nd 
the heap can extend dynamically by getting new chunks from UNIX.
The expansion occurs either automatically on the fly
or on user's explicit demand by calling <B>system:alloc</B> function.
When it is managed automatically, free memory size is kept
about 25% of total heap size. This ratio can be changed by setting a value between 0.1 and 0.9 to
the <B>sys:*gc-margin*</B> parameter.

<P>
When all the heap memory is exhausted, mark-and-sweep type garbage collection
runs.
Cells accessible from the root (pack