diff --git a/hardware/arduino/avr/cores/arduino/Arduino.h b/hardware/arduino/avr/cores/arduino/Arduino.h
index de1886d5dde..cf0555a564d 100644
--- a/hardware/arduino/avr/cores/arduino/Arduino.h
+++ b/hardware/arduino/avr/cores/arduino/Arduino.h
@@ -87,6 +87,10 @@ void yield(void);
 #define bitClear(value, bit) ((value) &= ~(1UL << (bit)))
 #define bitWrite(value, bit, bitvalue) (bitvalue ? bitSet(value, bit) : bitClear(value, bit))
 
+// avr-libc defines _NOP() since 1.6.2
+#ifndef _NOP
+#define _NOP() do { __asm__ volatile ("nop"); } while (0)
+#endif
 
 typedef unsigned int word;
 
@@ -195,6 +199,10 @@ extern const uint8_t PROGMEM digital_pin_to_timer_PGM[];
 #include "WCharacter.h"
 #include "WString.h"
 #include "HardwareSerial.h"
+#include "USBAPI.h"
+#if defined(HAVE_HWSERIAL0) && defined(HAVE_CDCSERIAL)
+#error "Targets with both UART0 and CDC serial not supported"
+#endif
 
 uint16_t makeWord(uint16_t w);
 uint16_t makeWord(byte h, byte l);
diff --git a/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp b/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
index 9a86fa069d0..c611492a1db 100644
--- a/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
+++ b/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
@@ -25,244 +25,158 @@
 #include <stdio.h>
 #include <string.h>
 #include <inttypes.h>
+#include <util/atomic.h>
+#include <avr/cpufunc.h>
 #include "Arduino.h"
 #include "wiring_private.h"
 
+#include "HardwareSerial.h"
+
 // this next line disables the entire HardwareSerial.cpp, 
 // this is so I can support Attiny series and any other chip without a uart
-#if defined(UBRRH) || defined(UBRR0H) || defined(UBRR1H) || defined(UBRR2H) || defined(UBRR3H)
-
-#include "HardwareSerial.h"
+#if defined(HAVE_HWSERIAL0) || defined(HAVE_HWSERIAL1) || defined(HAVE_HWSERIAL2) || defined(HAVE_HWSERIAL3)
 
-/*
- * on ATmega8, the uart and its bits are not numbered, so there is no "TXC0"
- * definition.
- */
+// Ensure that the various bit positions we use are available with a 0
+// postfix, so we can always use the values for UART0 for all UARTs. The
+// alternative, passing the various values for each UART to the
+// HardwareSerial constructor also works, but makes the code bigger and
+// slower.
 #if !defined(TXC0)
 #if defined(TXC)
+// On ATmega8, the uart and its bits are not numbered, so there is no TXC0 etc.
 #define TXC0 TXC
+#define RXEN0 RXEN
+#define TXEN0 TXEN
+#define RXCIE0 RXCIE
+#define UDRIE0 UDRIE
+#define U2X0 U2X
+#define UPE0 UPE
+#define UDRE0 UDRE
 #elif defined(TXC1)
 // Some devices have uart1 but no uart0
 #define TXC0 TXC1
+#define RXEN0 RXEN1
+#define TXEN0 TXEN1
+#define RXCIE0 RXCIE1
+#define UDRIE0 UDRIE1
+#define U2X0 U2X1
+#define UPE0 UPE1
+#define UDRE0 UDRE1
 #else
-#error TXC0 not definable in HardwareSerial.h
+#error No UART found in HardwareSerial.cpp
 #endif
+#endif // !defined TXC0
+
+// Check at compiletime that it is really ok to use the bit positions of
+// UART0 for the other UARTs as well, in case these values ever get
+// changed for future hardware.
+#if defined(TXC1) && (TXC1 != TXC0 || RXEN1 != RXEN0 || RXCIE1 != RXCIE0 || \
+		      UDRIE1 != UDRIE0 || U2X1 != U2X0 || UPE1 != UPE0 || \
+		      UDRE1 != UDRE0)
+#error "Not all bit positions for UART1 are the same as for UART0"
 #endif
-
-inline void store_char(unsigned char c, HardwareSerial *s)
-{
-  int i = (unsigned int)(s->_rx_buffer_head + 1) % SERIAL_BUFFER_SIZE;
-
-  // if we should be storing the received character into the location
-  // just before the tail (meaning that the head would advance to the
-  // current location of the tail), we're about to overflow the buffer
-  // and so we don't write the character or advance the head.
-  if (i != s->_rx_buffer_tail) {
-    s->_rx_buffer[s->_rx_buffer_head] = c;
-    s->_rx_buffer_head = i;
-  }
-}
-
-#if !defined(USART0_RX_vect) && defined(USART1_RX_vect)
-// do nothing - on the 32u4 the first USART is USART1
-#else
-#if !defined(USART_RX_vect) && !defined(USART0_RX_vect) && \
-    !defined(USART_RXC_vect)
-  #error "Don't know what the Data Received vector is called for the first UART"
-#else
-  void serialEvent() __attribute__((weak));
-  void serialEvent() {}
-  #define serialEvent_implemented
-#if defined(USART_RX_vect)
-  ISR(USART_RX_vect)
-#elif defined(USART0_RX_vect)
-  ISR(USART0_RX_vect)
-#elif defined(USART_RXC_vect)
-  ISR(USART_RXC_vect) // ATmega8
+#if defined(TXC2) && (TXC2 != TXC0 || RXEN2 != RXEN0 || RXCIE2 != RXCIE0 || \
+		      UDRIE2 != UDRIE0 || U2X2 != U2X0 || UPE2 != UPE0 || \
+		      UDRE2 != UDRE0)
+#error "Not all bit positions for UART2 are the same as for UART0"
 #endif
-  {
-  #if defined(UDR0)
-    if (bit_is_clear(UCSR0A, UPE0)) {
-      unsigned char c = UDR0;
-      store_char(c, &Serial);
-    } else {
-      unsigned char c = UDR0;
-    };
-  #elif defined(UDR)
-    if (bit_is_clear(UCSRA, PE)) {
-      unsigned char c = UDR;
-      store_char(c, &Serial);
-    } else {
-      unsigned char c = UDR;
-    };
-  #else
-    #error UDR not defined
-  #endif
-  }
+#if defined(TXC3) && (TXC3 != TXC0 || RXEN3 != RXEN0 || RXCIE3 != RXCIE0 || \
+		      UDRIE3 != UDRIE0 || U3X3 != U3X0 || UPE3 != UPE0 || \
+		      UDRE3 != UDRE0)
+#error "Not all bit positions for UART3 are the same as for UART0"
 #endif
+
+// SerialEvent functions are weak, so when the user doesn't define them,
+// the linker just sets their address to 0 (which is checked below).
+// The Serialx_available is just a wrapper around Serialx.available(),
+// but we can refer to it weakly so we don't pull in the entire
+// HardwareSerial instance if the user doesn't also refer to it.
+#if defined(HAVE_HWSERIAL0)
+  void serialEvent() __attribute__((weak));
+  bool Serial0_available() __attribute__((weak));
 #endif
 
