diff --git a/src/sys/nact.cpp b/src/sys/nact.cpp
index 10784b6..708feed 100644
--- a/src/sys/nact.cpp
+++ b/src/sys/nact.cpp
@@ -117,20 +117,12 @@ void NACT::quit(int code)
 EMSCRIPTEN_KEEPALIVE  // Prevent inlining, because this function is listed in ASYNCIFY_ADD
 void NACT::execute()
 {
-	if (!sco.is_addr_valid()) {
-		sys_error("Scenario error: invalid address %d:0x%x", sco.page(), sco.addr());
-		return;
-	}
-
-	if (game_id.sys_ver == 1 && sco.page() == 0 && sco.addr() == 2) {
+	if (game_id.sys_ver == 1 && sco.page() == 0 && sco.current_addr() == 2) {
 		opening();
 	}
 
-	sco.skip_syseng_rev_marker();
-
 	// １コマンド実行
-	sco.mark_cmd_start();
-	uint8 cmd = sco.getd();
+	uint8 cmd = sco.fetch_command();
 
 	if(set_palette && cmd != 'P') {
 		// パレット設定が終わった
@@ -323,7 +315,7 @@ void NACT::message(uint8 terminator)
 		}
 	}
 	if (!ags->draw_menu)
-		msgskip->on_message(sco.page(), sco.addr());
+		msgskip->on_message(sco.page(), sco.current_addr());
 
 	// TODO: Convert hankaku to zenkaku
 	output_console(buf);
diff --git a/src/sys/nact_sys1.cpp b/src/sys/nact_sys1.cpp
index 0a91494..9e93257 100644
--- a/src/sys/nact_sys1.cpp
+++ b/src/sys/nact_sys1.cpp
@@ -145,8 +145,7 @@ void NACT_Sys1::cmd_branch()
 	if(!condition) {
 		// 次の'}'命令までスキップする（ネストも考慮する）
 		for(;;) {
-			sco.mark_cmd_start();
-			uint8 cmd = sco.getd();
+			uint8 cmd = sco.fetch_command();
 
 			if(cmd == '!') {
 				int index = sco.getd();
@@ -785,7 +784,7 @@ void NACT_Sys1::cmd_q()
 			FPUTW(0);
 			FPUTW(mako->current_music);
 			FPUTW(0);
-			FPUTW(sco.addr());
+			FPUTW(sco.current_addr());
 			FPUTW(0);
 			for(int i = 0; i < 512; i++) {
 				FPUTW(var[i]);
@@ -1294,7 +1293,7 @@ uint16 NACT_Sys1::cali()
 		}
 	}
 	if (!ok) {
-		sys_error("cali: invalid expression at %d:%04x", sco.page(), sco.addr());
+		sys_error("cali: invalid expression at %d:%04x", sco.page(), sco.cmd_addr());
 	}
 	return (uint16)(cali[1] & 0xffff);
 }
@@ -1309,10 +1308,10 @@ uint16 NACT_Sys1::cali2()
 	} else if(0xc0 <= dat && dat <= 0xff) {
 		val = ((dat & 0x3f) << 8) | sco.getd();
 	} else {
-		sys_error("cali2: invalid expression at %d:%04x", sco.page(), sco.addr());
+		sys_error("cali2: invalid expression at %d:%04x", sco.page(), sco.cmd_addr());
 	}
 	if (sco.getd() != 0x7f) {
-		sys_error("cali2: invalid expression at %d:%04x", sco.page(), sco.addr());
+		sys_error("cali2: invalid expression at %d:%04x", sco.page(), sco.cmd_addr());
 	}
 	return val;
 }
diff --git a/src/sys/nact_sys2.cpp b/src/sys/nact_sys2.cpp
index c1cfb4a..48f8854 100644
--- a/src/sys/nact_sys2.cpp
+++ b/src/sys/nact_sys2.cpp
@@ -80,8 +80,7 @@ void NACT_Sys2::cmd_branch()
 	if(!condition) {
 		// 次の'}'命令までスキップする（ネストも考慮する）
 		for(;;) {
-			sco.mark_cmd_start();
-			uint8 cmd = sco.getd();
+			uint8 cmd = sco.fetch_command();
 
