diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4bf6dbd..ec0769e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -67,6 +67,10 @@ elseif (EMSCRIPTEN)
     )
 
 else()  # NOT (ANDROID OR EMSCRIPTEN)
+  if (NOT NINTENDO_SWITCH)
+    option(ENABLE_DEBUGGER "Enable built-in debugger" ON)
+  endif()
+
   add_executable(system3)
   target_link_libraries(system3 PRIVATE ymfm)
 
@@ -176,6 +180,14 @@ target_sources(system3 PRIVATE
   src/sys/nact_sys3.cpp
   src/sys/scenario.cpp
   )
+if (ENABLE_DEBUGGER)
+  target_compile_definitions(system3 PRIVATE ENABLE_DEBUGGER)
+  target_sources(system3 PRIVATE
+    src/debugger/debug_info.cpp
+    src/debugger/debugger.cpp
+    src/debugger/cli_frontend.cpp
+  )
+endif()
 
 if(NINTENDO_SWITCH)
   set(RESOURCE_PATH romfs:/)
diff --git a/src/config.cpp b/src/config.cpp
index 574e193..9c66c71 100644
--- a/src/config.cpp
+++ b/src/config.cpp
@@ -77,6 +77,23 @@ std::pair<char*, char*> parse_keyval(char *line)
 	return { trim(line), trim(eq + 1) };
 }
 
+#ifdef ENABLE_DEBUGGER
+DebuggerMode parse_debugger_mode(const char *s, int lineno = -1)
+{
+	if (!strcasecmp(s, "none"))
+		return DebuggerMode::DISABLED;
+	if (!strcasecmp(s, "cli"))
+		return DebuggerMode::CLI;
+	if (!strcasecmp(s, "dap"))
+		return DebuggerMode::DAP;
+	if (lineno == -1)
+		ERROR("Command line: Invalid debugger mode '%s'", s);
+	else
+		ERROR(INIFILENAME ":%d Invalid debugger mode '%s'", lineno, s);
+	return DebuggerMode::DISABLED;
+}
+#endif
+
 TexthookMode parse_texthook_mode(const char *s, int lineno = -1)
 {
 	if (!strcasecmp(s, "none"))
@@ -135,6 +152,10 @@ Config::Config(int argc, char *argv[])
 			title = argv[++i];
 		else if (strcmp(argv[i], "-scanline") == 0)
 			scanline = true;
+#ifdef ENABLE_DEBUGGER
+		else if (strcmp(argv[i], "-debugger") == 0)
+			debugger_mode = parse_debugger_mode(argv[++i]);
+#endif
 		else if (strcmp(argv[i], "-texthook") == 0)
 			texthook_mode = parse_texthook_mode(argv[++i]);
 		else if (strcmp(argv[i], "-texthook_suppress") == 0)
@@ -197,6 +218,10 @@ void Config::load_ini()
 				title = val;
 			else if (!strcasecmp(key, "scanline"))
 				scanline = to_bool(val, lineno);
+#ifdef ENABLE_DEBUGGER
+			else if (!strcasecmp(key, "debugger"))
+				debugger_mode = parse_debugger_mode(val, lineno);
+#endif
 			else if (!strcasecmp(key, "texthook"))
 				texthook_mode = parse_texthook_mode(val, lineno);
 			else if (!strcasecmp(key, "texthook_suppress"))
diff --git a/src/config.h b/src/config.h
index a89518a..7a0c617 100644
--- a/src/config.h
+++ b/src/config.h
@@ -11,6 +11,12 @@ enum class TexthookMode {
 	COPY,
 };
 
+enum class DebuggerMode {
+	DISABLED,
+	CLI,
+	DAP,
+};
+
 struct Strings {
 	std::string back;
 	std::string next_page;
@@ -39,6 +45,7 @@ struct Config {
 	bool no_antialias = false;
 	bool scanline = false;
 	TexthookMode texthook_mode = TexthookMode::NONE;
+	DebuggerMode debugger_mode = DebuggerMode::DISABLED;
 	std::string texthook_suppressions;
 