-#if defined(USART1_RX_vect)
+#if defined(HAVE_HWSERIAL1)
   void serialEvent1() __attribute__((weak));
-  void serialEvent1() {}
-  #define serialEvent1_implemented
-  ISR(USART1_RX_vect)
-  {
-    if (bit_is_clear(UCSR1A, UPE1)) {
-      unsigned char c = UDR1;
-      store_char(c, &Serial1);
-    } else {
-      unsigned char c = UDR1;
-    };
-  }
+  bool Serial1_available() __attribute__((weak));
 #endif
 
-#if defined(USART2_RX_vect) && defined(UDR2)
+#if defined(HAVE_HWSERIAL2)
   void serialEvent2() __attribute__((weak));
-  void serialEvent2() {}
-  #define serialEvent2_implemented
-  ISR(USART2_RX_vect)
-  {
-    if (bit_is_clear(UCSR2A, UPE2)) {
-      unsigned char c = UDR2;
-      store_char(c, &Serial2);
-    } else {
-      unsigned char c = UDR2;
-    };
-  }
+  bool Serial2_available() __attribute__((weak));
 #endif
 
-#if defined(USART3_RX_vect) && defined(UDR3)
+#if defined(HAVE_HWSERIAL3)
   void serialEvent3() __attribute__((weak));
-  void serialEvent3() {}
-  #define serialEvent3_implemented
-  ISR(USART3_RX_vect)
-  {
-    if (bit_is_clear(UCSR3A, UPE3)) {
-      unsigned char c = UDR3;
-      store_char(c, &Serial3);
-    } else {
-      unsigned char c = UDR3;
-    };
-  }
+  bool Serial3_available() __attribute__((weak));
 #endif
 
 void serialEventRun(void)
 {
-#ifdef serialEvent_implemented
-  if (Serial.available()) serialEvent();
+#if defined(HAVE_HWSERIAL0)
+  if (Serial0_available && serialEvent && Serial0_available()) serialEvent();
 #endif
-#ifdef serialEvent1_implemented
-  if (Serial1.available()) serialEvent1();
+#if defined(HAVE_HWSERIAL1)
+  if (Serial1_available && serialEvent1 && Serial1_available()) serialEvent1();
 #endif
-#ifdef serialEvent2_implemented
-  if (Serial2.available()) serialEvent2();
+#if defined(HAVE_HWSERIAL2)
+  if (Serial2_available && serialEvent2 && Serial2_available()) serialEvent2();
 #endif
-#ifdef serialEvent3_implemented
-  if (Serial3.available()) serialEvent3();
+#if defined(HAVE_HWSERIAL3)
+  if (Serial3_available && serialEvent2 && Serial3_available()) serialEvent3();
 #endif
 }
 
+// Actual interrupt handlers //////////////////////////////////////////////////////////////
 