 			if(cmd == '!') {
 				int index = sco.getd();
@@ -870,7 +869,7 @@ void NACT_Sys2::cmd_q()
 			FPUTW(0);
 			FPUTW(mako->current_music);
 			FPUTW(0);
-			FPUTW(sco.addr());
+			FPUTW(sco.current_addr());
 			FPUTW(0);
 			for(int i = 0; i < 512; i++) {
 				FPUTW(var[i]);
@@ -1263,7 +1262,7 @@ uint16 NACT_Sys2::cali()
 		}
 	}
 	if (!ok) {
-		sys_error("cali: invalid expression at %d:%04x", sco.page(), sco.addr());
+		sys_error("cali: invalid expression at %d:%04x", sco.page(), sco.cmd_addr());
 	}
 	return (uint16)(cali[1] & 0xffff);
 }
@@ -1278,10 +1277,10 @@ uint16 NACT_Sys2::cali2()
 	} else if(0xc0 <= dat && dat <= 0xff) {
 		val = ((dat & 0x3f) << 8) | sco.getd();
 	} else {
-		sys_error("cali2: invalid expression at %d:%04x", sco.page(), sco.addr());
+		sys_error("cali2: invalid expression at %d:%04x", sco.page(), sco.cmd_addr());
 	}
 	if (sco.getd() != 0x7f) {
-		sys_error("cali2: invalid expression at %d:%04x", sco.page(), sco.addr());
+		sys_error("cali2: invalid expression at %d:%04x", sco.page(), sco.cmd_addr());
 	}
 	return val;
 }
diff --git a/src/sys/nact_sys3.cpp b/src/sys/nact_sys3.cpp
index c0c8cd0..48aa7ec 100644
--- a/src/sys/nact_sys3.cpp
+++ b/src/sys/nact_sys3.cpp
@@ -31,7 +31,7 @@ void NACT_Sys3::cmd_calc()
 void NACT_Sys3::cmd_branch()
 {
 	int condition = cali();
-	int t_addr = sco.addr() + 2;
+	int t_addr = sco.current_addr() + 2;
 	int f_addr = sco.getw();
 
 	// sigmarion3 最適化誤爆の対策
@@ -823,7 +823,7 @@ void NACT_Sys3::cmd_q()
 			FPUTW(0);
 			FPUTW(mako->current_music);
 			FPUTW(0);
-			FPUTW(sco.addr());
+			FPUTW(sco.current_addr());
 			FPUTW(0);
 			for(int i = 0; i < 512; i++) {
 				FPUTW(var[i]);
@@ -1428,7 +1428,7 @@ uint16 NACT_Sys3::cali()
 		}
 	}
 	if (!ok) {
-		sys_error("cali: invalid expression at %d:%04x", sco.page(), sco.addr());
+		sys_error("cali: invalid expression at %d:%04x", sco.page(), sco.cmd_addr());
 	}
 	return (uint16)(cali[1] & 0xffff);
 }
@@ -1443,10 +1443,10 @@ uint16 NACT_Sys3::cali2()
 	} else if(0xc0 <= dat && dat <= 0xff) {
 		val = ((dat & 0x3f) << 8) | sco.getd();
 	} else {
-		sys_error("cali2: invalid expression at %d:%04x", sco.page(), sco.addr());
+		sys_error("cali2: invalid expression at %d:%04x", sco.page(), sco.cmd_addr());
 	}
 	if (sco.getd() != 0x7f) {
-		sys_error("cali2: invalid expression at %d:%04x", sco.page(), sco.addr());
+		sys_error("cali2: invalid expression at %d:%04x", sco.page(), sco.cmd_addr());
 	}
 	return val;
 }
diff --git a/src/sys/scenario.cpp b/src/sys/scenario.cpp
index 17a7614..c2176cb 100644
--- a/src/sys/scenario.cpp
+++ b/src/sys/scenario.cpp
@@ -3,6 +3,19 @@
 #include "common.h"
 #include "encoding.h"
 
