You should not eat into the other ISR memory location!! The RTC is no good by itself. The RTC usually fits into another program, for example if you build a digital clock, then you can use the same RTC to provide you the time. you'll need other routines to take care of the display, plus the time set and alarm set functions. for the set functions you can use the external interrupts which in turn will have its own ISR you have to take care that these do not overlap!! so you can use SJMP or ACALL as previously mentioned, but always end the SJMP with the RETI. and the ACALL with a RET and then in the ISR a RETI. (Is this required??)

cheers,

sushil.