-#if !defined(USART0_UDRE_vect) && defined(USART1_UDRE_vect)
-// do nothing - on the 32u4 the first USART is USART1
-#else
-#if !defined(UART0_UDRE_vect) && !defined(UART_UDRE_vect) && !defined(USART0_UDRE_vect) && !defined(USART_UDRE_vect)
-  #error "Don't know what the Data Register Empty vector is called for the first UART"
-#else
-#if defined(UART0_UDRE_vect)
-ISR(UART0_UDRE_vect)
-#elif defined(UART_UDRE_vect)
-ISR(UART_UDRE_vect)
-#elif defined(USART0_UDRE_vect)
-ISR(USART0_UDRE_vect)
-#elif defined(USART_UDRE_vect)
-ISR(USART_UDRE_vect)
-#endif
+void HardwareSerial::_rx_complete_irq(void)
 {
-  if (Serial._tx_buffer_head == Serial._tx_buffer_tail) {
-	// Buffer empty, so disable interrupts
-#if defined(UCSR0B)
-    cbi(UCSR0B, UDRIE0);
-#else
-    cbi(UCSRB, UDRIE);
-#endif
-  }
-  else {
-    // There is more data in the output buffer. Send the next byte
-    unsigned char c = Serial._tx_buffer[Serial._tx_buffer_tail];
-    Serial._tx_buffer_tail = (Serial._tx_buffer_tail + 1) % SERIAL_BUFFER_SIZE;
-	
-  #if defined(UDR0)
-    UDR0 = c;
-  #elif defined(UDR)
-    UDR = c;
-  #else
-    #error UDR not defined
-  #endif
-  }
+  if (bit_is_clear(*_ucsra, UPE0)) {
+    // No Parity error, read byte and store it in the buffer if there is
+    // room
+    unsigned char c = *_udr;
+    int i = (unsigned int)(_rx_buffer_head + 1) % SERIAL_BUFFER_SIZE;
+
+    // if we should be storing the received character into the location
+    // just before the tail (meaning that the head would advance to the
+    // current location of the tail), we're about to overflow the buffer
+    // and so we don't write the character or advance the head.
+    if (i != _rx_buffer_tail) {
+      _rx_buffer[_rx_buffer_head] = c;
+      _rx_buffer_head = i;
+    }
+  } else {
+    // Parity error, read byte but discard it
+    unsigned char c = *_udr;
+  };
 }
-#endif
-#endif
 
-#ifdef USART1_UDRE_vect
-ISR(USART1_UDRE_vect)
+void HardwareSerial::_tx_udr_empty_irq(void)
 {
-  if (Serial1._tx_buffer_head == Serial1._tx_buffer_tail) {
-	// Buffer empty, so disable interrupts
-    cbi(UCSR1B, UDRIE1);
-  }
-  else {
-    // There is more data in the output buffer. Send the next byte
-    unsigned char c = Serial1._tx_buffer[Serial1._tx_buffer_tail];
-    Serial1._tx_buffer_tail = (Serial1._tx_buffer_tail + 1) % SERIAL_BUFFER_SIZE;
-	
-    UDR1 = c;
-  }
-}
-#endif
+  // If interrupts are enabled, there must be more data in the output
+  // buffer. Send the next byte
+  unsigned char c = _tx_buffer[_tx_buffer_tail];
+  _tx_buffer_tail = (_tx_buffer_tail + 1) % SERIAL_BUFFER_SIZE;
 
-#ifdef USART2_UDRE_vect
-ISR(USART2_UDRE_vect)
-{
-  if (Serial2._tx_buffer_head == Serial2._tx_buffer_tail) {
-	// Buffer empty, so disable interrupts
-    cbi(UCSR2B, UDRIE2);
-  }
-  else {
-    // There is more data in the output buffer. Send the next byte
-    unsigned char c = Serial2._tx_buffer[Serial2._tx_buffer_tail];
-    Serial2._tx_buffer_tail = (Serial2._tx_buffer_tail + 1) % SERIAL_BUFFER_SIZE;
-	
-    UDR2 = c;
-  }
-}
-#endif
+  *_udr = c;
 
-#ifdef USART3_UDRE_vect
-ISR(USART3_UDRE_vect)
-{
-  if (Serial3._tx_buffer_head == Serial3._tx_buffer_tail) {
-	// Buffer empty, so disable interrupts
-    cbi(UCSR3B, UDRIE3);
-  }
-  else {
-    // There is more data in the output buffer. Send the next byte
-    unsigned char c = Serial3._tx_buffer[Serial3._tx_buffer_tail];
-    Serial3._tx_buffer_tail = (Serial3._tx_buffer_tail + 1) % SERIAL_BUFFER_SIZE;
-	
-    UDR3 = c;
+  // clear the TXC bit -- "can be cleared by writing a one to its bit
+  // location". This makes sure flush() won't return until the bytes
+  // actually got written
+  sbi(*_ucsra, TXC0);
+
+  if (_tx_buffer_head == _tx_buffer_tail) {
+    // Buffer empty, so disable interrupts
+    cbi(*_ucsrb, UDRIE0);
   }
 }
-#endif
 