  private:
diff --git a/src/debugger/cli_frontend.cpp b/src/debugger/cli_frontend.cpp
new file mode 100644
index 0000000..4ee799d
--- /dev/null
+++ b/src/debugger/cli_frontend.cpp
@@ -0,0 +1,408 @@
+#include "debugger/frontend.h"
+#include <stdio.h>
+#include "debugger/debug_info.h"
+#include "debugger/debugger.h"
+#include "encoding.h"
+#include "nact.h"
+#ifndef _WIN32
+#include <signal.h>
+#endif
+
+namespace debugger {
+
+namespace {
+
+const char whitespaces[] = " \t\r\n";
+const size_t CMD_BUFFER_SIZE = 256;
+
+#ifndef _WIN32
+void handle_sigint(int) {
+	g_debugger->set_state(State::STOPPED_INTERRUPT);
+}
+
+void set_sigint_handler() {
+	struct sigaction sa;
+	sa.sa_handler = handle_sigint;
+	sigemptyset(&sa.sa_mask);
+	sa.sa_flags = 0;
+	sigaction(SIGINT, &sa, nullptr);
+}
+#endif
+
+} // namespace
+
+class CliFrontend : public Frontend {
+public:
+	CliFrontend(Debugger* backend, const DebugInfo& symbols) : Frontend(backend, symbols) {
+		backend->set_state(State::STOPPED_ENTRY);
+#ifndef _WIN32
+		set_sigint_handler();
+#endif
+	}
+
+	~CliFrontend() override = default;
+
+	void repl(int bp_no) override {
+		if (backend->get_state() == State::STOPPED_BREAKPOINT && bp_no)
+			printf("Breakpoint %d\n", bp_no);
+		printf("Stopped at %s\n", format_address(g_nact->sco.page(), g_nact->sco.cmd_addr()).c_str());
+		backend->set_state(State::RUNNING);
+
+		char buf[CMD_BUFFER_SIZE];
+		while (printf("dbg> "), fflush(stdout), fgets(buf, CMD_BUFFER_SIZE, stdin)) {
+			// If input line is empty, repeat the last command.
+			if (buf[0] == '\n')
+				strcpy(buf, default_command);
+			else
+				strcpy(default_command, buf);
+
+			char *token = strtok(buf, whitespaces);
+			if (!token)
+				continue;
+
+			const Command *cmd = find_command(token);
+			if (cmd) {
+				if ((this->*(cmd->func))() == EXIT_REPL)
+					return;
+			}
+		}
+	}
+	void on_sleep() override  {
+		if (backend->get_state() == State::STOPPED_INTERRUPT)
+			backend->repl(0);
+	}
+
+	void on_palette_change() override {
+	}
+
+	void console_output(int lv, const char *output) override {
+	}
+
+private:
+	enum CommandResult { CONTINUE_REPL, EXIT_REPL };
+
+	char default_command[CMD_BUFFER_SIZE];
+
+	CommandResult cmd_backtrace();
+	CommandResult cmd_break();
+	CommandResult cmd_continue();
+	CommandResult cmd_delete();
+	CommandResult cmd_help();
+	CommandResult cmd_list();
+	CommandResult cmd_step();
+	CommandResult cmd_finish();
+	CommandResult cmd_next();
+	CommandResult cmd_print();
+	CommandResult cmd_quit();
+	CommandResult cmd_string();
+
+	struct Command {
+		const char *name;
+		const char *alias;
+		const char *description;
+		const char *help;
+		CommandResult (CliFrontend::* func)();
+	};
+
+	static const Command commands[];
+
+	const Command *find_command(const char *str);
+
+	void cmd_print_help(const Command& cmd) {
+		if (cmd.alias)
+			printf("%s, %s -- %s\n", cmd.name, cmd.alias, cmd.description);
+		else
+			printf("%s -- %s\n", cmd.name, cmd.description);
+	}
+
+	std::string format_address(int page, int addr) {
+		char buf[200];
+		const char *src = symbols.page2src(page);
+		int line = symbols.addr2line(page, addr);
+		if (src && line > 0)
+			snprintf(buf, sizeof(buf), "%s:%d", src, line);
+		else
+			snprintf(buf, sizeof(buf), "%d:0x%x", page, addr);
+		return buf;
+	}
+
+
+	bool parse_address(const char *str, int *page, int *addr) {
+		// <page>:<address>
+		if (sscanf(str, "%i:%i", page, addr) == 2)
+			return true;
+
+		// <filename>:<linenum>
+		char filename[100];
+		int line_no;
+		if (sscanf(str, "%[^:]:%i", filename, &line_no) == 2) {
+			*page = symbols.src2page(filename);
+			if (*page < 0) {
+				printf("No source file named %s.\n", filename);
+				return false;
+			}
+			*addr = symbols.line2addr(*page, line_no);
+			if (*addr < 0) {
+				printf("No line %d in file %s.\n", line_no, filename);
+				return false;
+			}
+			return true;
+		}
+
+		*page = g_nact->sco.page();
+
+		// <linenum>
+		char *endptr;
+		line_no = strtol(str, &endptr, 0);
+		if (!*endptr) {
+			*addr = symbols.line2addr(g_nact->sco.page(), line_no);
+			if (*addr < 0) {
+				printf("No line %d in current file.\n", line_no);
+				return false;
+			}
+			return true;
+		}
+
+		return false;
+	}
+};
+
+static const char desc_backtrace[] = "Print backtrace of stack frames.";
+static const char * const help_backtrace = nullptr;
+CliFrontend::CommandResult CliFrontend::cmd_backtrace() {
+	for (const StackFrame &frame : backend->stack_trace()) {
+		if (frame.src && frame.line > 0) {
+			printf("\t%s:%d\n", frame.src, frame.line);
+		} else {
+			printf("\t%d:%04x\n", frame.page, frame.addr);
+		}
+	}
+	return CONTINUE_REPL;
+}
+
+static const char desc_break[] = "Set breakpoint at specified location.";
+static const char help_break[] =
+	"Syntax: break <filename>:<linenum> [if <condition>]\n"
+	"        break <linenum> [if <condition>]\n"
+	"        break <page>:<address> [if <condition>]";
+
+CliFrontend::CommandResult CliFrontend::cmd_break() {
+	char *token = strtok(nullptr, whitespaces);
+	int page, addr;
+	if (!token || !parse_address(token, &page, &addr)) {
+		puts(help_break);
+		return CONTINUE_REPL;
+	}
+	int bp_no = backend->set_breakpoint(page, addr, false);
+	if (bp_no < 0) {
+		printf("Failed to set breakpoint at %d:%04x: invalid address\n", page, addr);
+		return CONTINUE_REPL;
+	}
+
+	printf("Breakpoint %d at %s\n", bp_no, format_address(page, addr).c_str());
+	return CONTINUE_REPL;
+}
+
+static const char desc_continue[] = "Continue program execution.";
+static const char * const help_continue = nullptr;
+CliFrontend::CommandResult CliFrontend::cmd_continue() {
+	return EXIT_REPL;
+}
+
+static const char desc_delete[] = "Delete breakpoints.";
+static const char help_delete[] =
+	"Syntax: delete <breakpoint_no>...";
+CliFrontend::CommandResult CliFrontend::cmd_delete() {
+	char *arg = strtok(nullptr, whitespaces);
+	if (!arg) {
+		puts(help_delete);
+		return CONTINUE_REPL;
+	}
+
+	while (arg) {
+		char *endptr;
+		int bp_no = strtol(arg, &endptr, 0);
+		if (*endptr)
+			printf("Bad breakpoint number %s\n", arg);
+		else {
+			if (!backend->delete_breakpoint(bp_no))
+				printf("No breakpoint number %d.\n", bp_no);
+		}
+		arg = strtok(nullptr, whitespaces);
+	}
+	return CONTINUE_REPL;
+}
+
+static const char desc_list[] = "List specified line.";
+static const char help_list[] =
+	"Syntax: list\n"
+	"        list <linenum>\n"
+	"        list <filename>:<linenum>\n"
+	"\n"
+	"With no argument, lists ten lines around current location.";
+
+CliFrontend::CommandResult CliFrontend::cmd_list() {
+	char *arg = strtok(NULL, whitespaces);
+	int page, addr;
+	if (arg) {
+		if (!parse_address(arg, &page, &addr)) {
+			puts(help_list);
+			return CONTINUE_REPL;
+		}
+	} else {
+		page = g_nact->sco.page();
+		addr = g_nact->sco.cmd_addr();
+	}
+
+	int line_no = symbols.addr2line(page, addr);
+	if (line_no <= 0) {
+		printf("Cannot determine source location for %x:%x\n", page, addr);
+		return CONTINUE_REPL;
+	}
+
+	if ((line_no -= 5) <= 0)
+		line_no = 1;
+
+	for (int i = 0; i < 10; i++) {
