sdcc -c file-1.c                          -- just compile file-1.c to file-1.rel
sdcc -c file-2.c                          -- just compile file-2.c to file-2.rel
:
sdcc -c file-n.c                          -- just compile file-n.c to file-n.rel

sdcc file-1.rel file-2.rel ... file-n.rel -- link all the objects together