There are times when a process requires only small amounts of memory rather than an entire memory object. It would be wasteful to allocate an entire page of memory when only a few bytes are necessary, so a mechanism is provided for applications to allocate a large block of memory and then suballocate portions of the memory as necessary to fulfill small requests from an application. This is done by creating a heap.
A heap is a region of storage within a memory object from which an application can allocate blocks of memory. A memory block is a piece of memory within a heap. The size of the memory block is rounded up to the next higher multiple of 8 bytes.
Because OS/2 allocates a 4KB page for each memory allocation, using a heap to suballocate amounts of memory smaller than 4KB is more efficient than using DosAllocMem.
When an application creates a heap, it can have OS/2 track the committing and decommitting of memory within the heap. When the application commits and decommits memory itself, it has to keep track of the access state of each page as they are accessed.
Applications use DosSubSetMem to initialize a memory object for suballocation, then use DosSubAllocMem and DosSubFreeMem to allocate and free the memory.
Memory is still committed in pages when an application uses suballocation. If the application suballocates 512 bytes, 4096 bytes will be committed. Accessing the 513th byte will not cause a protection violation, but you could be accessing memory that was suballocated by another thread in the process.