+		const char *line = symbols.source_line(page, line_no + i);
+		if (!line) {
+			if (i == 0)
+				printf("No source text for %s:%d\n", symbols.page2src(page), line_no);
+			break;
+		}
+		printf("%d\t%s\n", line_no + i, line);
+	}
+
+	// Empty REPL input will print the next 10 lines.
+	snprintf(default_command, CMD_BUFFER_SIZE, "l %s:%d", symbols.page2src(page), line_no + 15);
+
+	return CONTINUE_REPL;
+}
+
+static const char desc_step[] = "Step program until it reaches a different source line.";
+static const char * const help_step = nullptr;
+CliFrontend::CommandResult CliFrontend::cmd_step() {
+	backend->stepin();
+	return EXIT_REPL;
+}
+
+static const char desc_finish[] = "Execute until current function returns.";
+static const char * const help_finish = nullptr;
+CliFrontend::CommandResult CliFrontend::cmd_finish() {
+	backend->stepout();
+	return EXIT_REPL;
+}
+
+static const char desc_next[] = "Step program, proceeding through subroutine calls.";
+static const char * const help_next = nullptr;
+CliFrontend::CommandResult CliFrontend::cmd_next() {
+	backend->next();
+	return EXIT_REPL;
+}
+
+static const char desc_print[] = "Print value of variable.";
+static const char help_print[] =
+	"Syntax: print <variable>";
+
+CliFrontend::CommandResult CliFrontend::cmd_print() {
+	char *arg = strtok(NULL, whitespaces);
+	if (!arg) {
+		puts(help_print);
+		return CONTINUE_REPL;
+	}
+	int var = symbols.lookup_variable(arg);
+	if (var < 0) {
+		printf("Unrecognized variable name \"%s\".\n", arg);
+		return CONTINUE_REPL;
+	}
+	printf("%s = %d\n", arg, g_nact->get_var(var));
+	return CONTINUE_REPL;
+}
+
+static const char desc_quit[] = "Exit system3-sdl2.";
+static const char * const help_quit = nullptr;
+CliFrontend::CommandResult CliFrontend::cmd_quit() {
+	g_nact->quit(0);
+	return EXIT_REPL;
+}
+
+static const char desc_string[] = "Print value of string variable.";
+static const char help_string[] =
+	"Syntax: string <index>";
+
+CliFrontend::CommandResult CliFrontend::cmd_string() {
+	char *arg = strtok(NULL, whitespaces);
+	if (!arg) {
+		puts(help_string);
+		return CONTINUE_REPL;
+	}
+
+	while (arg) {
+		int no = atoi(arg);
+		if (no < 1 || no > MAX_STRVAR) {
+			printf("Bad string index %s\n", arg);
+		} else {
+			char *utf = g_nact->encoding->toUtf8(g_nact->get_string(no - 1));
+			printf("string[%d] = \"%s\"\n", no, utf);  // TODO: escaping
+			free(utf);
+		}
+		arg = strtok(NULL, whitespaces);
+	}
+	return CONTINUE_REPL;
+}
+
+static const char desc_help[] = "Print list of commands.";
+static const char * const help_help = nullptr;
+
+const CliFrontend::Command CliFrontend::commands[] = {
+	{"backtrace", "bt",    desc_backtrace, help_backtrace, &CliFrontend::cmd_backtrace},
+	{"break",     "b",     desc_break,     help_break,     &CliFrontend::cmd_break},
+	{"continue",  "c",     desc_continue,  help_continue,  &CliFrontend::cmd_continue},
+	{"delete",    "d",     desc_delete,    help_delete,    &CliFrontend::cmd_delete},
+	{"help",      "h",     desc_help,      help_help,      &CliFrontend::cmd_help},
+	{"list",      "l",     desc_list,      help_list,      &CliFrontend::cmd_list},
+	{"step",      "s",     desc_step,      help_step,      &CliFrontend::cmd_step},
+	{"finish",    nullptr, desc_finish,    help_finish,    &CliFrontend::cmd_finish},
+	{"next",      "n",     desc_next,      help_next,      &CliFrontend::cmd_next},
+	{"print",     "p",     desc_print,     help_print,     &CliFrontend::cmd_print},
+	{"quit",      "q",     desc_quit,      help_quit,      &CliFrontend::cmd_quit},
+	{"string",    "str",   desc_string,    help_string,    &CliFrontend::cmd_string},
+};
+
+const CliFrontend::Command *CliFrontend::find_command(const char *str) {
+	for (const Command &cmd : commands) {
+		if (!strcmp(str, cmd.name) || (cmd.alias && !strcmp(str, cmd.alias)))
+			return &cmd;
+	}
+	printf("Unknown command \"%s\". Try \"help\".\n", str);
+	return nullptr;
+}
+
+CliFrontend::CommandResult CliFrontend::cmd_help() {
+	char *token = strtok(nullptr, whitespaces);
+	if (!token) {
+		puts("List of commands:");
+		puts("");
+		for (auto& cmd : commands)
+			cmd_print_help(cmd);
+		puts("");
+		puts("Type \"help\" followed by command name for full documentation.");
+		return CONTINUE_REPL;
+	}
+	const Command *cmd = find_command(token);
+	if (cmd) {
+		cmd_print_help(*cmd);
+		if (cmd->help) {
+			puts("");
+			puts(cmd->help);
+		}
+	}
+	return CONTINUE_REPL;
+}
+
+// static
+Frontend* Frontend::create_cli(Debugger* backend, const DebugInfo& symbols) {
+	return new CliFrontend(backend, symbols);
+}
+
+} // namespace debugger
diff --git a/src/debugger/debug_info.cpp b/src/debugger/debug_info.cpp
new file mode 100644
index 0000000..c969996
--- /dev/null
+++ b/src/debugger/debug_info.cpp
@@ -0,0 +1,220 @@
+#include "debugger/debug_info.h"
+#include <algorithm>
+#include <assert.h>
+#include <string.h>
+#include "fileio.h"
+
+namespace debugger {
+
+namespace {
+
+inline uint32_t LittleEndian_getDW(uint8_t* buf, int ofs) {
+	return buf[ofs] | (buf[ofs + 1] << 8) | (buf[ofs + 2] << 16) | (buf[ofs + 3] << 24);
+}
+
+std::string fget4cc(FILEIO& fio) {
+	char buf[5] = {0};
+	if (!fio.read(buf, 4))
+		buf[0] = '\0';
+	return buf;
+}
+
+bool ensure_srcs(std::vector<SrcInfo>& srcs, uint32_t nr_srcs) {
+	if (srcs.empty()) {
+		srcs.resize(nr_srcs);
+	} else if (nr_srcs != srcs.size()) {
+		WARNING("malformed debug information");
+		return false;
+	}
+	return true;
+}
+
+bool load_srcs(std::vector<SrcInfo>& srcs, std::vector<uint8_t>& buf) {
+	uint32_t nr_srcs = LittleEndian_getDW(buf.data(), 0);
+	if (!ensure_srcs(srcs, nr_srcs))
+		return false;
+
+	char *p = reinterpret_cast<char*>(buf.data()) + 4;
+	for (uint32_t i = 0; i < nr_srcs; i++) {
+		srcs[i].filename = p;
+		p += strlen(p) + 1;
+	}
+	return true;
+}
+
+void store_src_lines(SrcInfo& info, char *src) {
+	while (src) {
+		char *eol = strchr(src, '\n');
+		if (eol) {
+			if (src < eol && eol[-1] == '\r')
+				eol[-1] = '\0';
+			else
+				eol[0] = '\0';
+			eol++;
+		}
+		info.lines.push_back(src);
+		src = eol;
+	}
+}
+
+bool load_scnt(std::vector<SrcInfo>& srcs, std::vector<uint8_t>& buf) {
+	uint32_t nr_srcs = LittleEndian_getDW(buf.data(), 0);
+	if (!ensure_srcs(srcs, nr_srcs))
+		return false;
+
+	char *p = reinterpret_cast<char*>(buf.data()) + 4;
+	for (uint32_t i = 0; i < nr_srcs; i++) {
+		size_t len = strlen(p);
+		store_src_lines(srcs[i], p);
+		p += len + 1;
+	}
+	return true;
+}
+
+bool load_line(std::vector<SrcInfo>& srcs, std::vector<uint8_t>& buf) {
+	uint32_t nr_srcs = LittleEndian_getDW(buf.data(), 0);
+	if (!ensure_srcs(srcs, nr_srcs))
+		return false;
+
+	int ofs = 4;
+	for (uint32_t i = 0; i < nr_srcs; i++) {
+		SrcInfo& info = srcs[i];
+		uint32_t nr_mappings = LittleEndian_getDW(buf.data(), ofs);
+		ofs += 4;
+		info.mappings.resize(nr_mappings);
+		for (uint32_t i = 0; i < nr_mappings; i++) {
+			info.mappings[i].line = LittleEndian_getDW(buf.data(), ofs);
+			info.mappings[i].addr = LittleEndian_getDW(buf.data(), ofs + 4);
+			ofs += 8;
+		}
+	}
+	return true;
+}
+