+uint8_t Scenario::fetch_command()
+{
+	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_;
+	return getd();
+}
+
 void Scenario::skip_syseng_string(Encoding *enc, uint8_t terminator)
 {
 	for (uint8_t c = getd(); c != terminator; c = getd()) {
@@ -14,15 +27,13 @@ void Scenario::skip_syseng_string(Encoding *enc, uint8_t terminator)
 
 void Scenario::get_syseng_string(char* buf, int size, Encoding *enc, uint8_t terminator)
 {
-	int start_addr = addr_;
-
 	int i = 0;
 	for (uint8_t c = getd(); c != terminator; c = getd()) {
 		if (c != '\\')
 			ungetd();
 		int len = enc->mblen(ptr());
 		if (i + len >= size)
-			sys_error("String buffer overrun at %d:0x%x", page_, start_addr);
+			sys_error("String buffer overrun at %d:%04x", page_, cmd_addr_);
 		memcpy(&buf[i], ptr(), len);
 		i += len;
 		skip(len);
@@ -63,8 +74,8 @@ void Scenario::page_call(int target_page)
 
 [[noreturn]] void Scenario::unknown_command(uint8_t cmd) {
 	if (cmd >= 0x20 && cmd < 0x7f) {
-		sys_error("Unknown Command: '%c' at %d:0x%d", cmd, page_, cmd_start_addr);
+		sys_error("Unknown Command: '%c' at %d:%04x", cmd, page_, cmd_addr_);
 	} else {
-		sys_error("Unknown Command: %02x at %d:0x%d", cmd, page_, cmd_start_addr);
+		sys_error("Unknown Command: %02x at %d:%04x", cmd, page_, cmd_addr_);
 	}
 }
\ No newline at end of file
diff --git a/src/sys/scenario.h b/src/sys/scenario.h
index 3f33101..9f52891 100644
--- a/src/sys/scenario.h
+++ b/src/sys/scenario.h
@@ -14,7 +14,7 @@ public:
 	void page_jump(int page, int addr) {
 		data_ = adisk.load(page + 1);
 		page_ = page;
-		addr_ = addr;
+		cmd_addr_ = addr_ = addr;
 	}
 	size_t size() const { return data_.size(); }
 	uint8_t& operator[](int i) { return data_[i]; }
@@ -22,18 +22,16 @@ public:
 	int default_addr() { return data_[0] | data_[1] << 8; }
 
 	int page() const { return page_; }
-	int addr() const { return addr_; }
-	bool is_addr_valid() const {
-		return addr_ >= 2 && static_cast<size_t>(addr_) < data_.size();
-	}
-	// Skip SysEng's "new style" marker
-	void skip_syseng_rev_marker() {
-		if (page_ == 0 && addr_ == 2 && data_[2] == 'R' && data_[3] == 'E' && data_[4] == 'V')
-			addr_ = 5;
-	}
+	// start address of the current command
+	int cmd_addr() const { return cmd_addr_; }
+	// current address, possibly in the middle of a command
+	int current_addr() const { return addr_; }
 
-	void jump_to(int addr) { addr_ = addr; }
+	void jump_to(int addr) { cmd_addr_ = addr_ = addr; }
 	void skip(int n) { addr_ += n; }
+
+	uint8_t fetch_command();
+
 	uint8_t getd() { return data_[addr_++]; }
 	uint16_t getw() {
 		uint16_t w = data_[addr_++];
@@ -54,7 +52,6 @@ public:
 	void page_stack_pop() { page_stack.pop_back(); }
 	void page_stack_clear() { page_stack.clear(); }
 
-	void mark_cmd_start() { cmd_start_addr = addr_; }
 	[[noreturn]] void unknown_command(uint8_t cmd);
 
 private:
@@ -62,7 +59,7 @@ private:
 	std::vector<uint8_t> data_;
 	int page_;
 	int addr_;
-	int cmd_start_addr;
+	int cmd_addr_;
 	std::vector<int> label_stack;
 	std::vector<std::pair<int, int>> page_stack;
 };