diff --git a/include/system4/ald.h b/include/system4/ald.h
index 0e4e382..ceb435f 100644
--- a/include/system4/ald.h
+++ b/include/system4/ald.h
@@ -24,23 +24,13 @@
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
+#include "system4/archive.h"
 
 #define ALD_FILEMAX 255
 #define ALD_DATAMAX 65535
 
-enum ald_error {
-	ALD_SUCCESS,
-	ALD_FILE_ERROR,
-	ALD_BAD_ARCHIVE_ERROR,
-	ALD_MAX_ERROR
-};
-
-enum {
-	ALD_MMAP = 1
-};
-
 struct ald_archive {
-	bool mmapped;
+	struct archive ar;
 	int nr_files;
 	struct {
 		uint8_t *data;
@@ -57,14 +47,6 @@ struct ald_archive {
 	int *fileptr[ALD_FILEMAX];
 };
 
-struct archive_data {
-	size_t size;
-	uint8_t *data;
-	uint8_t *data_raw;
-	char *name;
-	struct ald_archive *archive;
-};
-
 enum {
 	ALDFILE_BGM,
 	ALDFILE_CG,
@@ -72,36 +54,11 @@ enum {
 	ALDFILETYPE_MAX
 };
 
-struct ald_archive *ald[ALDFILETYPE_MAX];
-
-/*
- * Returns a human readable description of an error.
- */
-const char *ald_strerror(int error);
+struct archive *ald[ALDFILETYPE_MAX];
 
 /*
  * Open an ALD archive.
  */
-struct ald_archive *ald_open(char **files, int count, int flags, int *error);
-
-/*
- * Check if data exists in an ALD archive.
- */
-bool ald_data_exists(struct ald_archive *ar, int no);
-
-/*
- * Retrieve a piece of data from an ALD archive.
- */
-struct archive_data *ald_get(struct ald_archive *archive, int no);
-
-/*
- * Free an ald_data structure returned by `ald_get`.
- */
-void ald_free_data(struct archive_data *data);
-
-/*
- * Free an ald_archive structure returned by `ald_open`.
- */
-void ald_free_archive(struct ald_archive *ar);
+struct archive *ald_open(char **files, int count, int flags, int *error);
 
 #endif /* SYSTEM4_ALD_H */
diff --git a/include/system4/archive.h b/include/system4/archive.h
new file mode 100644
index 0000000..f061928
--- /dev/null
+++ b/include/system4/archive.h
@@ -0,0 +1,92 @@
+/* Copyright (C) 2019 Nunuhara Cabbage <nunuhara@haniwa.technology>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://gnu.org/licenses/>.
+ */
+
+#ifndef SYSTEM4_ARCHIVE_H
+#define SYSTEM4_ARCHIVE_H
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <stddef.h>
+
+enum ald_error {
+	ARCHIVE_SUCCESS,
+	ARCHIVE_FILE_ERROR,
+	ARCHIVE_BAD_ARCHIVE_ERROR,
+	ARCHIVE_MAX_ERROR
+};
+
+enum {
+	ARCHIVE_MMAP = 1
+};
+
+struct archive {
+	bool mmapped;
+	struct archive_ops *ops;
+};
+
+struct archive_ops {
+	bool (*exists)(struct archive *ar, int no);
+	struct archive_data *(*get)(struct archive *ar, int no);
+	void (*free_data)(struct archive_data *data);
+	void (*free)(struct archive *ar);
+};
+
+struct archive_data {
+	size_t size;
+	uint8_t *data;
+	uint8_t *data_raw;
+	char *name;
+	struct archive *archive;
+};
+
+/*
+ * Returns a human readable description of an error.
+ */
+const char *archive_strerror(int error);
+
+/*
+ * Check if data exists in an archive.
+ */
+static inline bool archive_exists(struct archive *ar, int no)
+{
+	return ar->ops->exists(ar, no);
+}
+
+/*
+ * Retrieve a piece of data from an ALD archive.
+ */
+static inline struct archive_data *archive_get(struct archive *ar, int no)
+{
+	return ar->ops->get(ar, no);
+}
+
+/*
+ * Free an archive_data structure returned by `archive_get`.
+ */
+static inline void archive_free_data(struct archive_data *data)
+{
+	data->archive->ops->free_data(data);
+}
+
+/*
+ * Free an ald_archive structure returned by `ald_open`.
+ */
+static inline void archive_free(struct archive *ar)
+{
+	ar->ops->free(ar);
+}
+
+#endif /* SYSTEM4_ARCHIVE_H */
diff --git a/include/system4/cg.h b/include/system4/cg.h
index 9874991..cb718f9 100644
--- a/include/system4/cg.h
+++ b/include/system4/cg.h
@@ -22,7 +22,7 @@
 #include <stdbool.h>
 #include <stdint.h>
 