+bool load_vari(std::vector<std::string>& variables, std::vector<uint8_t>& buf) {
+	uint32_t nr_vars = LittleEndian_getDW(buf.data(), 0);
+	char *p = reinterpret_cast<char*>(buf.data()) + 4;
+	for (uint32_t i = 0; i < nr_vars; i++) {
+		variables.push_back(p);
+		p += strlen(p) + 1;
+	}
+	return true;
+}
+
+} // namespace
+
+int SrcInfo::line2addr(int line) const {
+	// Find the first mapping with line >= `line`.
+	auto it = std::lower_bound(mappings.begin(), mappings.end(), line,
+        [](const Mapping& mapping, int line) {
+            return mapping.line < line;
+        });
+	return it != mappings.end() ? it->addr : -1;
+}
+
+int SrcInfo::addr2line(int addr) const {
+	// Find the last mapping with addr <= `addr`.
+	auto it = std::upper_bound(mappings.begin(), mappings.end(), addr,
+		[](int addr, const Mapping& mapping) {
+			return addr < mapping.addr;
+		});
+	return it != mappings.begin() ? (it - 1)->line : -1;
+}
+
+void DebugInfo::load(const char *path) {
+	auto fio = FILEIO::open(path, FILEIO_READ_BINARY);
+	if (!fio) {
+		WARNING("Cannot open %s: %s", path, strerror(errno));
+		return;
+	}
+	if (fget4cc(*fio) != "DSYM") {
+		WARNING("%s: wrong signature", path);
+		return;
+	}
+	int version = fio->getdw();
+	if (version != 0) {
+		WARNING("%s: unsupported debug info version", path);
+		return;
+	}
+
+	uint32_t nr_sections = fio->getdw();
+	for (uint32_t i = 0; i < nr_sections; i++) {
+		std::string tag = fget4cc(*fio);
+		uint32_t section_size = fio->getdw();
+		std::vector<uint8_t> section_content(section_size - 8);
+		if (!fio->read(section_content.data(), section_content.size())) {
+			WARNING("%s: %s", path, strerror(errno));
+			return;
+		}
+
+		if (tag == "SRCS") {
+			if (!load_srcs(srcs, section_content)) return;
+		} else if (tag == "SCNT") {
+			if (!load_scnt(srcs, section_content)) return;
+		} else if (tag == "LINE") {
+			if (!load_line(srcs, section_content)) return;
+		} else if (tag == "VARI") {
+			if (!load_vari(variables, section_content)) return;
+		} else {
+			WARNING("%s: unrecognized section %s", path, tag.c_str());
+		}
+	}
+	if (fio->getc() != EOF) {
+		WARNING("%s: broken debug information structure", path);
+		return;
+	}
+	loaded_ = !srcs.empty() && !variables.empty();
+}
+
+int DebugInfo::src2page(const char *fname) const {
+	for (size_t i = 0; i < srcs.size(); i++) {
+		if (!strcasecmp(srcs[i].filename.c_str(), fname))
+			return static_cast<int>(i);
+	}
+	return -1;
+}
+
+const char *DebugInfo::page2src(int page) const {
+	if (static_cast<size_t>(page) >= srcs.size())
+		return NULL;
+	return srcs[page].filename.c_str();
+}
+
+int DebugInfo::line2addr(int page, int line) const {
+	if (static_cast<size_t>(page) >= srcs.size())
+		return -1;
+	return srcs[page].line2addr(line);
+}
+
+int DebugInfo::addr2line(int page, int addr) const {
+	if (static_cast<size_t>(page) >= srcs.size())
+		return -1;
+	return srcs[page].addr2line(addr);
+}
+
+const char *DebugInfo::source_line(int page, int line) const {
+	if (static_cast<size_t>(page) >= srcs.size())
+		return NULL;
+	const SrcInfo& info = srcs[page];
+
+	line--; // 1-based to 0-based index
+	if (static_cast<size_t>(line) >= info.lines.size())
+		return NULL;
+	return info.lines[line].c_str();
+}
+
+const char *DebugInfo::variable_name(int i) const {
+	if (static_cast<size_t>(i) >= variables.size())
+		return NULL;
+	return variables[i].c_str();
+}
+
+int DebugInfo::lookup_variable(const char *name) const {
+	for (size_t i = 0; i < variables.size(); i++) {
+		if (variables[i] == name)
+			return static_cast<int>(i);
+	}
+	return -1;
+}
+
+} // namespace debugger
diff --git a/src/debugger/debug_info.h b/src/debugger/debug_info.h
new file mode 100644
index 0000000..5848c78
--- /dev/null
+++ b/src/debugger/debug_info.h
@@ -0,0 +1,43 @@
+#ifndef _DEBUG_INFO_H_
+#define _DEBUG_INFO_H_
+
+#include <string>
+#include <vector>
+
+namespace debugger {
+
+struct SrcInfo {
+	struct Mapping {
+		int line;
+		int addr;
+	};
+	std::string filename;
+	std::vector<std::string> lines;
+	std::vector<Mapping> mappings;
+
+	int line2addr(int line) const;
+	int addr2line(int addr) const;
+};
+
+class DebugInfo {
+public:
+	void load(const char *filename);
+	bool loaded() const { return loaded_; }
+	int src2page(const char *fname) const;
+	const char *page2src(int page) const;
+	int line2addr(int page, int line) const;
+	int addr2line(int page, int addr) const;
+	const char *source_line(int page, int line) const;
+	size_t num_variables() const { return variables.size(); }
+	const char *variable_name(int i) const;
+	int lookup_variable(const char *name) const;
+
+private:
+	bool loaded_ = false;
+	std::vector<SrcInfo> srcs;
+	std::vector<std::string> variables;
+};
+
+} // namespace debugger
+
+#endif // _DEBUG_INFO_H_
diff --git a/src/debugger/debugger.cpp b/src/debugger/debugger.cpp
new file mode 100644
index 0000000..121cdfd
--- /dev/null
+++ b/src/debugger/debugger.cpp
@@ -0,0 +1,219 @@
+#include "debugger/debugger.h"
+#include <algorithm>
+#include <map>
+#include "common.h"
+#include "nact.h"
+#include "debugger/frontend.h"
+
+namespace debugger {
+
+namespace {
+
+const int INTERNAL_BREAKPOINT_NO = -1;
+
+struct Breakpoint {
+	int page;
+	int addr;
+	int no;
+	uint8_t restore_op;
+};
+
+std::map<std::pair<int, int>, Breakpoint> breakpoints;
+int next_breakpoint_no = 1;
+
+// Used in State::STOPPED_STEP and State::STOPPED_NEXT
+struct {
+	int page;
+	int line;
+} step_exec_state;
+
+bool should_continue_step(const DebugInfo& symbols) {
+	if (step_exec_state.line < 0)
+		return false;  // line info was not available
+	return step_exec_state.page == g_nact->sco.page()
+		&& step_exec_state.line == symbols.addr2line(step_exec_state.page, g_nact->sco.cmd_addr());
+}
+
+int get_retaddr_if_funcall() {
+	int orig_addr = g_nact->sco.current_addr();
+	int retaddr = -1;
+
+	g_nact->sco.jump_to(g_nact->sco.cmd_addr());
+	int c0 = g_nact->sco.getd();
+	if (c0 == BREAKPOINT_INSTRUCTION) {
+		auto bp = breakpoints.find({g_nact->sco.page(), g_nact->sco.cmd_addr()});
+		if (bp == breakpoints.end())
+			sys_error("Illegal BREAKPOINT instruction");
+		c0 = bp->second.restore_op;
+	}
+	switch (c0) {
+	case '%':
+		if (g_nact->cali() != 0)
+			retaddr = g_nact->sco.current_addr();
+		break;
+	case '\\':
+		if (g_nact->sco.getw() != 0)
+			retaddr = g_nact->sco.current_addr();
+		break;
+	}
+	g_nact->sco.jump_to(orig_addr);
+	return retaddr;
+}
+
+} // namespace
+
+Debugger::Debugger(const char *symbols_path, DebuggerMode mode) {
+	symbols.load(symbols_path);
+	if (mode == DebuggerMode::DAP) {
+		// frontend = std::make_unique<DapFrontend>(this);
+	} else {
+		frontend = std::unique_ptr<Frontend>(Frontend::create_cli(this, symbols));
+	}
+}
+
+Debugger::~Debugger() = default;
+
+void Debugger::repl(int bp_no) {
+	delete_breakpoint(INTERNAL_BREAKPOINT_NO);
+
+	switch (state) {
+	case State::STOPPED_STEP:
+		if (should_continue_step(symbols))
+			return;
+		break;
+	case State::STOPPED_NEXT:
+		if (should_continue_step(symbols)) {
+			do_next();
+			return;
+		}
+		break;
+	default:
+		break;
+	}
+	frontend->repl(bp_no);
+}
+
