diff --git a/include/xsystem4.h b/include/xsystem4.h
index bcfc9a9..513a810 100644
--- a/include/xsystem4.h
+++ b/include/xsystem4.h
@@ -43,6 +43,7 @@ struct config {
 	bool echo;
 	float text_x_scale;
 	bool manual_text_x_scale;
+	bool rsm_save;
 };
 
 extern struct config config;
diff --git a/src/resume.c b/src/resume.c
index 99d4357..385abbe 100644
--- a/src/resume.c
+++ b/src/resume.c
@@ -17,6 +17,7 @@
 // enable private VM interface
 #define VM_PRIVATE
 
+#include <assert.h>
 #include <stdio.h>
 #include <string.h>
 #include <errno.h>
@@ -24,6 +25,7 @@
 
 #include "system4.h"
 #include "system4/file.h"
+#include "system4/savefile.h"
 #include "system4/string.h"
 
 #include "savedata.h"
@@ -148,7 +150,229 @@ static cJSON *vm_image_to_json(const char *key)
 	return image;
 }
 
-int vm_save_image(const char *key, const char *path)
+static struct rsave_heap_frame *frame_page_to_rsave(struct page *page, int ref)
+{
+	struct rsave_heap_frame *o = xcalloc(1, sizeof(struct rsave_heap_frame) + page->nr_vars * sizeof(int32_t));
+	o->ref = ref;
+	struct ain_variable *vars;
+	if (page->type == GLOBAL_PAGE) {
+		o->tag = RSAVE_GLOBALS;
+		o->func.id = -1;
+		assert(page->nr_vars == ain->nr_globals);
+		vars = ain->globals;
+	} else {
+		struct ain_function *f = &ain->functions[page->index];
+		o->tag = RSAVE_LOCALS;
+		o->func.name = strdup(f->name);
+		assert(page->nr_vars == f->nr_vars);
+		vars = f->vars;
+	}
+	o->nr_types = page->nr_vars;
+	o->types = xcalloc(o->nr_types, sizeof(int32_t));
+	for (int i = 0; i < o->nr_types; i++)
+		o->types[i] = vars[i].type.data;
+	o->nr_slots = page->nr_vars;
+	for (int i = 0; i < o->nr_slots; i++)
+		o->slots[i] = page->values[i].i;
+	return o;
+}
+
+static struct rsave_heap_struct *struct_page_to_rsave(struct page *page, int ref)
+{
+	struct rsave_heap_struct *o = xcalloc(1, sizeof(struct rsave_heap_struct) + page->nr_vars * sizeof(int32_t));
+	o->tag = RSAVE_STRUCT;
+	o->ref = ref;
+	struct ain_struct *s = &ain->structures[page->index];
+	o->ctor.name = strdup(s->constructor >= 0 ? ain->functions[s->constructor].name : "");
+	o->dtor.name = strdup(s->destructor >= 0 ? ain->functions[s->destructor].name : "");
+	o->uk = 0;
+	o->struct_type.name = strdup(s->name);
+	o->nr_types = s->nr_members;
+	o->types = xcalloc(o->nr_types, sizeof(int32_t));
+	for (int i = 0; i < o->nr_types; i++)
+		o->types[i] = s->members[i].type.data;
+	o->nr_slots = page->nr_vars;
+	for (int i = 0; i < o->nr_slots; i++)
+		o->slots[i] = page->values[i].i;
+	return o;
+}
+
+static struct rsave_heap_array *array_page_to_rsave(struct page *page, int ref)
+{
+	struct rsave_heap_array *o = xcalloc(1, sizeof(struct rsave_heap_array) + page->nr_vars * sizeof(int32_t));
+	o->tag = RSAVE_ARRAY;
+	o->ref = ref;
+	o->rank_minus_1 = page->array.rank - 1;
+	o->data_type = page->index;
+	if (page->array.struct_type >= 0)
+		o->struct_type.name = strdup(ain->structures[page->array.struct_type].name);
+	else
+		o->struct_type.name = strdup("");
+	o->root_rank = page->array.rank;  // FIXME: this is incorrect for subarrays
+	o->is_not_empty = page->nr_vars ? 1 : 0;
+	o->nr_slots = page->nr_vars;
+	for (int i = 0; i < o->nr_slots; i++)
+		o->slots[i] = page->values[i].i;
+	return o;
+}
+
+static void *heap_item_to_rsave(int i)
+{
+	if (!heap[i].ref)
+		return rsave_null;
+	if (heap[i].type == VM_STRING) {
+		int len = heap[i].s->size + 1;
+		struct rsave_heap_string *s = xmalloc(sizeof(struct rsave_heap_string) + len);
+		s->tag = RSAVE_STRING;
+		s->ref = heap[i].ref;
+		s->uk = 0;
+		s->len = len;
+		memcpy(s->text, heap[i].s->text, len);
+		return s;
+	}
+	struct page *page = heap[i].page;
+	if (!page) {
+		// Empty array.
+		struct rsave_heap_array *o = xcalloc(1, sizeof(struct rsave_heap_array));
+		o->tag = RSAVE_ARRAY;
+		o->ref = heap[i].ref;
+		o->rank_minus_1 = -1;
+
+		// FIXME: System40.exe populates them but we don't have the type
+		// information of the array here.
+		o->data_type = 0;
+		o->struct_type.name = strdup("");
+
+		o->root_rank = -1;
+		o->is_not_empty = 0;
+		o->nr_slots = 0;
+		return o;
+	}
+	switch (page->type) {