-struct ald_archive;
+struct archive;
 
 /*
  * Available CG formats
@@ -53,8 +53,8 @@ struct cg {
 	void *pixels;
 };
 
-bool cg_get_metrics(struct ald_archive *ar, int no, struct cg_metrics *dst);
-struct cg *cg_load(struct ald_archive *ar, int no);
+bool cg_get_metrics(struct archive *ar, int no, struct cg_metrics *dst);
+struct cg *cg_load(struct archive *ar, int no);
 void cg_free(struct cg *cg);
 
 #endif /* SYSTEM4_CG_H */
diff --git a/src/ald.c b/src/ald.c
index 72d3dd1..d06c47f 100644
--- a/src/ald.c
+++ b/src/ald.c
@@ -36,19 +36,17 @@
 #include <sys/mman.h>
 #endif
 
-static const char *errtab[ALD_MAX_ERROR] = {
-	[ALD_SUCCESS]           = "Success",
-	[ALD_FILE_ERROR]        = "Error opening ALD file",
-	[ALD_BAD_ARCHIVE_ERROR] = "Invalid ALD file"
-};
+static bool ald_exists(struct archive *ar, int no);
+static struct archive_data *ald_get(struct archive *ar, int no);
+static void ald_free_data(struct archive_data *data);
+static void ald_free(struct archive *ar);
 
-/* Get a message describing an error. */
-const char *ald_strerror(int error)
-{
-	if (error < ALD_MAX_ERROR)
-		return errtab[error];
-	return "Invalid error number";
-}
+struct archive_ops ald_archive_ops = {
+	.exists = ald_exists,
+	.get = ald_get,
+	.free_data = ald_free_data,
+	.free = ald_free,
+};
 
 /* Get the size of a file in bytes. */
 static long get_file_size(FILE *fp)
@@ -188,25 +186,26 @@ static int _ald_get(struct ald_archive *ar, int no, int *disk_out, int *dataptr_
 	return dataptr2 - dataptr;
 }
 
-bool ald_data_exists(struct ald_archive *ar, int no)
+static bool ald_exists(struct archive *ar, int no)
 {
 	int disk, dataptr;
-	return ar && !!_ald_get(ar, no, &disk, &dataptr);
+	return ar && !!_ald_get((struct ald_archive*)ar, no, &disk, &dataptr);
 }
 
 /* Get a piece of data from an ALD archive. */
-struct archive_data *ald_get(struct ald_archive *ar, int no)
+struct archive_data *ald_get(struct archive *_ar, int no)
 {
-	if (!ar)
+	if (!_ar)
 		return NULL;
 
 	uint8_t *data;
 	struct archive_data *dfile;
 	int disk, dataptr, ptr, size;
+	struct ald_archive *ar = (struct ald_archive*)_ar;
 	int readsize = _ald_get(ar, no, &disk, &dataptr);
 
 	// get data top
-	if (ar->mmapped) {
+	if (ar->ar.mmapped) {
 		data = ar->files[disk].data + dataptr;
 	} else {
 		//int readsize = dataptr2 - dataptr;
@@ -228,28 +227,31 @@ struct archive_data *ald_get(struct ald_archive *ar, int no)
 	dfile->data = data + ptr;
 	dfile->size = size;
 	dfile->name = strdup((char*)data + 16);
-	dfile->archive = ar;
+	dfile->archive = &ar->ar;
 	return dfile;
 }
 
 /* Free an ald_data strcture returned by `ald_get`. */
-void ald_free_data(struct archive_data *data)
+static void ald_free_data(struct archive_data *data)
 {
 	if (!data)
 		return;
-	if (!((struct ald_archive*)data->archive)->mmapped)
+	if (!data->archive->mmapped)
 		free(data->data);
 	free(data->name);
 	free(data);
 }
 
 /* Free an ald_archive structure returned by `ald_open`. */
-void ald_free_archive(struct ald_archive *ar)
+static void ald_free(struct archive *_ar)
 {
-	if (!ar)
+	if (!_ar)
 		return;
+
+	struct ald_archive *ar = (struct ald_archive*)_ar;
+
 	// unmap mmap files
-	if (ar->mmapped) {
+	if (ar->ar.mmapped) {
 		for (int i = 0; i < ALD_FILEMAX; i++) {
 			if (ar->files[i].data)
 				munmap(ar->files[i].data, ar->files[i].size);
@@ -259,7 +261,7 @@ void ald_free_archive(struct ald_archive *ar)
 }
 
