;Code in the 8051 book i'm reading: 

ORG 0 

LJMP MAIN 

ORG 0BH ;or other interrupt address 
LJMP timer1_ISR 

ORG 23H ;serial interrupt for example 
LJMP serial_ISR 

ORG 30H 
MAIN: 
      ; initialize 
      ; initialize ports, timers, variables etc
.
.
.

      ; do forever
do_forever:
      ; do what you have to do between interrupts
.
.
. 

jmp do_forever


ORG 100H; 
timer1_ISR: 
. 
. 
RETI

serial_ISR: 
;. 
;. 
RETI 

end