answering some of your questions by expanding on joergs example:


org 0000h
ljmp start
org 0003h ;interrupt service routine
ljmp ex0ISR

start:
clr it0 ;set ext. int. on "falling edge"
setb ea ;enable interrupts
setb ex0 ;enable external interrupt 0
workloop:
; here is room for more code
sjmp workloop

ex0ISR:
push psw ;save psw to stack
cpl p3.7 ;toggle LED
; here is room for more code
pop psw ;load psw from stack
reti ;return to where we came from when the interrupt happened