DR and MS DOS 5 and later have a 386 memory manager which can put an x86-32 CPU into Virtual 86 mode, remap RAM over any unused blocks in the I/O and ROM area (between 641kB-1024kB) to make Upper Memory Blocks, and load small resident parts of the OS into those blocks in order to keep as much base memory (0kb-640kB) free.

These memory managers could also turn the extended memory (above 1024kB) into managed XMS or EMS memory. LIM EMS was accessible to DOS apps and XMS to DOS-extender apps.

These were killer features in 1990 or so, and I made good money from optimising DOS memory for clients around that time.