diff --git a/src/common.c b/src/common.c
index 6c8de42..d32313f 100644
--- a/src/common.c
+++ b/src/common.c
@@ -1 +1,29 @@
-void *TODO;
+#include "common.h"
+#include "serial.h"
+
+#define PREFIX_ERROR(fmt) "Error:" fmt
+
+static bool state;
+
+void Info(const char *fmt, ...)
+{
+	va_list args;
+
+	va_start(args, fmt);
+	USART_Print(state, "", fmt, args);
+	va_end(args);
+}
+
+void Error(const char *fmt, ...)
+{
+	va_list args;
+
+	va_start(args, fmt);
+	USART_Print(state, "Error: ", fmt, args);
+	va_end(args);
+}
+
+void SetStateFlag(bool is_running)
+{
+	state = is_running;
+}
diff --git a/src/common.h b/src/common.h
index be3c756..9a26b73 100644
--- a/src/common.h
+++ b/src/common.h
@@ -5,10 +5,16 @@
 #define UNUSED(s) (void)(s)
 
 #include <stdlib.h>
+#include <stdarg.h>
 #include <stdbool.h>
+
 #include <util/delay.h>
 #define Sleep(ms) _delay_ms(ms)
 
+void Info(const char *fmt, ...);
+void Error(const char *fmt, ...);
+void SetStateFlag(bool is_running);
+
 void Enable(void);
 void Disable(void);
 void SetTemp(long val);
diff --git a/src/main.c b/src/main.c
index 8efc39d..f03070a 100644
--- a/src/main.c
+++ b/src/main.c
@@ -9,8 +9,8 @@ int main(void)
 	char ch;
 	unsigned long i = 1;
 
-	running = true;
 	USART_Init();
+	Enable();
 
 	for (;;) {
 		// Process rx ring buffer
@@ -19,13 +19,11 @@ int main(void)
 		}
 
 		Sleep(3000);
-
-		if (!running)
-			continue;
-
-		// TODO: Main program
-		if (i >= 99999) i = 1;
-		USART_Printf("[CORE] Fetching sensors #%05lu...\r\n", i++);
+		if (running) {
+			// TODO: Main program
+			if (i >= 99999) i = 1;
+			Info("Fetching sensors #%05lu...", i++);
+		}
 	}
 
 	return 0;
@@ -33,39 +31,48 @@ int main(void)
 
 void Enable(void)
 {
-	USART_Printf("[CORE] Parsed 'CMD_RUN' token.\r\n");
+	Info("Parsed 'CMD_RUN' token.");
 	if (running == true) {
-		USART_Printf("[CORE] Error: Already running.\r\n");
+		Error("Already running.");
+		return;
 	}
-
 	running = true;
+	SetStateFlag(running);
+	Info("Program started.");
 }
 
 void Disable(void)
 {
-	USART_Printf("[CORE] Parsed 'CMD_STOP' token.\r\n");
+	Info("Parsed 'CMD_STOP' token.");
 	if (running == false) {
-		USART_Printf("[CORE] Error: Already stopped.\r\n");
+		Error("Already stopped.");
+		return;
 	}
 	running = false;
+	SetStateFlag(running);
+	Info("Program stopped.");
 }
 
 void SetTemp(long val)
 {
-	USART_Printf("[CORE] Parsed 'CMD_SET_TEMP', VAL='%ld'.\r\n", val);
+	Info("Parsed 'CMD_SET_TEMP', VAL='%ld'.", val);
 	if (val < 10) {
-		USART_Printf("[CORE] Error: Given temperature is too low.\r\n");
+		Error("Given temperature is too low.");
+		return;
 	} else if (val > 40) {
-		USART_Printf("[CORE] Error: Given temperature is too high.\r\n");
+		Error("Given temperature is too high.");
+		return;
 	}
 }
 
