The Zone memory system is an internal memory allocator used by Doom for memory management. Due to technology at the time of Doom's release, memory allocation was considered an expensive operation. Presumably due to the inadequacies of memory management functions available in DOS at the time of Doom's development, Doom includes its own memory allocator. In a normal C program, memory is allocated and deallocated using the C functions malloc and free. The zone memory code contains its own implementations of these, Z_Malloc and Z_Free.