 // Constructors ////////////////////////////////////////////////////////////////
 
 HardwareSerial::HardwareSerial(
   volatile uint8_t *ubrrh, volatile uint8_t *ubrrl,
   volatile uint8_t *ucsra, volatile uint8_t *ucsrb,
-  volatile uint8_t *ucsrc, volatile uint8_t *udr,
-  uint8_t rxen, uint8_t txen, uint8_t rxcie, uint8_t udrie, uint8_t u2x)
+  volatile uint8_t *ucsrc, volatile uint8_t *udr)
 {
   _tx_buffer_head = _tx_buffer_tail = 0;
   _rx_buffer_head = _rx_buffer_tail = 0;
@@ -272,102 +186,48 @@ HardwareSerial::HardwareSerial(
   _ucsrb = ucsrb;
   _ucsrc = ucsrc;
   _udr = udr;
-  _rxen = rxen;
-  _txen = txen;
-  _rxcie = rxcie;
-  _udrie = udrie;
-  _u2x = u2x;
 }
 
 // Public Methods //////////////////////////////////////////////////////////////
 
 void HardwareSerial::begin(unsigned long baud)
 {
-  uint16_t baud_setting;
-  bool use_u2x = true;
-
-#if F_CPU == 16000000UL
-  // hardcoded exception for compatibility with the bootloader shipped
-  // with the Duemilanove and previous boards and the firmware on the 8U2
-  // on the Uno and Mega 2560.
-  if (baud == 57600) {
-    use_u2x = false;
-  }
-#endif
-
-try_again:
-  
-  if (use_u2x) {
-    *_ucsra = 1 << _u2x;
-    baud_setting = (F_CPU / 4 / baud - 1) / 2;
-  } else {
-    *_ucsra = 0;
-    baud_setting = (F_CPU / 8 / baud - 1) / 2;
-  }
-  
-  if ((baud_setting > 4095) && use_u2x)
-  {
-    use_u2x = false;
-    goto try_again;
-  }
-
-  // assign the baud_setting, a.k.a. ubbr (USART Baud Rate Register)
-  *_ubrrh = baud_setting >> 8;
-  *_ubrrl = baud_setting;
-
-  transmitting = false;
-
-  sbi(*_ucsrb, _rxen);
-  sbi(*_ucsrb, _txen);
-  sbi(*_ucsrb, _rxcie);
-  cbi(*_ucsrb, _udrie);
+  begin(baud, SERIAL_8N1);
 }
 
 void HardwareSerial::begin(unsigned long baud, byte config)
 {
-  uint16_t baud_setting;
-  uint8_t current_config;
-  bool use_u2x = true;
-
-#if F_CPU == 16000000UL
-  // hardcoded exception for compatibility with the bootloader shipped
-  // with the Duemilanove and previous boards and the firmware on the 8U2
-  // on the Uno and Mega 2560.
-  if (baud == 57600) {
-    use_u2x = false;
-  }
-#endif
-
-try_again:
-  
-  if (use_u2x) {
-    *_ucsra = 1 << _u2x;
-    baud_setting = (F_CPU / 4 / baud - 1) / 2;
-  } else {
+  // Try u2x mode first
+  uint16_t baud_setting = (F_CPU / 4 / baud - 1) / 2;
+  *_ucsra = 1 << U2X0;
+
+  // hardcoded exception for 57600 for compatibility with the bootloader
+  // shipped with the Duemilanove and previous boards and the firmware
+  // on the 8U2 on the Uno and Mega 2560. Also, The baud_setting cannot
+  // be > 4095, so switch back to non-u2x mode if the baud rate is too
+  // low.
+  if (((F_CPU == 16000000UL) && (baud == 57600)) || (baud_setting >4095))
+  {
     *_ucsra = 0;
     baud_setting = (F_CPU / 8 / baud - 1) / 2;
   }
-  
-  if ((baud_setting > 4095) && use_u2x)
-  {
-    use_u2x = false;
-    goto try_again;
-  }
 
   // assign the baud_setting, a.k.a. ubbr (USART Baud Rate Register)
   *_ubrrh = baud_setting >> 8;
   *_ubrrl = baud_setting;
 
+  _written = false;
+
   //set the data bits, parity, and stop bits
 #if defined(__AVR_ATmega8__)
   config |= 0x80; // select UCSRC register (shared with UBRRH)
 #endif
   *_ucsrc = config;
   
-  sbi(*_ucsrb, _rxen);
-  sbi(*_ucsrb, _txen);
-  sbi(*_ucsrb, _rxcie);
-  cbi(*_ucsrb, _udrie);
+  sbi(*_ucsrb, RXEN0);
+  sbi(*_ucsrb, TXEN0);
+  sbi(*_ucsrb, RXCIE0);
+  cbi(*_ucsrb, UDRIE0);
 }
 
 void HardwareSerial::end()
@@ -376,10 +236,10 @@ void HardwareSerial::end()
   while (_tx_buffer_head != _tx_buffer_tail)
     ;
 
-  cbi(*_ucsrb, _rxen);
-  cbi(*_ucsrb, _txen);
-  cbi(*_ucsrb, _rxcie);  
-  cbi(*_ucsrb, _udrie);
+  cbi(*_ucsrb, RXEN0);
+  cbi(*_ucsrb, TXEN0);
+  cbi(*_ucsrb, RXCIE0);
+  cbi(*_ucsrb, UDRIE0);
   