+	case GLOBAL_PAGE:
+	case LOCAL_PAGE:
+		return frame_page_to_rsave(page, heap[i].ref);
+	case STRUCT_PAGE:
+		return struct_page_to_rsave(page, heap[i].ref);
+	case ARRAY_PAGE:
+		return array_page_to_rsave(page, heap[i].ref);
+	default:
+		ERROR("unsupported type %d", page->type);
+	}
+}
+
+static void save_heap_to_rsave(struct rsave *rs)
+{
+	rs->nr_heap_objs = heap_size;
+	rs->heap = xcalloc(heap_size, sizeof(void*));
+	for (size_t i = 0; i < heap_size; i++) {
+		rs->heap[i] = heap_item_to_rsave(i);
+	}
+}
+
+static void save_call_stack_to_rsave(struct rsave *rs)
+{
+	rs->nr_call_frames = call_stack_ptr + 1;
+	rs->call_frames = xcalloc(rs->nr_call_frames, sizeof(struct rsave_call_frame));
+	rs->call_frames[0].type = RSAVE_CALL_STACK_BOTTOM;
+	rs->call_frames[0].local_ptr = -1;
+	rs->nr_return_records = call_stack_ptr;
+	rs->return_records = xcalloc(rs->nr_return_records + 1, sizeof(struct rsave_return_record));
+	rs->return_records[0].return_addr = -1;
+
+	struct ain_function *prev_func = NULL;
+	for (int i = 0; i < call_stack_ptr; i++) {
+		struct function_call *call = &call_stack[i];
+		struct ain_function *func = &ain->functions[call->fno];
+		if (call->struct_page >= 0) {
+			rs->call_frames[i + 1].type = RSAVE_METHOD_CALL;
+		} else if (!strcmp(func->name, "main")) {
+			rs->call_frames[i + 1].type = RSAVE_ENTRY_POINT;
+		} else {
+			rs->call_frames[i + 1].type = RSAVE_FUNCTION_CALL;
+		}
+		rs->call_frames[i + 1].local_ptr = call->page_slot;
+		rs->call_frames[i + 1].struct_ptr = call->struct_page;
+		if (i > 0) {
+			rs->return_records[i].return_addr = call->return_address;
+			rs->return_records[i].local_addr = call->return_address - prev_func->address;
+		}
+		rs->return_records[i + 1].caller_func = strdup(func->name);
+		rs->return_records[i + 1].crc = func->crc;
+		prev_func = func;
+	}
+	rs->ip = rs->return_records[call_stack_ptr];
+	rs->ip.return_addr = instr_ptr + 6;
+	rs->ip.local_addr = instr_ptr - prev_func->address + 6;
+}
+
+static void save_stack_to_rsave(struct rsave *rs)
+{
+	// For compatibility with the AliceSoft's implementation, exclude two values
+	// (the SYS_RESUME_SAVE arguments) at the top of the stack.
+	rs->stack_size = stack_ptr - 2;
+
+	rs->stack = xcalloc(rs->stack_size, sizeof(int32_t));
+	for (int i = 0; i < rs->stack_size; i++) {
+		rs->stack[i] = stack[i].i;
+	}
+}
+
+static void save_func_names_to_rsave(struct rsave *rs)
+{
+	rs->nr_func_names = ain->nr_functions;
+	rs->func_names = xcalloc(rs->nr_func_names, sizeof(char *));
+	for (int i = 0; i < ain->nr_functions; i++) {
+		rs->func_names[i] = strdup(ain->functions[i].name);
+	}
+}
+
+static struct rsave *make_rsave(const char *key)
+{
+	struct rsave *save = xcalloc(1, sizeof(struct rsave));
+	// FIXME: This is not always correct. RSM version cannot be determined from
+	// the ain version alone.
+	save->version = ain->version <= 4 ? 6 : 7;
+	save->key = strdup(key);
+	return save;
+}
+
+static struct rsave *vm_image_to_rsave(const char *key)
+{
+	struct rsave *save = make_rsave(key);
+	save_heap_to_rsave(save);
+	save_call_stack_to_rsave(save);
+	save_stack_to_rsave(save);
+	save_func_names_to_rsave(save);
+	return save;
+}
+
+static int save_rsave_image(const char *key, const char *path)
+{
+	struct rsave *save = vm_image_to_rsave(key);
+	if (!save)
+		return 0;
+	char *full_path = savedir_path(path);
+	FILE *fp = file_open_utf8(full_path, "wb");
+	if (!fp) {
+		WARNING("Failed to open save file %s: %s", display_utf0(full_path), strerror(errno));
+		free(full_path);
+		rsave_free(save);
+		return 0;
+	}
+	free(full_path);
+
+	bool encrypt = true;
+	int compression_level = 1;
+	enum savefile_error error = rsave_write(save, fp, encrypt, compression_level);
+	if (error != SAVEFILE_SUCCESS)
+		WARNING("Failed to write save file: %s", savefile_strerror(error));
+	fclose(fp);
+	rsave_free(save);
+	return 1;
+}
+
+static int save_json_image(const char *key, const char *path)
 {
 	cJSON *image = vm_image_to_json(key);
 	int r = save_json(path, image);
@@ -156,6 +380,14 @@ int vm_save_image(const char *key, const char *path)
 	return r;
 }
 
