flock(int(f.Fd()), how|syscall.LOCK_NB); err != nil {
+		_ = f.Close()
+		return nil, fmt.Errorf("tiny-bitcask: database lock: %w", err)
+	}
+	return f, nil
+}