   // clear any received data
   _rx_buffer_head = _rx_buffer_tail;
@@ -387,83 +247,109 @@ void HardwareSerial::end()
 
 int HardwareSerial::available(void)
 {
-  return (unsigned int)(SERIAL_BUFFER_SIZE + _rx_buffer_head - _rx_buffer_tail) % SERIAL_BUFFER_SIZE;
+  ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+    return (unsigned int)(SERIAL_BUFFER_SIZE + _rx_buffer_head - _rx_buffer_tail) % SERIAL_BUFFER_SIZE;
+  }
 }
 
 int HardwareSerial::peek(void)
 {
-  if (_rx_buffer_head == _rx_buffer_tail) {
-    return -1;
-  } else {
-    return _rx_buffer[_rx_buffer_tail];
+  ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+    if (_rx_buffer_head == _rx_buffer_tail) {
+      return -1;
+    } else {
+      return _rx_buffer[_rx_buffer_tail];
+    }
   }
 }
 
 int HardwareSerial::read(void)
 {
-  // if the head isn't ahead of the tail, we don't have any characters
-  if (_rx_buffer_head == _rx_buffer_tail) {
-    return -1;
-  } else {
-    unsigned char c = _rx_buffer[_rx_buffer_tail];
-    _rx_buffer_tail = (unsigned int)(_rx_buffer_tail + 1) % SERIAL_BUFFER_SIZE;
-    return c;
+  ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+    // if the head isn't ahead of the tail, we don't have any characters
+    if (_rx_buffer_head == _rx_buffer_tail) {
+      return -1;
+    } else {
+      unsigned char c = _rx_buffer[_rx_buffer_tail];
+      _rx_buffer_tail = (unsigned int)(_rx_buffer_tail + 1) % SERIAL_BUFFER_SIZE;
+      return c;
+    }
   }
 }
 
 void HardwareSerial::flush()
 {
-  // UDR is kept full while the buffer is not empty, so TXC triggers when EMPTY && SENT
-  while (transmitting && ! (*_ucsra & _BV(TXC0)));
-  transmitting = false;
+  // If we have never written a byte, no need to flush. This special
+  // case is needed since there is no way to force the TXC (transmit
+  // complete) bit to 1 during initialization
+  if (!_written)
+    return;
+
+  while (bit_is_set(*_ucsrb, UDRIE0) || bit_is_clear(*_ucsra, TXC0)) {
+    if (bit_is_clear(SREG, SREG_I) && bit_is_set(*_ucsrb, UDRIE0))
+	// Interrupts are globally disabled, but the DR empty
+	// interrupt should be enabled, so poll the DR empty flag to
+	// prevent deadlock
+	if (bit_is_set(*_ucsra, UDRE0))
+	  _tx_udr_empty_irq();
+  }
+  // If we get here, nothing is queued anymore (DRIE is disabled) and
+  // the hardware finished tranmission (TXC is set).
 }
 
 size_t HardwareSerial::write(uint8_t c)
 {
-  int i = (_tx_buffer_head + 1) % SERIAL_BUFFER_SIZE;
-	
-  // If the output buffer is full, there's nothing for it other than to 
-  // wait for the interrupt handler to empty it a bit
-  // ???: return 0 here instead?
-  while (i == _tx_buffer_tail)
-    ;
-	
-  _tx_buffer[_tx_buffer_head] = c;
-  _tx_buffer_head = i;
-	
-  sbi(*_ucsrb, _udrie);
-  // clear the TXC bit -- "can be cleared by writing a one to its bit location"
-  transmitting = true;
-  sbi(*_ucsra, TXC0);
-  
-  return 1;
+  bool interrupts_enabled = bit_is_set(SREG, SREG_I);
+  ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+    // If the buffer and the data register is empty, just write the byte
+    // to the data register and be done. This shortcut helps
+    // significantly improve the effective datarate at high (>
+    // 500kbit/s) bitrates, where interrupt overhead becomes a slowdown.
+    if (_tx_buffer_head == _tx_buffer_tail && bit_is_set(*_ucsra, UDRE0)) {
+      *_udr = c;
+      sbi(*_ucsra, TXC0);
+      return 1;
+    }
+    int i = (_tx_buffer_head + 1) % SERIAL_BUFFER_SIZE;
+
+    // If the output buffer is full, there's nothing for it other than to 
+    // wait for the interrupt handler to empty it a bit
+    while (i == _tx_buffer_tail) {
+      if (!interrupts_enabled) {
+	// Interrupts were disabled at the start of this function, so we
+	// can't just enable them (that would allow other interrupts to
+	// trigger as well!). We'll have to poll the data register empty
+	// flag ourselves. If it is set, pretend an interrupt has happened
+	// and call the handler to free up space for us.
+	if(bit_is_set(*_ucsra, UDRE0))
+	  _tx_udr_empty_irq();
+      } else {
+	// Interrupts were enabled, so we can temporarily enable them so
+	// our tx interrupt can fire and free up some space. Note that we
+	// can't just use the UDRE polling approach here instead of
+	// enabling interrupts, since that would prevent other
+	// interrupts from triggering while we wait for room.
+	NONATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+	  _NOP(); // Any pending interrupts will trigger here
+	}
+	// Recalculate i, since other interrupt handlers might have
+	// written to our buffer
+	i = (_tx_buffer_head + 1) % SERIAL_BUFFER_SIZE;
+      }
+    }
+
+    _tx_buffer[_tx_buffer_head] = c;
+    _tx_buffer_head = i;
+
+    sbi(*_ucsrb, UDRIE0);
+    _written = true;
+
+    return 1;
+  }
 }
 