+int vm_save_image(const char *key, const char *path)
+{
+	if (config.rsm_save)
+		return save_rsave_image(key, path);
+	else
+		return save_json_image(key, path);
+}
+
 #define _invalid_save_data(file, func, line, fmt, ...)	\
 	_vm_error("*ERROR*(%s:%s:%d): " fmt "\n", file, func, line, ##__VA_ARGS__)
 #define invalid_save_data(fmt, ...) \
@@ -187,7 +419,7 @@ static cJSON *_type_check(const char *file, const char *func, int line, int type
 
 #define type_check(type, json) _type_check(__FILE__, __func__, __LINE__, type, json)
 
-static void load_page(int slot, cJSON *json)
+static void load_json_page(int slot, cJSON *json)
 {
 	int struct_type = 0, rank = 0;
 
@@ -221,7 +453,7 @@ static void load_page(int slot, cJSON *json)
 	heap[slot].type = VM_PAGE;
 }
 
-static void load_string(int slot, cJSON *json)
+static void load_json_string(int slot, cJSON *json)
 {
 	const char *str = cJSON_GetStringValue(json);
 	heap[slot].s = make_string(str, strlen(str));
@@ -279,7 +511,7 @@ void delegate_list_add(struct delegate_list *list, int slot)
 	list->slots[list->n++] = slot;
 }
 