+void Debugger::on_sleep() {
+	frontend->on_sleep();
+}
+
+void Debugger::on_palette_change() {
+	frontend->on_palette_change();
+}
+
+uint8_t Debugger::handle_breakpoint(int page, int addr) {
+	auto bp = breakpoints.find({page, addr});
+	if (bp == breakpoints.end())
+		sys_error("Illegal BREAKPOINT instruction");
+
+	uint8_t restore_op = bp->second.restore_op;
+
+	state = bp->second.no == INTERNAL_BREAKPOINT_NO ? State::STOPPED_NEXT : State::STOPPED_BREAKPOINT;
+
+	repl(bp->second.no);  // this may destroy bp
+	return restore_op;
+}
+
+bool Debugger::console_vprintf(int lv, const char *format, va_list ap) {
+	return false;
+}
+void Debugger::post_command(void *data) {
+}
+
+int Debugger::set_breakpoint(int page, int addr, bool is_internal)
+{
+	int restore_op = g_nact->sco.write_instruction(page, addr, BREAKPOINT_INSTRUCTION);
+	if (restore_op == -1)
+		return -1;
+	if (restore_op == BREAKPOINT_INSTRUCTION) {
+		auto bp = breakpoints.find({page, addr});
+		if (bp != breakpoints.end()) {
+			sys_error("Illegal BREAKPOINT instruction");
+		} else {
+			return bp->second.no;
+		}
+	}
+	int no = is_internal ? INTERNAL_BREAKPOINT_NO : next_breakpoint_no++;
+	breakpoints[{page, addr}] = {page, addr, no, static_cast<uint8_t>(restore_op)};
+	return no;
+}
+
+bool Debugger::delete_breakpoint(int no)
+{
+	for (auto it = breakpoints.begin(); it != breakpoints.end(); ++it) {
+		if (it->second.no == no) {
+			g_nact->sco.write_instruction(it->second.page, it->second.addr, it->second.restore_op);
+			breakpoints.erase(it);
+			return true;
+		}
+	}
+	return false;
+}
+
+void Debugger::delete_breakpoints_in_page(int page)
+{
+	auto lower = breakpoints.lower_bound({page, 0});
+	auto upper = breakpoints.upper_bound({page, 0xffff});
+	for (auto it = lower; it != upper; ++it) {
+		g_nact->sco.write_instruction(it->second.page, it->second.addr, it->second.restore_op);
+	}
+	breakpoints.erase(lower, upper);
+}
+
+void Debugger::stepin()
+{
+	step_exec_state.page = g_nact->sco.page();
+	step_exec_state.line = symbols.addr2line(step_exec_state.page, g_nact->sco.cmd_addr());
+	state = State::STOPPED_STEP;
+}
+
+void Debugger::stepout()
+{
+	// Set an internal breakpoint at the return address of current frame.
+	auto& call_stack = g_nact->sco.get_call_stack();
+	if (call_stack.empty()) {
+		return;
+	}
+	const auto& frame = call_stack.back();
+	set_breakpoint(frame.page, frame.addr, true);
+}
+
+void Debugger::next()
+{
+	step_exec_state.page = g_nact->sco.page();
+	step_exec_state.line = symbols.addr2line(step_exec_state.page, g_nact->sco.cmd_addr());
+	state = State::STOPPED_NEXT;
+	do_next();
+}
+
+void Debugger::do_next()
+{
+	assert(state == State::STOPPED_NEXT);
+
+	int retaddr = get_retaddr_if_funcall();
+	if (retaddr >= 0) {
+		set_breakpoint(step_exec_state.page, retaddr, true);
+		state = State::RUNNING;
+	}
+}
+
+std::vector<StackFrame> Debugger::stack_trace()
+{
+	std::vector<StackFrame> frames;
+	const char *src = symbols.page2src(g_nact->sco.page());
+	int line = symbols.addr2line(g_nact->sco.page(), g_nact->sco.cmd_addr());
+	frames.emplace_back(g_nact->sco.page(), g_nact->sco.cmd_addr(), src, line);
+
+	auto& call_stack = g_nact->sco.get_call_stack();
+	std::for_each(call_stack.rbegin(), call_stack.rend(), [&](const auto& frame) {
+		const char* src = symbols.page2src(frame.page);
+		// -1 because the return address is the address of the next command
+		int line = symbols.addr2line(frame.page, frame.addr - 1);
+		frames.emplace_back(frame.page, frame.addr, src, line);
+	});
+	return frames;
+}
+
+} // namespace debugger
+
+std::unique_ptr<debugger::Debugger> g_debugger;
diff --git a/src/debugger/debugger.h b/src/debugger/debugger.h
new file mode 100644
index 0000000..b2749f6
--- /dev/null
+++ b/src/debugger/debugger.h
@@ -0,0 +1,76 @@
+#ifndef _DEBUGGER_H_
+#define _DEBUGGER_H_
+
+#ifdef ENABLE_DEBUGGER
+
+#include <memory>
+#include <stdarg.h>
+#include "config.h"
+#include "debugger/debug_info.h"
+
+namespace debugger {
+
+const uint8_t BREAKPOINT_INSTRUCTION = 0x0f;
+
+class Frontend;
+
+enum class State {
+	RUNNING,
+	STOPPED_ENTRY,
+	STOPPED_STEP,
+	STOPPED_NEXT,
+	STOPPED_BREAKPOINT,
+	STOPPED_INTERRUPT,
+	STOPPED_EXCEPTION,
+};
+
+struct StackFrame {
+	int page;
+	int addr;
+	const char *src;
+	int line;
+
+	StackFrame(int page, int addr, const char *src, int line)
+		: page(page), addr(addr), src(src), line(line) {}
+};
+
+class Debugger {
+public:
+	Debugger(const char *symbols_path, DebuggerMode mode);
+	~Debugger();
+	void repl(int bp_no);
+
+	// API for VM
+	bool trapped() const { return state != State::RUNNING; }
+	void on_sleep();
+	void on_palette_change();
+	uint8_t handle_breakpoint(int page, int addr);
+	bool console_vprintf(int lv, const char *format, va_list ap);
+	void post_command(void *data);
+
+	// API for frontend
+	State get_state() const { return state; }
+	void set_state(State s) { state = s; }
+	int set_breakpoint(int page, int addr, bool is_internal);
+	bool delete_breakpoint(int no);
+	void delete_breakpoints_in_page(int page);
+	void stepin();
+	void stepout();
+	void next();
+	std::vector<StackFrame> stack_trace();
+
+private:
+	State state = State::RUNNING;
+	DebugInfo symbols;
+	std::unique_ptr<Frontend> frontend;
+
+	void do_next();
+};
+
+} // namespace debugger
+
+extern std::unique_ptr<debugger::Debugger> g_debugger;
+
+#endif // ENABLE_DEBUGGER
+
+#endif /* _DEBUGGER_H_ */
diff --git a/src/debugger/frontend.h b/src/debugger/frontend.h
new file mode 100644
index 0000000..f441349
--- /dev/null
+++ b/src/debugger/frontend.h
@@ -0,0 +1,26 @@
+#ifndef _DEBUGGER_FRONTEND_H_
+#define _DEBUGGER_FRONTEND_H_
+
+namespace debugger {
+
+class Debugger;
+class DebugInfo;
+
+class Frontend {
+public:
+	static Frontend* create_cli(Debugger* backend, const DebugInfo& symbols);
+
+	Frontend(Debugger* backend, const DebugInfo& symbols) : backend(backend), symbols(symbols) {}
+	virtual ~Frontend() = default;
+	virtual void repl(int bp_no) = 0;
+	virtual void on_sleep() = 0;
+	virtual void on_palette_change() = 0;
+	virtual void console_output(int lv, const char *output) = 0;
+protected:
+	Debugger* backend;
+	const DebugInfo& symbols;
+};
+
+} // namespace debugger
+
+#endif /* _DEBUGGER_FRONTEND_H_ */
diff --git a/src/fileio.h b/src/fileio.h
index 6b4e90f..248f388 100644
--- a/src/fileio.h
+++ b/src/fileio.h
@@ -44,6 +44,13 @@ public:
 		int val = getc();
 		return val | (getc() << 8);
 	}
+	uint32_t getdw() {
+		uint32_t val = getc();
+		val |= getc() << 8;
+		val |= getc() << 16;
+		val |= getc() << 24;
+		return val;
+	}
 	void putc(int c) { fputc(c, fp); }
 	void putw(int w) {
 		putc(w & 0xff);
diff --git a/src/sdlmain.cpp b/src/sdlmain.cpp
index f29884f..0e0a14b 100644
--- a/src/sdlmain.cpp
+++ b/src/sdlmain.cpp
@@ -4,6 +4,7 @@
 #include "fileio.h"
 #include "texthook.h"
 #include "sys/nact.h"
+#include "debugger/debugger.h"
 #ifdef _WIN32
 #include <direct.h>
 #else
@@ -108,6 +109,12 @@ int main(int argc, char *argv[])
 	texthook_set_mode(config.texthook_mode);
 	texthook_set_suppression_list(config.texthook_suppressions.c_str());
 