 HardwareSerial::operator bool() {
 	return true;
 }
 
-// Preinstantiate Objects //////////////////////////////////////////////////////
-
-#if defined(UBRRH) && defined(UBRRL)
-  HardwareSerial Serial(&UBRRH, &UBRRL, &UCSRA, &UCSRB, &UCSRC, &UDR, RXEN, TXEN, RXCIE, UDRIE, U2X);
-#elif defined(UBRR0H) && defined(UBRR0L)
-  HardwareSerial Serial(&UBRR0H, &UBRR0L, &UCSR0A, &UCSR0B, &UCSR0C, &UDR0, RXEN0, TXEN0, RXCIE0, UDRIE0, U2X0);
-#elif defined(USBCON)
-  // do nothing - Serial object and buffers are initialized in CDC code
-#else
-  #error no serial port defined  (port 0)
-#endif
-
-#if defined(UBRR1H)
-  HardwareSerial Serial1(&UBRR1H, &UBRR1L, &UCSR1A, &UCSR1B, &UCSR1C, &UDR1, RXEN1, TXEN1, RXCIE1, UDRIE1, U2X1);
-#endif
-#if defined(UBRR2H)
-  HardwareSerial Serial2(&UBRR2H, &UBRR2L, &UCSR2A, &UCSR2B, &UCSR2C, &UDR2, RXEN2, TXEN2, RXCIE2, UDRIE2, U2X2);
-#endif
-#if defined(UBRR3H)
-  HardwareSerial Serial3(&UBRR3H, &UBRR3L, &UCSR3A, &UCSR3B, &UCSR3C, &UDR3, RXEN3, TXEN3, RXCIE3, UDRIE3, U2X3);
-#endif
-
 #endif // whole file
-
diff --git a/hardware/arduino/avr/cores/arduino/HardwareSerial.h b/hardware/arduino/avr/cores/arduino/HardwareSerial.h
index 0f622625320..8bf7744a2b7 100644
--- a/hardware/arduino/avr/cores/arduino/HardwareSerial.h
+++ b/hardware/arduino/avr/cores/arduino/HardwareSerial.h
@@ -46,14 +46,9 @@ class HardwareSerial : public Stream
     volatile uint8_t *_ucsrb;
     volatile uint8_t *_ucsrc;
     volatile uint8_t *_udr;
-    uint8_t _rxen;
-    uint8_t _txen;
-    uint8_t _rxcie;
-    uint8_t _udrie;
-    uint8_t _u2x;
-    bool transmitting;
+    // Has any byte been written to the UART since begin()
+    bool _written;
 
-  public:
     volatile uint8_t _rx_buffer_head;
     volatile uint8_t _rx_buffer_tail;
     volatile uint8_t _tx_buffer_head;
@@ -65,11 +60,11 @@ class HardwareSerial : public Stream
     unsigned char _rx_buffer[SERIAL_BUFFER_SIZE];
     unsigned char _tx_buffer[SERIAL_BUFFER_SIZE];
 
+  public:
     HardwareSerial(
       volatile uint8_t *ubrrh, volatile uint8_t *ubrrl,
       volatile uint8_t *ucsra, volatile uint8_t *ucsrb,
-      volatile uint8_t *ucsrc, volatile uint8_t *udr,
-      uint8_t rxen, uint8_t txen, uint8_t rxcie, uint8_t udrie, uint8_t u2x);
+      volatile uint8_t *ucsrc, volatile uint8_t *udr);
     void begin(unsigned long);
     void begin(unsigned long, uint8_t);
     void end();
@@ -84,6 +79,10 @@ class HardwareSerial : public Stream
     inline size_t write(int n) { return write((uint8_t)n); }
     using Print::write; // pull in write(str) and write(buf, size) from Print
     operator bool();
+
+    // Interrupt handlers - Not intended to be called externally
+    void _rx_complete_irq(void);
+    void _tx_udr_empty_irq(void);
 };
 
 // Define config for Serial.begin(baud, config);
@@ -114,18 +113,19 @@ class HardwareSerial : public Stream
 
 #if defined(UBRRH) || defined(UBRR0H)
   extern HardwareSerial Serial;
-#elif defined(USBCON)
-  #include "USBAPI.h"
-//  extern HardwareSerial Serial_;  
+  #define HAVE_HWSERIAL0
 #endif
 #if defined(UBRR1H)
   extern HardwareSerial Serial1;
+  #define HAVE_HWSERIAL1
 #endif
 #if defined(UBRR2H)
   extern HardwareSerial Serial2;
+  #define HAVE_HWSERIAL2
 #endif
 #if defined(UBRR3H)
   extern HardwareSerial Serial3;
+  #define HAVE_HWSERIAL3
 #endif
 
