STORAGE-CONDITION error

Hi all, I'm getting the following error in Allegro Common Lisp 6.2:

Error: An explicit gc call caused tenuring and a need for 524288 more
       bytes of heap.  The operating system will not make the space
       available because of a lack of swap space or some other
       operating system imposed limit or memory mapping collision.
  [condition type: STORAGE-CONDITION]

from the documentation it seems that all the memory on the system can be
used by alisp but in my case alisp uses 757M of memory when the error
happens on a machine with 2G+2G of main memory plus swap space (and the
memory is actually free). I'm using the option (setq
EXCL:*GLOBAL-GC-BEHAVIOR* nil) because otherwise the global GC is using a
lot of time.

Is there some variable that fix an upper bound for the memory used by the
lisp process?

Thanks, Fabrizio.