checksum question/help

I have unpacked a dos program BUT the upacked exe won't run so i think it must have some checksum routine in it my question is if i understand checksum right you can't have the checksum value be written to the exe, meaning you have to store the checksum value in a file right? cuase if you have it store the checksum in the exe that would change the checksum. cuase when i play with a checksum code IF YOU change ONE BYTE then re-compile it the checksum is diffenert... this is what i have seen so far unless there is a way to have it wite itself to exe can this be done?


thanks