+#ifdef ENABLE_DEBUGGER
+	if (config.debugger_mode != DebuggerMode::DISABLED) {
+		g_debugger = std::make_unique<debugger::Debugger>("ADISK.DAT.symbols", config.debugger_mode);
+	}
+#endif
+
 	int exit_code = 0;
 	while (g_nact) {
 		exit_code = g_nact->mainloop();
diff --git a/src/sys/msgskip.cpp b/src/sys/msgskip.cpp
index c3c0a6a..61301b0 100644
--- a/src/sys/msgskip.cpp
+++ b/src/sys/msgskip.cpp
@@ -39,8 +39,7 @@ void MsgSkip::load_from_file()
 {
 	auto fio = FILEIO::open(MSGSKIP_FILENAME, FILEIO_READ_BINARY | FILEIO_SAVEDATA);
 	if (fio) {
-		uint32 size = fio->getw();
-		size |= fio->getw() << 16;
+		uint32 size = fio->getdw();
 		if (size == BLOOM_FILTER_SIZE)
 			fio->read(bloom, BLOOM_FILTER_SIZE / 8);
 	}
diff --git a/src/sys/nact.cpp b/src/sys/nact.cpp
index 133560e..5542d09 100644
--- a/src/sys/nact.cpp
+++ b/src/sys/nact.cpp
@@ -16,6 +16,7 @@
 #include "../config.h"
 #include "../fileio.h"
 #include "game_id.h"
+#include "debugger/debugger.h"
 