 extern void serialEventRun(void) __attribute__((weak));
diff --git a/hardware/arduino/avr/cores/arduino/HardwareSerial0.cpp b/hardware/arduino/avr/cores/arduino/HardwareSerial0.cpp
new file mode 100644
index 00000000000..dd4d806c756
--- /dev/null
+++ b/hardware/arduino/avr/cores/arduino/HardwareSerial0.cpp
@@ -0,0 +1,54 @@
+#include "Arduino.h"
+#include "HardwareSerial.h"
+
+// Each HardwareSerial is defined in its own file, sine the linker pulls
+// in the entire file when any element inside is used. --gc-sections can
+// additionally cause unused symbols to be dropped, but ISRs have the
+// "used" attribute so are never dropped and they keep the
+// HardwareSerial instance in as well. Putting each instance in its own
+// file prevents the linker from pulling in any unused instances in the
+// first place.
+
+#if defined(HAVE_HWSERIAL0)
+
+#if defined(USART_RX_vect)
+  ISR(USART_RX_vect)
+#elif defined(USART0_RX_vect)
+  ISR(USART0_RX_vect)
+#elif defined(USART_RXC_vect)
+  ISR(USART_RXC_vect) // ATmega8
+#else
+  #error "Don't know what the Data Received vector is called for the first UART"
+#endif
+  {
+    Serial._rx_complete_irq();
+  }
+
+#if defined(UART0_UDRE_vect)
+ISR(UART0_UDRE_vect)
+#elif defined(UART_UDRE_vect)
+ISR(UART_UDRE_vect)
+#elif defined(USART0_UDRE_vect)
+ISR(USART0_UDRE_vect)
+#elif defined(USART_UDRE_vect)
+ISR(USART_UDRE_vect)
+#else
+  #error "Don't know what the Data Register Empty vector is called for the first UART"
+#endif
+{
+  Serial._tx_udr_empty_irq();
+}
+
+#if defined(UBRRH) && defined(UBRRL)
+  HardwareSerial Serial(&UBRRH, &UBRRL, &UCSRA, &UCSRB, &UCSRC, &UDR);
+#else
+  HardwareSerial Serial(&UBRR0H, &UBRR0L, &UCSR0A, &UCSR0B, &UCSR0C, &UDR0);
+#endif
+
+// Function that can be weakly referenced by serialEventRun to prevent
+// pulling in this file if it's not otherwise used.
+bool Serial0_available() {
+  return Serial.available();
+}
+
+#endif // HAVE_HWSERIAL0
diff --git a/hardware/arduino/avr/cores/arduino/HardwareSerial1.cpp b/hardware/arduino/avr/cores/arduino/HardwareSerial1.cpp
new file mode 100644
index 00000000000..c04263e52c4
--- /dev/null
+++ b/hardware/arduino/avr/cores/arduino/HardwareSerial1.cpp
@@ -0,0 +1,54 @@
+#include "Arduino.h"
+#include "HardwareSerial.h"
+
+// Each HardwareSerial is defined in its own file, sine the linker pulls
+// in the entire file when any element inside is used. --gc-sections can
+// additionally cause unused symbols to be dropped, but ISRs have the
+// "used" attribute so are never dropped and they keep the
+// HardwareSerial instance in as well. Putting each instance in its own
+// file prevents the linker from pulling in any unused instances in the
+// first place.
+
+#if defined(HAVE_HWSERIAL1)
+
+#if defined(USART_RX_vect)
+  ISR(USART_RX_vect)
+#elif defined(USART1_RX_vect)
+  ISR(USART1_RX_vect)
+#elif defined(USART_RXC_vect)
+  ISR(USART_RXC_vect) // ATmega8
+#else
+  #error "Don't know what the Data Received vector is called for the first UART"
+#endif
+  {
+    Serial1._rx_complete_irq();
+  }
+
+#if defined(UART1_UDRE_vect)
+ISR(UART1_UDRE_vect)
+#elif defined(UART_UDRE_vect)
+ISR(UART_UDRE_vect)
+#elif defined(USART1_UDRE_vect)
+ISR(USART1_UDRE_vect)
+#elif defined(USART_UDRE_vect)
+ISR(USART_UDRE_vect)
+#else
+  #error "Don't know what the Data Register Empty vector is called for the first UART"
+#endif
+{
+  Serial1._tx_udr_empty_irq();
+}
+
+#if defined(UBRRH) && defined(UBRRL)
+  HardwareSerial Serial1(&UBRRH, &UBRRL, &UCSRA, &UCSRB, &UCSRC, &UDR);
+#else
+  HardwareSerial Serial1(&UBRR1H, &UBRR1L, &UCSR1A, &UCSR1B, &UCSR1C, &UDR1);
+#endif
+
+// Function that can be weakly referenced by serialEventRun to prevent
+// pulling in this file if it's not otherwise used.
+bool Serial1_available() {
+  return Serial1.available();
+}
+
+#endif // HAVE_HWSERIAL1
diff --git a/hardware/arduino/avr/cores/arduino/HardwareSerial2.cpp b/hardware/arduino/avr/cores/arduino/HardwareSerial2.cpp
new file mode 100644
index 00000000000..2c448f9372b
--- /dev/null
+++ b/hardware/arduino/avr/cores/arduino/HardwareSerial2.cpp