-static void load_heap(cJSON *json)
+static void load_json_heap(cJSON *json)
 {
 	struct delegate_list delegates = {0};
 	delete_heap();
@@ -298,9 +530,9 @@ static void load_heap(cJSON *json)
 		heap[slot].ref = ref;
 
 		if (cJSON_IsString(value)) {
-			load_string(slot, value);
+			load_json_string(slot, value);
 		} else if (cJSON_IsObject(value)) {
-			load_page(slot, value);
+			load_json_page(slot, value);
 			if (heap[slot].page->type == DELEGATE_PAGE)
 				delegate_list_add(&delegates, slot);
 		} else if (cJSON_IsNull(value)) {
@@ -319,7 +551,137 @@ static void load_heap(cJSON *json)
 	free(delegates.slots);
 }
 
-static void load_call_stack(cJSON *json)
+static int resolve_func_symbol(struct rsave_symbol *sym)
+{
+	if (!sym->name)
+		return sym->id;
+	return ain_get_function(ain, sym->name);
+}
+
+static int resolve_struct_symbol(struct rsave_symbol *sym)
+{
+	if (!sym->name)
+		return sym->id;
+	return ain_get_struct(ain, sym->name);
+}
+
+static void load_rsave_frame(int slot, struct rsave_heap_frame *f)
+{
+	struct page *page;
+	int nr_vars;
+	struct ain_variable *vars;
+	if (f->tag == RSAVE_GLOBALS) {
+		page = alloc_page(GLOBAL_PAGE, 0, f->nr_slots);
+		nr_vars = ain->nr_globals;
+		vars = ain->globals;
+	} else {
+		int func = resolve_func_symbol(&f->func);
+		page = alloc_page(LOCAL_PAGE, func, f->nr_slots);
+		nr_vars = ain->functions[func].nr_vars;
+		vars = ain->functions[func].vars;
+	}
+
+	// type check
+	if (nr_vars < f->nr_types) {
+		invalid_save_data("call frame has too many variables");
+	}
+	for (int i = 0; i < f->nr_types; i++) {
+		if (f->types[i] != vars[i].type.data) {
+			invalid_save_data(
+				"variable type mismatch. Expected %d, got %d",
+				vars[i].type.data, f->types[i]);
+		}
+	}
+
+	for (int i = 0; i < f->nr_slots; i++) {
+		// TODO: update function pointers using rsave->func_names
+		page->values[i].i = f->slots[i];
+	}
+	alloc_heap_slot(slot);
+	heap[slot].ref = f->ref;
+	heap[slot].type = VM_PAGE;
+	heap[slot].page = page;
+}
+
+static void load_rsave_string(int slot, struct rsave_heap_string *s)
+{
+	alloc_heap_slot(slot);
+	heap[slot].ref = s->ref;
+	heap[slot].type = VM_STRING;
+	heap[slot].s = make_string(s->text, s->len - 1);
+}
+
+static void load_rsave_array(int slot, struct rsave_heap_array *a)
+{
+	if (a->rank_minus_1 < 0) {
+		alloc_heap_slot(slot);
+		heap[slot].ref = a->ref;
+		heap[slot].type = VM_PAGE;
+		heap[slot].page = NULL;
+		return;
+	}
+	struct page *page = alloc_page(ARRAY_PAGE, a->data_type, a->nr_slots);
+	page->array.struct_type = resolve_struct_symbol(&a->struct_type);
+	page->array.rank = a->rank_minus_1 + 1;
+	for (int i = 0; i < a->nr_slots; i++)
+		page->values[i].i = a->slots[i];
+
+	alloc_heap_slot(slot);
+	heap[slot].ref = a->ref;
+	heap[slot].type = VM_PAGE;
+	heap[slot].page = page;
+}
+
+static void load_rsave_struct(int slot, struct rsave_heap_struct *s)
+{
+	int struct_index = resolve_struct_symbol(&s->struct_type);
+	struct page *page = alloc_page(STRUCT_PAGE, struct_index, s->nr_slots);
+	page->struc.delegates = NULL;
+	page->struc.nr_delegates = 0;
+
+	// type check
+	struct ain_struct *as = &ain->structures[struct_index];
+	if (as->nr_members < s->nr_types) {
+		invalid_save_data("Struct %s has too many members", as->name);
+	}
+	for (int i = 0; i < s->nr_types; i++) {
+		if (s->types[i] != as->members[i].type.data) {
+			invalid_save_data(
+				"%s.%s: member type mismatch. Expected %d, got %d",
+				as->name, as->members[i].name,
+				as->members[i].type.data, s->types[i]);
+		}
+	}
+
+	for (int i = 0; i < s->nr_slots; i++) {
+		// TODO: update function pointers using rsave->func_names
+		page->values[i].i = s->slots[i];
+	}
+
+	alloc_heap_slot(slot);
+	heap[slot].ref = s->ref;
+	heap[slot].type = VM_PAGE;
+	heap[slot].page = page;
+}
+
+static void load_rsave_heap(struct rsave *save)
+{
+	delete_heap();
+
+	for (int slot = 0; slot < save->nr_heap_objs; slot++) {
+		enum rsave_heap_tag *tag = save->heap[slot];
+		switch (*tag) {
+		case RSAVE_GLOBALS:
+		case RSAVE_LOCALS: load_rsave_frame(slot, save->heap[slot]);  break;
+		case RSAVE_STRING: load_rsave_string(slot, save->heap[slot]); break;
+		case RSAVE_ARRAY:  load_rsave_array(slot, save->heap[slot]);  break;
+		case RSAVE_STRUCT: load_rsave_struct(slot, save->heap[slot]); break;
+		case RSAVE_NULL: break;
+		}
+	}
+}
+
+static void load_json_call_stack(cJSON *json)
 {
 	call_stack_ptr = 0;
 	cJSON *item;
@@ -336,7 +698,46 @@ static void load_call_stack(cJSON *json)
 	}
 }
 
