SS Ben Sunn:

Please check out the "code library" link on this page for some good multibyte math routines. If they are not immediately useful, they should give you good ideas about the way to write your own.

Although it works, the

org interruptvector
call handler
reti

handler:
.
.
ret

construct is not necessary. Why waste the stack space and the CPU time of the calls and returns if you can write:

org interruptvector
jmp handler

handler:
.
.
reti