in the bootloader:

$NOMOD51

$include   (CAPPBASE.h)

EXTRN      CODE (?C_STARTUP)
EXTRN      CODE (ISR_EI0)
............

EXTRN      BIT  (GBboot)

CSEG       AT    000h     ;reset
           ljmp  startup
CSEG       AT    003h     ;int 0
           ljmp  ISC_EI0
.......

// here switches between boot and app.
                              
ISC_EI0:   jnb   GBboot,appET0          //int 0
           ljmp  ISR_EI0   
appET0:    ljmp  APPBASE + 000h
....

startup:;  setb  GBboot
           mov   dptr,#0fbffh
           mov   a,#0
           movc  a,@a+dptr
           jnz   startAP               ; if last page of flash is erased we are in app mode
           ljmp   ?C_STARTUP
startAP:   clr   GBboot 
           ljmp  APPBASE + 060h

           end

in the app

$NOMOD51

$include   (MAPPBASE.h)

EXTRN      CODE (ISR_EI0)
EXTRN      CODE (?C_STARTUP)

CSEG       AT    APPBASE + 000h    ;int 0
           ljmp  ISR_EI0
.......

CSEG       AT    APPBASE + 060h    ;int 20
           ljmp  ?C_STARTUP

           end

startup modified like this

$INCLUDE   (cyf120.h)

?C_C51STARTUP    SEGMENT   CODE
?STACK           SEGMENT   IDATA

RSEG       ?STACK
DS         1

EXTRN      CODE (?C_START)
PUBLIC     ?C_STARTUP

           RSEG  ?C_C51STARTUP

?C_STARTUP:
;; disable watchdog (should be enabled after initialization)

Erik