-static void load_stack(cJSON *json)
+static void load_rsave_call_stack(struct rsave *save)
+{
+	call_stack_ptr = 0;
+
+	if (save->nr_return_records == 0 || save->return_records[0].return_addr != -1)
+		ERROR("invalid resume save");
+	struct rsave_return_record *rr = save->return_records;
+	if (save->nr_return_records == save->nr_call_frames)
+		rr++;
+	else if (save->nr_return_records != save->nr_call_frames - 1)
+		ERROR("invalid resume save");
+
+	int32_t return_address = -1;
+	for (int i = 0; i < save->nr_call_frames; i++) {
+		if (save->call_frames[i].type != RSAVE_CALL_STACK_BOTTOM) {
+			int fno = ain_get_function(ain, rr->caller_func);
+			if (fno < 0)
+				VM_ERROR("Failed to load VM image: function '%s' not found", display_sjis0(rr->caller_func));
+			if (ain->functions[fno].crc != rr->crc)
+				VM_ERROR("Failed to load VM image: CRC mismatch for function '%s'", display_sjis0(rr->caller_func));
+			call_stack[call_stack_ptr++] = (struct function_call) {
+				.fno            = fno,
+				.return_address = return_address,
+				.page_slot      = save->call_frames[i].local_ptr,
+				.struct_page    = save->call_frames[i].struct_ptr,
+				.delegate       = -1,
+			};
+			// Calculate return address from the function address and offset
+			// to make it robust to ain changes.
+			return_address = ain->functions[fno].address + rr->local_addr;
+		}
+		if (++rr == save->return_records + save->nr_return_records)
+			rr = &save->ip;
+	}
+	// RSM records instruction pointer after the CALLSYS instruction, but
+	// xsystem4 expects the address of the CALLSYS instruction, so -6.
+	instr_ptr = return_address - 6;
+}
+
+static void load_json_stack(cJSON *json)
 {
 	stack_ptr = 0;
 
@@ -345,9 +746,23 @@ static void load_stack(cJSON *json)
 		type_check(cJSON_Number, item);
 		stack_push_value(vm_int(item->valueint));
 	}
+	// Pop the arguments of SYS_RESUME_SAVE.
+	//heap_unref(stack_pop().i);
+	//heap_unref(stack_pop().i);
+	stack_pop();
+	stack_pop();
 }
 
