allegro heap size limit?

This is not strictly a MATLISP question, but I though people on this
list might have some experience with very large Lisp heaps.

I'm using Allegro 6.0, on a P4 machine running RedHat with 1.5GB of RAM 
and 3GB of swap.  I'm unable to get a heap-size larger than about 500MB. 
  As soon as an allocation attempts to grow the heap above that level, I 
get something like this:

---
Error: An allocation request for 4016 bytes caused a need for 171442176 
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]
---

When I got this error the lisp process size was about 360MB.  Allocating 
171MB would have put it at 530MB.

A C program on the same machine has no problem allocating blocks of 1GB
or more, which makes me wonder if it's really the OS generating the error...

J.