Heap Characteristics

Each heap has the following characteristics:

The system assigns a unique heap identifier to each heap within the activation group.

The heap identifier for the default heap is always zero.

A storage management-bindable API, called by a program or procedure, uses the heap identifier to identify the heap on which it is to act. The bindable API must run within the activation group that owns the heap.
The activation group that creates a heap also owns it.

Because activation groups own heaps, the lifetime of a heap is no longer than that of the owning activation group. The heap identifier is meaningful and unique only within the activation group that owns it.
The size of a heap is dynamically extended to satisfy allocation requests.

The maximum size of the heap is 4 gigabytes minus 512K bytes. This is the maximum heap size if the total number of allocations (at any one time) does not exceed 128 000.
The maximum size of any single allocation from a heap is limited to 16 megabytes minus 64K bytes.