-static cJSON *read_image(const char *keyname, const char *path)
+static void load_rsave_stack(struct rsave *save)
+{
+	stack_ptr = 0;
+
+	for (int i = 0; i < save->stack_size; i++) {
+		stack_push_value(vm_int(save->stack[i]));
+	}
+}
+
+static cJSON *read_json_image(const char *keyname, const char *path)
 {
 	char *full_path = savedir_path(path);
 	char *save_file = file_read(full_path, NULL);
@@ -367,23 +782,56 @@ static cJSON *read_image(const char *keyname, const char *path)
 	return type_check(cJSON_Object, save);
 }
 
-void vm_load_image(const char *key, const char *path)
+static void load_json_image(const char *key, const char *path)
 {
-	cJSON *save = read_image(key, path);
+	cJSON *save = read_json_image(key, path);
 	if (!save) {
 		VM_ERROR("Failed to read VM image: '%s'", display_sjis0(path));
 	}
 	cJSON *ip = type_check(cJSON_Number, cJSON_GetObjectItem(save, "ip"));
-	load_heap(type_check(cJSON_Array, cJSON_GetObjectItem(save, "heap")));
-	load_call_stack(type_check(cJSON_Array, cJSON_GetObjectItem(save, "call-stack")));
-	load_stack(type_check(cJSON_Array, cJSON_GetObjectItem(save, "stack")));
+	load_json_heap(type_check(cJSON_Array, cJSON_GetObjectItem(save, "heap")));
+	load_json_call_stack(type_check(cJSON_Array, cJSON_GetObjectItem(save, "call-stack")));
+	load_json_stack(type_check(cJSON_Array, cJSON_GetObjectItem(save, "stack")));
 	instr_ptr = ip->valueint;
 	cJSON_Delete(save);
 }
 
-struct page *vm_load_image_comments(const char *key, const char *path, int *success)
+static enum savefile_error load_rsave_image(const char *key, const char *path)
 {
-	cJSON *save = read_image(key, path);
+	char *full_path = savedir_path(path);
+	enum savefile_error error;
+	struct rsave *save = rsave_read(full_path, &error);
+	free(full_path);
+	if (error != SAVEFILE_SUCCESS)
+		return error;
+	if (strcmp(key, save->key))
+		invalid_save_data("Key doesn't match");
+	load_rsave_heap(save);
+	load_rsave_call_stack(save);
+	load_rsave_stack(save);
+	rsave_free(save);
+	return SAVEFILE_SUCCESS;
+}
+
+void vm_load_image(const char *key, const char *path)
+{
+	// First, try to read as a rsave.
+	enum savefile_error error = load_rsave_image(key, path);
+	switch (error) {
+	case SAVEFILE_SUCCESS:
+		return;
+	case SAVEFILE_INVALID_SIGNATURE:
+		// If not a System4 resume save file, read as a json.
+		load_json_image(key, path);
+		return;
+	default:
+		VM_ERROR("Failed to read VM image '%s': %s", display_sjis0(path), savefile_strerror(error));
+	}
+}
+
+static struct page *load_json_image_comments(const char *key, const char *path, int *success)
+{
+	cJSON *save = read_json_image(key, path);
 	if (!save) {
 		*success = 0;
 		return NULL;
@@ -418,9 +866,103 @@ struct page *vm_load_image_comments(const char *key, const char *path, int *succ
 	return array;
 }
 