@@ -0,0 +1,54 @@
+#include "Arduino.h"
+#include "HardwareSerial.h"
+
+// Each HardwareSerial is defined in its own file, sine the linker pulls
+// in the entire file when any element inside is used. --gc-sections can
+// additionally cause unused symbols to be dropped, but ISRs have the
+// "used" attribute so are never dropped and they keep the
+// HardwareSerial instance in as well. Putting each instance in its own
+// file prevents the linker from pulling in any unused instances in the
+// first place.
+
+#if defined(HAVE_HWSERIAL2)
+
+#if defined(USART_RX_vect)
+  ISR(USART_RX_vect)
+#elif defined(USART2_RX_vect)
+  ISR(USART2_RX_vect)
+#elif defined(USART_RXC_vect)
+  ISR(USART_RXC_vect) // ATmega8
+#else
+  #error "Don't know what the Data Received vector is called for the first UART"
+#endif
+  {
+    Serial2._rx_complete_irq();
+  }
+
+#if defined(UART2_UDRE_vect)
+ISR(UART2_UDRE_vect)
+#elif defined(UART_UDRE_vect)
+ISR(UART_UDRE_vect)
+#elif defined(USART2_UDRE_vect)
+ISR(USART2_UDRE_vect)
+#elif defined(USART_UDRE_vect)
+ISR(USART_UDRE_vect)
+#else
+  #error "Don't know what the Data Register Empty vector is called for the first UART"
+#endif
+{
+  Serial2._tx_udr_empty_irq();
+}
+
+#if defined(UBRRH) && defined(UBRRL)
+  HardwareSerial Serial2(&UBRRH, &UBRRL, &UCSRA, &UCSRB, &UCSRC, &UDR);
+#else
+  HardwareSerial Serial2(&UBRR2H, &UBRR2L, &UCSR2A, &UCSR2B, &UCSR2C, &UDR2);
+#endif
+
+// Function that can be weakly referenced by serialEventRun to prevent
+// pulling in this file if it's not otherwise used.
+bool Serial2_available() {
+  return Serial2.available();
+}
+
+#endif // HAVE_HWSERIAL2
diff --git a/hardware/arduino/avr/cores/arduino/HardwareSerial3.cpp b/hardware/arduino/avr/cores/arduino/HardwareSerial3.cpp
new file mode 100644
index 00000000000..012839df8a6
--- /dev/null
+++ b/hardware/arduino/avr/cores/arduino/HardwareSerial3.cpp
@@ -0,0 +1,54 @@
+#include "Arduino.h"
+#include "HardwareSerial.h"
+
+// Each HardwareSerial is defined in its own file, sine the linker pulls
+// in the entire file when any element inside is used. --gc-sections can
+// additionally cause unused symbols to be dropped, but ISRs have the
+// "used" attribute so are never dropped and they keep the
+// HardwareSerial instance in as well. Putting each instance in its own
+// file prevents the linker from pulling in any unused instances in the
+// first place.
+
+#if defined(HAVE_HWSERIAL3)
+
+#if defined(USART_RX_vect)
+  ISR(USART_RX_vect)
+#elif defined(USART3_RX_vect)
+  ISR(USART3_RX_vect)
+#elif defined(USART_RXC_vect)
+  ISR(USART_RXC_vect) // ATmega8
+#else
+  #error "Don't know what the Data Received vector is called for the first UART"
+#endif
+  {
+    Serial3._rx_complete_irq();
+  }
+
+#if defined(UART3_UDRE_vect)
+ISR(UART3_UDRE_vect)
+#elif defined(UART_UDRE_vect)
+ISR(UART_UDRE_vect)
+#elif defined(USART3_UDRE_vect)
+ISR(USART3_UDRE_vect)
+#elif defined(USART_UDRE_vect)
+ISR(USART_UDRE_vect)
+#else
+  #error "Don't know what the Data Register Empty vector is called for the first UART"
+#endif
+{
+  Serial3._tx_udr_empty_irq();
+}
+
+#if defined(UBRRH) && defined(UBRRL)
+  HardwareSerial Serial3(&UBRRH, &UBRRL, &UCSRA, &UCSRB, &UCSRC, &UDR);
+#else
+  HardwareSerial Serial3(&UBRR3H, &UBRR3L, &UCSR3A, &UCSR3B, &UCSR3C, &UDR3);
+#endif
+
+// Function that can be weakly referenced by serialEventRun to prevent
+// pulling in this file if it's not otherwise used.
+bool Serial3_available() {
+  return Serial3.available();
+}
+
+#endif // HAVE_HWSERIAL3
diff --git a/hardware/arduino/avr/cores/arduino/USBAPI.h b/hardware/arduino/avr/cores/arduino/USBAPI.h
index da2e86961cc..5a330024cab 100644
--- a/hardware/arduino/avr/cores/arduino/USBAPI.h
+++ b/hardware/arduino/avr/cores/arduino/USBAPI.h
@@ -55,6 +55,8 @@ class Serial_ : public Stream
 };
 extern Serial_ Serial;
 
+#define HAVE_CDCSERIAL
+
 //================================================================================
 //================================================================================
 //	Mouse