The UMA memory is between 640 kilobytes( KB) and 1 megabyte( MB) and is reserved as memory for the BIOS and hardware extensions. Part of the UMA memory can be used for device drivers or the like. This part is called Upper Memory Block( UMB).
The Upper Memory Area is managed by the Extendend Memory Manager( EMM). Depending on the memory partitioning, the EMM divides the UMA into several Upper Memory Blocks (UMB).