.include "m328def.inc"; include library for the AVR ATmega328 microcontroller
.org 0x0000
          RJMP begin; jump to begin
.org 0x0002; address of enternal interrupt through pin INT0
          RJMP   EXT_INT0; jump to the ISR code 'EXT_INT0' when external interrupt is read through the pin INT0
.org 0x0034; sets initial program address at 0x0034  
 begin:  SEI; enables interrupt bit
         LDI    R16,low(RAMEND); loads the low value of RAMEND into a register
         OUT    SPL,R16; sets stack pointer SPL
         LDI    R16,high(RAMEND); loads the high value of RAMEND into a register
         OUT    SPH, R16; sets stack pointer SPH    
               
         LDI    R16, 0x02
         STS    0x69, R16; sets the EICRA to generate interrupt on the falling edge of INT0
                 
         LDI    R16, 0x01
         STS    0x3D, R16; sets the EIMSK to enable interrupt through pin INT0

         LDI    R16,0xFB
         OUT    DDRD, R16; sets DDRD as 0xFB to set all pins of PORTD to output and pin 2 to input
looper:  LDI    R16,0x80
         OUT    PORTD, R16; sets pin 7 of PORTD to high
         RCALL  Delay; calls a subroutine starting at 'Delay' to create a delay of a second while the LED is on
         LDI    R16,0x04
         OUT    PORTD, R16; sets all pins of PORTD except pin 2 to low
         RCALL  Delay; calls a subroutine starting at 'Delay' to create a delay of a second while the LED is off
         RJMP   looper; jumps back to 'looper' to the next iteration of the loop

EXT_INT0:LDI    R16,0x84; starts the ISR
         OUT    PORTD, R16; sets pins 7 and 2 of PORTD to high
         RCALL  Delay_Int; calls a subroutine starting at 'Delay' to create a delay of a third of a second while the LED is on during the ISR
         LDI    R16,0x04
         OUT    PORTD, R16; sets all pins of PORTD except pin 2 to low
         RCALL  Delay_Int; calls a subroutine starting at 'Delay' to create a delay of a third of a second while the LED is off during the ISR
         LDI    R16,0x84
         OUT    PORTD, R16; sets pins 7 and 2 of PORTD to high
         RCALL  Delay_Int; calls a subroutine starting at 'Delay' to create a delay of a third of a second while the LED is on during the ISR
         LDI    R16,0x04
         OUT    PORTD, R16; sets all pins of PORTD except pin 2 to low
         RCALL  Delay_Int; calls a subroutine starting at 'Delay' to create a delay of a third of a second while the LED is off during the ISR
         RETI; Stops the ISR to return to the original task

Delay:   LDI    R17, 0x99; defines the starting of the subroutine loop with the looping variable at 0x99 (=153)
loop:    RCALL  Delay2; calls a nested subroutine 'Delay2'
         DEC    R17; decreases the looping value stored in the register
         BRNE   loop; if value at R17 is not zero, jumps back to 'loop' else continues
         RET; returns to address where subroutine was called from
Delay_Int:LDI   R17, 0x33; defines the starting of the subroutine loop with the looping variable at 0x33 (=51)
loopint: RCALL  Delay2; calls a nested subroutine 'Delay2'
         DEC    R17; decreases the looping value stored in the register
         BRNE   loopint; if value at R17 is not zero, jumps back to 'loopint' else continues
         RET; returns to address where subroutine was called from
Delay2:  LDI    R18, 0xFF; defines the starting of the subroutine loop with the looping variable at 0xFF (=1255)
loop2:   RCALL  Delay3; calls a nested subroutine 'Delay3'
         DEC    R18; decreases the looping value stored in the register
         BRNE   loop2; if value at R18 is not zero, jumps back to 'loop2' else continues
         RET; returns to address where subroutine was called from  
Delay3:  LDI    R19, 0xFF; defines the starting of the subroutine loop with the looping variable at 0xFF (=255)
loop3:   DEC    R19; decreases the looping value stored in the register
         BRNE   loop3; if value at R19 is not zero, jumps back to 'loop3' else continues    
         RET; returns to address where subroutine was called from