Mark bit set/clear semantics

Hi,
 In the case of gen0 collection what is the purpose of using the mark
bit? like for example it is set in copy_object_simple(..) when the object
is first copied, but reset back in scavenge_object_simple(..) by the
clear_marked_pinned call and
Relocate(..) which comes after scavenge calls get_copied_object that has
an assert that the bit be marked.{assert(marked(o))}

what my changes are doing is - clear the mark bits of objects
once they are copied, so when they hit the assert in get_copied_object
they fail.
so which are the ones that need to have the mark bits set?

since i am not sure of the mark-bit semantics i have made an error
somewhere, can someone kindly clarify this

Thank you
archana