Application requests to allocate, reallocate, or deallocate Extended Memory Blocks are translated into corresponding call to the memory manager. A free handle table entry, indicated by a start address of zero, is selected and updated to contain the start address and size of the memory object. The total XMS memory size for the system and for each VDM is checked at this point to ensure that these limits are not exceeded.
Reallocation requests are serviced by passing the request to a VDH service and recording the new size and start address. When an application reallocates to zero, VXMS requests the memory manager to deallocate the memory object, and changes the handle table entry so it has zero pages with a sentinel non-zero address to indicate the handle is still in use. Objects of size zero are allowed in VXMS, but not in OS/2, so VXMS will deallocate but retain its own data for the handle. When a non-zero reallocation is performed on an object of size zero, a new object is transparently allocated by the memory manager.
All allocations (and reallocations) are rounded up to the nearest integral page multiple. Since there is no facility for telling the applications how much memory was actually reserved, the extra memory is wasted.
High Memory Area
There is only one HMA per VDM, so a single pointer suffices to manage the state of the HMA. If the pointer is zero, then the HMA is not in use. Otherwise, the pointer contains the linear address of the block of memory being used to simulate the HMA. Whether the HMA region is mapped to this block of memory is determined by the state of the simulated A20 line; see section High Memory Area (HMA).
When a request for the HMA is made, the pointer is tested against zero. If the pointer is non-zero, then the HMA is in use, and the request fails. Otherwise, the pointer is set to a newly allocated 64KB block of memory, and the HMA region is mapped to this block if the A20 address line is active.
When the HMA is released, the block of memory is freed, and the pointer is reset to 0. The HMA is then mapped to a black hole if the A20 address line is active. Once an HMA is freed, all information previously stored therein becomes invalid.
Upper Memory Blocks
For UMB allocation, a linked list of reserved address ranges is maintained. This list contains information about the start address of each reserved range, the base address of the physical memory block allocated and mapped into address range, and the length of the block. If the base address is zero, then the address range is not in use and is instead mapped to a black hole.
Allocations are made by searching through the list to find an address range for which the base address is zero and which is large enough to satisfy the request. If the address range exceeds the required size, it is split into two parts and a new object is allocated to hold the unused portion.
Deallocations are made by searching the list to find a structure whose starting address matches the one being deallocated. The physical memory into which the address range was mapped is freed, and the address range is instead remapped to a black hole. Finally, the newly freed object has its base address set to zero to signify that it is not in use. It is then coalesced with any adjacent free blocks.
Extended Memory Blocks
Each VDM has a fixed table of up to 255 EMB handles, the exact number of which is under user control. Each entry of the table describes a single Extended Memory Block.
Each entry contains a field which records the number of active locks on the Extended Memory Block. Locking a handle prevents the corresponding Extended Memory Block form being reallocated or freed, and also prevents the base address from changing. As part of its function, the lock subfunction returns the 32-bit base address.
If an allocation is of size zero, no physical memory allocation is requested, but a sentinel non-zero address and zero size are recorded in the handle entry. The lock count for the newly created Extended Memory Block is reset to zero.
When a deallocation request is made for an Extended Memory Block with zero lock count, the address in the handle is changed to zero, and the memory manager is called to free the memory.