Nothing much to understand here, LINK/UNLK are for creating local workspace so subroutines that have to be re-entrant (or are just written in C or any other high level language) work correctly. If you don't write recursive/re-entrant routines, you can safely ignore these commands as you don't need them anyway. Pus, what PMC said except for the addq.l #-512,a7 bit
(addq only works with 3 bit values where value >0 and <=8)