Josh,

Jumper P1.0 to P3.2 so that when you set it low it causes an interrupt.

$mod51

org 0
jmp init

org 3
jmp blue

org 40H
init:
setb EX0
setb PX0
setb IT0
setb p1.0
setb EA

START:
mov r2,#0
reload:
mov r3,#0
loop:
mul ab
djnz r3,loop
djnz r2,reload
clr P1.0
AJMP START


BLUE:
INC R3
INC R3
INC R3
DEC R3
DEC R3
DEC R3
setb p1.0
RETI

end

regards,
p