CSEG       AT    000h     ;reset
           ljmp  app_cksm
CSEG       AT    003h     ;int 0
           ljmp  ISC_EI0
CSEG       AT    00bh     ;int 1
           ljmp  ISC_T0
CSEG       AT    013h     ;int 2
           ljmp  ISC_EI1
CSEG       AT    01bh     ;int 3  ;;T1
           ljmp  ISC_T1
........
// here switches between boot and app.
                              
ISC_EI0:   jnb   GBboot,appET0          //int 0
           ljmp  ISR_EI0   
appET0:    ljmp  APPBASE + 000h

ISC_T0:    jnb   GBboot,appT0           //int 1
           ljmp  ISR_T0
appT0:     ljmp   APPBASE + 004h

ISC_EI1:   jnb   GBboot,appEI1          //int 2
           ljmp  ISR_EI1   
appEI1:    ljmp   APPBASE + 008h

ISC_T1:    jnb   GBboot,appT1           //int 3
           lcall momma_spin0   
           reti               
.......
app_cksm:  mov dptr, #4000h		; app area extends from 4000h to fbffh
startup:   setb  GBboot
           mov   dptr,#0fb1fh
           mov   a,#0
           movc  a,@a+dptr
           jz    startAP               ; if last page of flash is erased we are in app mode
           ljmp   ?C_STARTUP
startAP:   clr   GBboot 
           ljmp  APPBASE + 060h