Rowid Structure in Oracle8.*

I am developing a script in PL/SQL and as a part of it, I need to use rowids. But unfortunatelly, its structure (the rowid structure) has changed since Oracle7, and I can't find a place with a description of the new structure, something like:

1.- First 4 bytes = block
2.- Byte 6 to x = segment

etc.

Does anybody know about a place where I can find some information? It's really important.

Thanks a lot for your answers

-- 
Atte.
Álvaro Palma Aste
Grupo de Ing Biomédica - U. de Chile
apalma_at_cipres.cec.uchile.cl