-int vm_write_image_comments(const char *key, const char *path, struct page *comments)
+static struct page *load_rsave_image_comments(const char *key, const char *path, enum savefile_error *error)
 {
-	cJSON *save = read_image(key, path);
+	char *full_path = savedir_path(path);
+	struct rsave *save = rsave_read(full_path, error);
+	free(full_path);
+	if (*error != SAVEFILE_SUCCESS)
+		return NULL;
+	if (strcmp(key, save->key))
+		invalid_save_data("Key doesn't match");
+	if (save->nr_comments == 0) {
+		rsave_free(save);
+		return NULL;
+	}
+	union vm_value dims = { .i = save->nr_comments };
+	struct page *array = alloc_array(1, &dims, AIN_ARRAY_STRING, 0, false);
+	for (int i = 0; i < dims.i; i++) {
+		int slot = heap_alloc_slot(VM_STRING);
+		if (!save->comments[i][0]) {
+			heap[slot].s = string_ref(&EMPTY_STRING);
+		} else {
+			heap[slot].s = make_string(save->comments[i], strlen(save->comments[i]));
+		}
+		array->values[i].i = slot;
+	}
+	rsave_free(save);
+	return array;
+}
+
+struct page *vm_load_image_comments(const char *key, const char *path, int *success)
+{
+	// First, try to read as a rsave.
+	enum savefile_error error;
+	struct page *page = load_rsave_image_comments(key, path, &error);
+	if (error == SAVEFILE_SUCCESS) {
+		*success = 1;
+		return page;
+	}
+
+	if (error == SAVEFILE_INVALID_SIGNATURE) {
+		// If not a System4 resume save file, read as a json.
+		return load_json_image_comments(key, path, success);
+	}
+	*success = 0;
+	return NULL;
+}
+
+static int write_rsave_image_comments(const char *key, const char *path, struct page *comments)
+{
+	char *full_path = savedir_path(path);
+	enum savefile_error error;
+	struct rsave *save = rsave_read(full_path, &error);
+	switch (error) {
+	case SAVEFILE_SUCCESS:
+		if (strcmp(key, save->key))
+			invalid_save_data("Key doesn't match");
+		break;
+	case SAVEFILE_FILE_ERROR:
+		save = make_rsave(key);
+		save->comments_only = true;
+		break;
+	default:
+		free(full_path);
+		return 0;
+	}
+
+	for (int i = 0; i < save->nr_comments; i++) {
+		free(save->comments[i]);
+	}
+	free(save->comments);
+	save->nr_comments = comments->nr_vars;
+	save->comments = xcalloc(save->nr_comments, sizeof(char*));
+	for (int i = 0; i < save->nr_comments; i++) {
+		save->comments[i] = xstrdup(heap_get_string(comments->values[i].i)->text);
+	}
+
+	FILE *fp = file_open_utf8(full_path, "wb");
+	if (!fp) {
+		WARNING("Failed to open save file %s: %s", display_utf0(full_path), strerror(errno));
+		free(full_path);
+		rsave_free(save);
+		return 0;
+	}
+	free(full_path);
+
+	bool encrypt = true;
+	int compression_level = 1;
+	error = rsave_write(save, fp, encrypt, compression_level);
+	if (error != SAVEFILE_SUCCESS)
+		WARNING("Failed to write save file: %s", savefile_strerror(error));
+	fclose(fp);
+	rsave_free(save);
+	return 1;
+}
+
+static int write_json_image_comments(const char *key, const char *path, struct page *comments)
+{
+	cJSON *save = read_json_image(key, path);
 	if (!save) {
 		// create blank save
 		save = cJSON_CreateObject();
@@ -440,3 +982,11 @@ int vm_write_image_comments(const char *key, const char *path, struct page *comm
 	cJSON_Delete(save);
 	return 1;
 }
+
+int vm_write_image_comments(const char *key, const char *path, struct page *comments)
+{
+	if (config.rsm_save)
+		return write_rsave_image_comments(key, path, comments);
+	else
+		return write_json_image_comments(key, path, comments);
+}
diff --git a/src/system4.c b/src/system4.c
index 474e757..6f6fd0c 100644
--- a/src/system4.c
+++ b/src/system4.c
@@ -60,6 +60,7 @@ struct config config = {
 	.echo = false,
 	.text_x_scale = 1.0,
 	.manual_text_x_scale = false,
+	.rsm_save = false,
 
