 If the object
was previously unmarked, the object is pushed on the mark stack.
The descriptor is read from the page descriptor.  (This is computed
from information <TT>GC_obj_kinds</tt> when the page is first allocated.)
</nl>
<P>
At the end of the mark phase, mark bits for left-over free lists are cleared,
in case a free list was accidentally marked due to a stray pointer.

<H2>Sweep phase</h2>

At the end of the mark phase, all blocks in the heap are examined.
Unmarked large objects are immediately retur