 extern SDL_Window* g_window;
 
@@ -86,6 +87,12 @@ int NACT::mainloop()
 
 	int sleep_cnt = 0;
 	while(!terminate) {
+#ifdef ENABLE_DEBUGGER
+		if (g_debugger && g_debugger->trapped()) {
+			sco.update_cmd_addr();
+			g_debugger->repl(0);
+		}
+#endif
 		execute();
 		// 512コマンド実行毎にSleep(10)
 		if(!(sleep_cnt = (sleep_cnt + 1) & 0x1ff)) {
@@ -132,6 +139,14 @@ void NACT::execute()
 		return;
 	}
 
+#ifdef ENABLE_DEBUGGER
+	if (cmd == debugger::BREAKPOINT_INSTRUCTION) {
+		if (!g_debugger)
+			sys_error("Illegal BREAKPOINT instruction");
+		cmd = g_debugger->handle_breakpoint(sco.page(), sco.cmd_addr());
+	}
+#endif
+
 	switch(cmd) {
 		case '!':
 			cmd_calc();
@@ -817,6 +832,10 @@ void NACT::wait_after_open_menu()
 void NACT::sys_sleep(int ms) {
 	pump_events();
 	ags->update_screen();
+#ifdef ENABLE_DEBUGGER
+	if (g_debugger)
+		g_debugger->on_sleep();
+#endif
 #ifdef __EMSCRIPTEN__
 	emscripten_sleep(ms);
 #else
diff --git a/src/sys/nact.h b/src/sys/nact.h
index 73bcfe1..d04c67a 100644
--- a/src/sys/nact.h
+++ b/src/sys/nact.h
@@ -80,6 +80,7 @@
 #define MAX_VERB 128
 #define MAX_OBJ 256
 #define MAX_PCM 256
+#define MAX_STRVAR 10
 
 class AGS;
 class MAKO;
@@ -97,6 +98,8 @@ public:
 	NACT(const Config& config, const GameId& game_id);
 	virtual ~NACT();
 