 	.bgi_path = NULL,
 	.wai_path = NULL,
@@ -175,6 +176,15 @@ static void read_user_config_file(const char *path)
 		} else if (!strcmp(ini[i].name->text, "save-folder")) {
 			free(config.save_dir);
 			config.save_dir = xstrdup(ini_string(&ini[i])->text);
+		} else if (!strcmp(ini[i].name->text, "save-format")) {
+			if (!strcmp(ini_string(&ini[i])->text, "json")) {
+				config.rsm_save = false;
+			} else if (!strcmp(ini_string(&ini[i])->text, "rsm")) {
+				config.rsm_save = true;
+			} else {
+				WARNING("Invalid value for save-format in config: \"%s\"",
+						ini_string(&ini[i])->text);
+			}
 		}
 		ini_free_entry(&ini[i]);
 	}
@@ -344,6 +354,7 @@ static void usage(void)
 	puts("        --font-x-scale  Specify the x scale for text rendering (1.0 = default scale)");
 	puts("    -j, --joypad        Enable joypad");
 	puts("        --save-folder   Override save folder location");
+	puts("        --save-format   Specify the resume save file format. json (default) or rsm");
 #ifdef DEBUGGER_ENABLED
 	puts("        --nodebug       Disable debugger");
 	puts("        --debug         Start in debugger");
@@ -372,6 +383,7 @@ enum {
 	LOPT_FONT_X_SCALE,
 	LOPT_JOYPAD,
 	LOPT_SAVE_FOLDER,
+	LOPT_SAVE_FORMAT,
 #ifdef DEBUGGER_ENABLED
 	LOPT_NODEBUG,
 	LOPT_DEBUG,
@@ -417,6 +429,7 @@ int main(int argc, char *argv[])
 			{ "font-x-scale", required_argument, 0, LOPT_FONT_X_SCALE },
 			{ "joypad",       optional_argument, 0, LOPT_JOYPAD },
 			{ "save-folder",  required_argument, 0, LOPT_SAVE_FOLDER },
+			{ "save-format",  required_argument, 0, LOPT_SAVE_FORMAT },
 #ifdef DEBUGGER_ENABLED
 			{ "nodebug",      no_argument,       0, LOPT_NODEBUG },
 			{ "debug",        no_argument,       0, LOPT_DEBUG },
@@ -471,6 +484,15 @@ int main(int argc, char *argv[])
 		case LOPT_SAVE_FOLDER:
 			savedir = optarg;
 			break;
+		case LOPT_SAVE_FORMAT:
+			if (!strcmp(optarg, "json")) {
+				config.rsm_save = false;
+			} else if (!strcmp(optarg, "rsm")) {
+				config.rsm_save = true;
+			} else {
+				WARNING("Invalid value for --save-format option: \"%s\"", optarg);
+			}
+			break;
 #ifdef DEBUGGER_ENABLED
 		case LOPT_NODEBUG:
 			dbg_enabled = false;
diff --git a/src/vm.c b/src/vm.c
index 5498784..6b177ee 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -530,10 +530,6 @@ static void system_call(enum syscall_code code)
 		int filename_slot = stack_pop().i;
 		int key_slot = stack_pop().i;
 		vm_load_image(heap_get_string(key_slot)->text, heap_get_string(filename_slot)->text);
-		//heap_unref(stack_pop().i);
-		//heap_unref(stack_pop().i);
-		stack_pop();
-		stack_pop();
 		stack_push(0);
 		break;
 	}
@@ -998,7 +994,7 @@ static enum opcode execute_instruction(enum opcode opcode)
 			sys_message("Assertion failed at %s:%d: %s\n",
 					display_sjis0(heap_get_string(file)->text),
 					line,
-					display_sjis0(heap_get_string(expr)->text));
+					display_sjis1(heap_get_string(expr)->text));
 			vm_exit(1);
 		}
 		heap_unref(file);
diff --git a/subprojects/libsys4 b/subprojects/libsys4
index c5d3cb9..0409f62 160000
--- a/subprojects/libsys4
+++ b/subprojects/libsys4
@@ -1 +1 @@
-Subproject commit c5d3cb955e2595456d5833918e55987f9c91fc88
+Subproject commit 0409f62789bd2f9b4364d4e9a755aaff7725cfba