 /* Open an ALD archive, optionally memory-mapping it. */
-struct ald_archive *ald_open(char **files, int count, int flags, int *error)
+struct archive *ald_open(char **files, int count, int flags, int *error)
 {
 	FILE *fp;
 	long filesize;
@@ -267,7 +269,7 @@ struct ald_archive *ald_open(char **files, int count, int flags, int *error)
 	bool gotmap = false;
 
 #ifdef _WIN32
-	flags &= ~ALD_MMAP;
+	flags &= ~ARCHIVE_MMAP;
 #endif
 
 	for (int i = 0; i < count; i++) {
@@ -275,12 +277,12 @@ struct ald_archive *ald_open(char **files, int count, int flags, int *error)
 		if (!files[i])
 			continue;
 		if (!(fp = fopen(files[i], "r"))) {
-			*error = ALD_FILE_ERROR;
+			*error = ARCHIVE_FILE_ERROR;
 			goto exit_err;
 		}
 		// check if it's a valid archive
 		if (!file_check(fp)) {
-			*error = ALD_BAD_ARCHIVE_ERROR;
+			*error = ARCHIVE_BAD_ARCHIVE_ERROR;
 			fclose(fp);
 			goto exit_err;
 		}
@@ -297,16 +299,16 @@ struct ald_archive *ald_open(char **files, int count, int flags, int *error)
 		ar->files[i].name = strdup(files[i]);
 		// close
 		fclose(fp);
-		if (flags & ALD_MMAP) {
+		if (flags & ARCHIVE_MMAP) {
 			int fd;
 			if (0 > (fd = open(files[i], O_RDONLY))) {
-				*error = ALD_FILE_ERROR;
+				*error = ARCHIVE_FILE_ERROR;
 				goto exit_err;
 			}
 			ar->files[i].data = mmap(0, filesize, PROT_READ, MAP_SHARED, fd, 0);
 			close(fd);
 			if (ar->files[i].data == MAP_FAILED) {
-				*error = ALD_FILE_ERROR;
+				*error = ARCHIVE_FILE_ERROR;
 				goto exit_err;
 			}
 			ar->files[i].size = filesize;
@@ -318,10 +320,12 @@ struct ald_archive *ald_open(char **files, int count, int flags, int *error)
 			continue;
 		c++;
 	}
-	ar->mmapped = flags & ALD_MMAP;
+	ar->ar.mmapped = flags & ARCHIVE_MMAP;
 	ar->nr_files = count;
-	return ar;
+	ar->ar.ops = &ald_archive_ops;
+	return &ar->ar;
 exit_err:
 	free(ar);
 	return NULL;
 }
+
diff --git a/src/archive.c b/src/archive.c
new file mode 100644
index 0000000..2c62401
--- /dev/null
+++ b/src/archive.c
@@ -0,0 +1,32 @@
+/* Copyright (C) 2019 Nunuhara Cabbage <nunuhara@haniwa.technology>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://gnu.org/licenses/>.
+ */
+
+#include "system4.h"
+#include "system4/ald.h"
+
+static const char *errtab[ARCHIVE_MAX_ERROR] = {
+	[ARCHIVE_SUCCESS]           = "Success",
+	[ARCHIVE_FILE_ERROR]        = "Error opening ALD file",
+	[ARCHIVE_BAD_ARCHIVE_ERROR] = "Invalid ALD file"
+};
+
+/* Get a message describing an error. */
+const char *archive_strerror(int error)
+{
+	if (error < ARCHIVE_MAX_ERROR)
+		return errtab[error];
+	return "Invalid error number";
+}
diff --git a/src/audio.c b/src/audio.c
index eb32826..ea7ba3f 100644
--- a/src/audio.c
+++ b/src/audio.c
@@ -101,7 +101,7 @@ static int audio_type_to_archive_type(enum audio_type type)
 bool audio_exists(enum audio_type type, int no)
 {
 	int archive = audio_type_to_archive_type(type);
-	return ald[archive] && ald_data_exists(ald[archive], no);
+	return ald[archive] && archive_exists(ald[archive], no);
 }
 
 static Mix_Chunk *load_chunk(enum audio_type type, int no)
@@ -109,13 +109,13 @@ static Mix_Chunk *load_chunk(enum audio_type type, int no)
 	struct archive_data *dfile;
 	int archive = audio_type_to_archive_type(type);
 