+	Scenario sco;
+
 protected:
 	AGS* ags;
 	MAKO* mako;
@@ -105,13 +108,11 @@ protected:
 	// コマンドパーサ
 	void execute();
 
-	Scenario sco;
-
 	// 変数
 	uint16 var[512] = {};
 	uint16 var_stack[30][20] = {};
-	char tvar[10][33] = {};
-	char tvar_stack[30][10][22] = {};
+	char tvar[MAX_STRVAR][33] = {};
+	char tvar_stack[30][MAX_STRVAR][22] = {};
 	int tvar_index = 0;
 	int tvar_maxlen;
 
@@ -194,7 +195,6 @@ protected:
 	int pcm[MAX_PCM] = {};
 
 	// 下位関数
-	virtual uint16 cali() = 0;
 	bool is_message(uint8_t c) { return c == ' ' || c & 0x80; }
 
 	uint16 random(uint16 range);
@@ -237,6 +237,8 @@ public:
 	void text_wait();
 	void set_skip_menu_state(bool enabled, bool checked);
 
+	virtual uint16 cali() = 0;
+
 	bool mouse_move_enabled = true;
 	const Config& config;
 	const GameId& game_id;
@@ -247,6 +249,8 @@ public:
 	void output_console(const char *format, ...);
 
 	int get_scenario_page() const { return sco.page(); }
+	uint16 get_var(int index) const { return var[index]; }
+	const char* get_string(int index) const { return tvar[index]; }
 
 private:
 	void pump_events();
diff --git a/src/sys/scenario.cpp b/src/sys/scenario.cpp
index c78d1da..027ab43 100644
--- a/src/sys/scenario.cpp
+++ b/src/sys/scenario.cpp
@@ -6,17 +6,44 @@
 
 uint8_t Scenario::fetch_command()
 {
-	if (addr_ < 2 || static_cast<size_t>(addr_) >= data_.size())
+	if (addr_ < 2 || static_cast<size_t>(addr_) >= data_size_)
 		sys_error("Scenario error: invalid address %d:%04x", page_, addr_);
 
 	// Skip SysEng's "new style" marker
 	if (page_ == 0 && addr_ == 2 && data_[2] == 'R' && data_[3] == 'E' && data_[4] == 'V')
 		addr_ = 5;
 
-	cmd_addr_ = addr_;
+	update_cmd_addr();
 	return getd();
 }
 
+std::vector<uint8_t>* Scenario::load_page(int page)
+{
+	if (static_cast<size_t>(page) < pages_.size() && !pages_[page].empty())
+		return &pages_[page];
+	std::vector<uint8_t> data = adisk.load(page + 1);
+	if (data.empty())
+		return nullptr;
+	if (static_cast<size_t>(page) >= pages_.size())
+		pages_.resize(page + 1);
+	pages_[page] = std::move(data);
+	return &pages_[page];
+}
+
+void Scenario::page_jump(int page, int addr)
+{
+	std::vector<uint8_t>* page_data = load_page(page);
+	if (page_data) {
+		data_ = page_data->data();
+		data_size_ = page_data->size();
+	} else {
+		data_ = nullptr;
+		data_size_ = 0;
+	}
+	page_ = page;
+	cmd_addr_ = addr_ = addr;
+}
+
 void Scenario::skip_syseng_string(Encoding *enc, uint8_t terminator)
 {
 	for (uint8_t c = getd(); c != terminator; c = getd()) {
@@ -136,4 +163,16 @@ void Scenario::page_stack_clear()
 	} else {
 		sys_error("Unknown Command: %02x at %d:%04x", cmd, page_, cmd_addr_);
 	}
-}
\ No newline at end of file
+}
+
+int Scenario::write_instruction(int page, int addr, uint8_t op)
+{
+	std::vector<uint8_t>* page_data = load_page(page);
+	if (!page_data)
+		return -1;
+	if (addr < 0 || addr >= static_cast<int>(page_data->size()))
+		return -1;
+	uint8_t old_op = (*page_data)[addr];
+	(*page_data)[addr] = op;
+	return old_op;
+}
diff --git a/src/sys/scenario.h b/src/sys/scenario.h
index f73e5ce..8900fe3 100644
--- a/src/sys/scenario.h
+++ b/src/sys/scenario.h
@@ -11,11 +11,7 @@ class Encoding;
 class Scenario {
 public:
 	void open(const char* file_name) { adisk.open(file_name); }
-	void page_jump(int page, int addr) {
-		data_ = adisk.load(page + 1);
-		page_ = page;
-		cmd_addr_ = addr_ = addr;
-	}
+	void page_jump(int page, int addr);
 	int default_addr() { return data_[0] | data_[1] << 8; }
 
 	int page() const { return page_; }
@@ -28,6 +24,7 @@ public:
 	void skip(int n) { addr_ += n; }
 
 	uint8_t fetch_command();
+	void update_cmd_addr() { cmd_addr_ = addr_; }
 
 	uint8_t getd() { return data_[addr_++]; }
 	uint16_t getw() {
@@ -63,8 +60,12 @@ public:
 	int write_instruction(int page, int addr, uint8_t op);
 
 private:
+	std::vector<uint8_t>* load_page(int page);
+
 	Dri adisk;
-	std::vector<uint8_t> data_;
+	std::vector<std::vector<uint8_t>> pages_;
+	const uint8_t* data_;
+	size_t data_size_;
 	int page_;
 	int addr_;
 	int cmd_addr_;