 void SetDewp(long val)
 {
-	USART_Printf("[CORE] Parsed 'CMD_SET_DEWP', VAL='%ld'.\r\n", val);
+	Info("Parsed 'CMD_SET_DEWP', VAL='%ld'.", val);
 	if (val < 10) {
-		USART_Printf("[CORE] Error: Given dew point is too low.\r\n");
+		Error("Given dew point is too low.");
+		return;
 	} else if (val > 80) {
-		USART_Printf("[CORE] Error: Given dew point is too high.\r\n");
+		Error("Given dew point is too high.");
+		return;
 	}
 }
diff --git a/src/parser.c b/src/parser.c
index bbd2ee8..24bcc35 100644
--- a/src/parser.c
+++ b/src/parser.c
@@ -110,7 +110,7 @@ void CMD_Parse(char ch)
 
 		// Unexpected character
 		else {
-			USART_Printf("[CORE] Error: Unexpected character '%c'.\r\n", ch);
+			Error("Unexpected character '%c'.", ch);
 			state = 0;
 		}
 		break;
@@ -163,7 +163,7 @@ void CMD_Parse(char ch)
 
 		// Unexpected character
 		else {
-			USART_Printf("[CORE] Error: Unexpected character '%c'.\r\n", ch);
+			Error("Unexpected character '%c'.", ch);
 			state = 0;
 		}
 		break;
diff --git a/src/serial.c b/src/serial.c
index bc78f0a..507a7bc 100644
--- a/src/serial.c
+++ b/src/serial.c
@@ -1,7 +1,6 @@
 #include "common.h"
 #include "serial.h"
 
-#include <stdarg.h>
 #include <stdio.h>
 #include <avr/io.h>
 #include <avr/interrupt.h>
@@ -12,6 +11,7 @@
 #define USART_BAUD_PRESCALE ((((F_CPU / 16) + \
 	(USART_BAUDRATE / 2)) / (USART_BAUDRATE)) - 1)
 
+// TODO: TX ring buffer
 static unsigned char rxbuf[USART_BUFSIZE];
 static unsigned char rxhead, rxtail;
 
@@ -30,6 +30,11 @@ void USART_Init(void)
 	UBRRL = USART_BAUD_PRESCALE; // Baud rate lower byte
 }
 
+bool USART_IsDataAvailable(void)
+{
+	return rxhead != rxtail;
+}
+
 char USART_GetChar(void)
 {
 	unsigned char newtail;
@@ -51,24 +56,21 @@ void USART_PutChar(const char ch)
 	UDR = ch;
 }
 
-void USART_Printf(const char *fmt, ...)
+void USART_Print(bool state, const char *pre, const char *fmt, va_list ap)
 {
+	int len;
 	char buf[256], *ptr;
-	va_list args;
 
-	va_start(args, fmt);
-	vsprintf(buf, fmt, args);
-	va_end(args);
+	len = sprintf(buf, "[CORE][%c] %s", state ? 'E' : 'D', pre);
+	vsnprintf(buf + len, sizeof(buf) - len, fmt, ap);
 
 	ptr = buf;
 	while (*ptr != '\0') {
 		USART_PutChar(*ptr++);
 	}
-}
 
-bool USART_IsDataAvailable(void)
-{
-	return rxhead != rxtail;
+	USART_PutChar('\r');
+	USART_PutChar('\n');
 }
 
 // Interrupt Handler
diff --git a/src/serial.h b/src/serial.h
index 44138ed..3a4c2b2 100644
--- a/src/serial.h
+++ b/src/serial.h
@@ -1,12 +1,13 @@
 #ifndef MAD_SERIAL_H
 #define MAD_SERIAL_H
 
+#include <stdarg.h>
 #include <stdbool.h>
 
 void USART_Init(void);
 char USART_GetChar(void);
 void USART_PutChar(const char ch);
-void USART_Printf(const char *fmt, ...);
 bool USART_IsDataAvailable(void);
+void USART_Print(bool state, const char *pre, const char *fmt, va_list ap);
 
 #endif // MAD_SERIAL_H