-	if (!(dfile = ald_get(ald[archive], no))) {
+	if (!(dfile = archive_get(ald[archive], no))) {
 		WARNING("Failed to load %s %d", archive == ALDFILE_BGM ? "BGM" : "WAV", no);
 		return NULL;
 	}
 
 	Mix_Chunk *chunk = Mix_LoadWAV_RW(SDL_RWFromConstMem(dfile->data, dfile->size), 1);
-	ald_free_data(dfile);
+	archive_free_data(dfile);
 
 	if (!chunk) {
 		WARNING("%s %d: not a valid audio file", archive == ALDFILE_BGM ? "BGM" : "WAV", no);
diff --git a/src/cg.c b/src/cg.c
index d06d0bd..0d793f5 100644
--- a/src/cg.c
+++ b/src/cg.c
@@ -18,7 +18,7 @@
 #include <stdint.h>
 #include <stdlib.h>
 #include "system4.h"
-#include "system4/ald.h"
+#include "system4/archive.h"
 #include "system4/cg.h"
 #include "system4/qnt.h"
 
@@ -35,11 +35,11 @@ static enum cg_type check_cgformat(uint8_t *data)
 	return ALCG_UNKNOWN;
 }
 
-bool cg_get_metrics(struct ald_archive *ar, int no, struct cg_metrics *dst)
+bool cg_get_metrics(struct archive *ar, int no, struct cg_metrics *dst)
 {
 	struct archive_data *dfile;
 
-	if (!(dfile = ald_get(ar, no)))
+	if (!(dfile = archive_get(ar, no)))
 		return false;
 
 	switch (check_cgformat(dfile->data)) {
@@ -48,10 +48,10 @@ bool cg_get_metrics(struct ald_archive *ar, int no, struct cg_metrics *dst)
 		break;
 	default:
 		WARNING("Unknown CG type (CG %d)", no);
-		ald_free_data(dfile);
+		archive_free_data(dfile);
 		return false;
 	}
-	ald_free_data(dfile);
+	archive_free_data(dfile);
 	return true;
 }
 
@@ -72,13 +72,13 @@ void cg_free(struct cg *cg)
  *  no: file no ( >= 0)
  *  return: cg object(extracted)
 */
-struct cg *cg_load(struct ald_archive *ar, int no)
+struct cg *cg_load(struct archive *ar, int no)
 {
 	struct cg *cg;
 	struct archive_data *dfile;
 	int type;
 
-	if (!(dfile = ald_get(ar, no))) {
+	if (!(dfile = archive_get(ar, no))) {
 		WARNING("Failed to load CG %d", no);
 		return NULL;
 	}
@@ -104,7 +104,7 @@ struct cg *cg_load(struct ald_archive *ar, int no)
 	}
 
 	// ok to free
-	ald_free_data(dfile);
+	archive_free_data(dfile);
 
 	if (cg->pixels)
 		return cg;
diff --git a/src/hll/SACT2.c b/src/hll/SACT2.c
index 8030194..73d321b 100644
--- a/src/hll/SACT2.c
+++ b/src/hll/SACT2.c
@@ -114,7 +114,7 @@ int sact_Key_IsDown(int keycode)
 
 int sact_CG_IsExist(int cg_no)
 {
-	return ald[ALDFILE_CG] && ald_data_exists(ald[ALDFILE_CG], cg_no - 1);
+	return ald[ALDFILE_CG] && archive_exists(ald[ALDFILE_CG], cg_no - 1);
 }
 
 int sact_Music_IsExist(int n)
diff --git a/src/meson.build b/src/meson.build
index a8901dc..aa40788 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -1,6 +1,7 @@
 # sources for libsys4.a
 system4 = ['ain.c',
            'ald.c',
+           'archive.c',
            'buffer.c',
            'cg.c',
            'ex.c',
diff --git a/src/system4.c b/src/system4.c
index bf28e57..38f6f65 100644
--- a/src/system4.c
+++ b/src/system4.c
@@ -75,10 +75,10 @@ static int ald_count[ALDFILETYPE_MAX];
 
 void ald_init(int type, char **files, int count)
 {
-	int error = ALD_SUCCESS;
-	ald[type] = ald_open(files, count, ALD_MMAP, &error);
+	int error = ARCHIVE_SUCCESS;
+	ald[type] = ald_open(files, count, ARCHIVE_MMAP, &error);
 	if (error)
-		ERROR("Failed to open ALD file: %s\n", ald_strerror(error));
+		ERROR("Failed to open ALD file: %s\n", archive_strerror(error));
 }
 
 static void init_gamedata_dir(const char *path)