assembly - How external interrupts are recognized in AVR Atmega16? -

i'm working on assembly program turn led on/off each time pressing button(creating external interrupt). can't understand how external interrupts wake device or how impact program counter change current running code , going interrupt service routine. wrote below code , i'm using bootrst=0 , ivsel=0 options in code. guess have missed 2 things , don't know how implement them: 1. setup clock option: falling or rising edge 2. implementation of interrupt service routine(handle_sw1). need wait external interrupt in (start - jmp start) section , check pinb every time see wether it's 0 or 1, or microcontroller handle external interrupts it's own , route them directly handle_sw1 ?

; reserved 2 bytes: jump reset @ beginnning .org 0x00     jmp reset_isr  ; configuration: put interrupt 1 vector @ address $002 .org 0x02     jmp handle_sw1  handle_sw1:     ; turn on led here, set pd7 output led     ldi r16,(1 << pd7)     out ddrd,r16     out portd,r16      ; control data output, keep led on untill key-press interrupt comes in     ; skip if previous status of led on     sbis portd,7     ret       ; create delay     call short_delay      ; otherwise turn off led     ldi r16,(0 << pd7)     out portd,r16      ; enable global interrupt flag     sei     ret  .org $1c00 reset_isr:     ; set stack pointer top of ram      ldi r16,high(ramend)     out sph,r16     ldi r16,low(ramend)     out spl,r16      ; enable int1     ldi r16, (1 << int1)      out gicr, r16      ; configuration: ivsel = 0, bootrst = 0     ; make sure ivsel set 0     ldi r16,(0 << ivce)     out gicr,r16     ldi r16,(0 << ivsel)     out gicr,r16      ; enable global interrupt flag     sei start:     ; enable input direction pd3     ldi r16,(0 << pd3)     out ddrd,r16     rjmp start      ; short delay synchronization between press , reaction short_delay:     ldi r25,5 loop:     dec r25     brne loop     ret