ATMEGA328P: externer Interrupt INT0 funktioniert nicht

Hallo miteinander;
kann mir jemand sagen warum ich nicht in die Interruptroutine komme wenn 
ich den Arduino Uno PIN 2 auf GND lege ?

.include "m328Pdef.inc"

.def temp = r16

rjmp start    ; 1     Reset
rjmp interrupt0          ; 2   External Interrupt Request 0   INT0_vect
reti      ; 3   External Interrupt Request 1   INT1_vect
reti      ; 4   Pin Change Interrupt Request 0   PCINT0_vect
reti      ; 5   Pin Change Interrupt Request 1   PCINT1_vect
reti      ; 6   Pin Change Interrupt Request 2   PCINT2_vect
reti      ; 7   Watchdog Time-out Interrupt   WDT_vect
reti      ; 8   Timer/Counter2 Compare Match A   TIMER2_COMPA_vect
reti      ; 9   Timer/Counter2 Compare Match B   TIMER2_COMPB_vect
reti      ; 10  Timer/Counter2 Overflow   TIMER2_OVF_vect
reti      ; 11  Timer/Counter1 Capture Event   TIMER1_CAPT_vect
reti      ; 12  Timer/Counter1 Compare Match A   TIMER1_COMPA_vect
reti      ; 13  Timer/Counter1 Compare Match B   TIMER1_COMPB_vect
reti      ; 14  Timer/Counter1 Overflow   TIMER1_OVF_vect
reti      ; 15  Timer/Counter0 Compare Match A   TIMER0_COMPA_vect
reti      ; 16  Timer/Counter0 Compare Match B   TIMER0_COMPB_vect
reti      ; 17  Timer/Counter0 Overflow   TIMER0_OVF_vect
reti      ; 18  SPI Serial Transfer Complete   SPI_STC_vect
reti      ; 19  USART Rx Complete   USART_RX_vect
reti      ; 20  USART Data Register Empty   USART_UDRE_vect
reti      ; 21  USART Tx Complete   USART_TX_vect
reti      ; 22  ADC Conversion Complete   ADC_vect
reti      ; 23  EEPROM Ready   EE_READY_vect
reti      ; 24  Analog Comparator   ANALOG_COMP_vect
reti      ; 25  Two-wire Serial Interface   TWI_vect
reti      ; 26  Store Program Memory Read   SPM_READY_vect

start:

ldi temp, high(RAMEND)
out SPH , temp
ldi temp, low(RAMEND)
out SPL , temp

ldi temp, 0b00000000
out DDRD, temp
ldi temp, 0b00100000
out DDRB, temp

ldi temp, 0
sts EICRA, temp

ldi temp, 1
out EIMSK, temp

sei

loop:
rjmp loop

interrupt0:
sbi  PORTB, 5
reti

Wenn ich das Pendant über die Arduino-IDE laufen lasse dann klappt es :

#include <avr/interrupt.h>
void setup()
{
pinMode(13, OUTPUT);

EICRA=0;
EIMSK=1;
sei();
}

void loop()
{
}

void blink()
{
Serial.println(millis());
}

ISR (INT0_vect)
{
digitalWrite(13, 1);
}