diff --git a/module00/ex01/Makefile b/module00/ex01/Makefile
index 40e9cb4..56b54a5 100644
--- a/module00/ex01/Makefile
+++ b/module00/ex01/Makefile
@@ -19,7 +19,7 @@ hex: $(TARGET).hex
 
 # https://gcc.gnu.org/onlinedocs/gcc/AVR-Options.html
 $(TARGET).bin: $(TARGET).c
-	$(CC) -mmcu=$(AVRGCC_MCU_TYPE) -D F_CPU=$(F_CPU) $(TARGET).c -o $(TARGET).bin
+	$(CC) -mmcu=$(AVRGCC_MCU_TYPE) -D F_CPU=$(F_CPU) $(TARGET).c -Os -o $(TARGET).bin
 
 
 # https://linux.die.net/man/1/avr-objcopy
diff --git a/module00/ex01/main.c b/module00/ex01/main.c
index fe04856..9fea211 100644
--- a/module00/ex01/main.c
+++ b/module00/ex01/main.c
@@ -1,4 +1,5 @@
 #include <avr/io.h>
+// #include <util/delay.h>
 
 #define SET(REGISTER, BIT)		REGISTER |= 1 << BIT
 #define CLEAR(REGISTER, BIT)	REGISTER &= ~(1 << BIT)
@@ -12,19 +13,22 @@
 
 int main()
 {
-    // DDRx : select direction of the pin
+	// registers :
+    // DDRx : (Data Direction Register) Controls whether each pin is an input or output
     //      - 1 = output (pull-up off)
     //      - 0 = input
-    // PORTx :
-    //      if DDRx = 1 (output mode) :
-    //      - 1 = HIGH (5V)
-    //      - 0 = LOW (0V)
-    //      if DDRx = 0 (input mode) : 
+    // PORTx : (Port Data Register)
+    //      if DDRx = 1 (output mode) : Controls the output state of a pin when it's configured as an output
+    //      - 1 = HIGH (5V?) (VCC)
+    //      - 0 = LOW (0V) (GND)
+    //      if DDRx = 0 (input mode) : Controls the internal pull-up resistor when the pin is an input
     //      - 1 = pull-up on
-    //      - 0 = pull-up off
-    // PINx : 
-
+    //      - 0 = pull-up off (pin floats)
+    // PINx : (Pin Input Register) Reads the current logical state of a pin
+	// 		Reflects the actual voltage on the pin
+	// 		Special feature: Writing 1 to PINxn toggles the corresponding PORTxn bit (regardless of DDRx)
 
+	// turns on led 1
     SET(DDRB, D1);		// make PB0 as OUTPUT (pullup is off)
     CLEAR(PORTB, D1);	// make PB0 as HIGH (LED turns ON)
 
@@ -37,5 +41,45 @@ int main()
     // CLEAR(DDRB, D4);		// make PB4 as INPUT
     // CLEAR(PORTB, D4);	// make PB4 pullup off
 
+	////////////////////////////////
+	// TEST weird behavior of PIN register :
+	// 		in initial tests, using PIN register to toggle ON two leds only worked for the second one
+	// 		after some tests, this behavior is not anymore
+	//
+	// test 1 : only led 3 turns on
+    // SET(DDRB, D1);
+    // CLEAR(PORTB, D1);
+    // SET(PINB, D1);
+    // SET(DDRB, D3);
+    // CLEAR(PORTB, D3);
+    // SET(PINB, D3);
+	//
+	// test 2 : only led 3 turns on
+    // SET(DDRB, D1);
+    // CLEAR(PORTB, D1);
+    // SET(DDRB, D3);
+    // CLEAR(PORTB, D3);
+    // SET(PINB, D1);
+    // SET(PINB, D3);
+	//
+	// test 3 : both leds turns on
+    // SET(DDRB, D1);
+    // CLEAR(PORTB, D1);
+    // SET(DDRB, D3);
+    // CLEAR(PORTB, D3);
+	// PINB = (1 << D1) | (1 << D3);
+	//
+	// test 4 : both leds turns on
+    // SET(DDRB, D1);
+    // CLEAR(PORTB, D1);
+    // SET(DDRB, D3);
+    // CLEAR(PORTB, D3);
+	// SET(PINB, D1);
+	// _delay_us(500);
+	// SET(PINB, D3);
+	//
+	////////////////////////////////
+
+
     return 0;
 }
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