DOS Memory Management
If your program is so large that either its data or code (or both) won't fit into 64 Kbytes then it must have more than one segment of the appropriate type.
Whenever an address in another segment is accessed, the appropriate segment register must be reloaded. Many compilers will generate code to do this automatically according to the memory model. The 80286, 386, 486 and Pentium processors make larger memory segments available
Memory above the 1 Mbyte mark is called extended memory.
Special registers are provided to map logical addresses (which still must be within the 1 Mb range) into physical addresses within the extended memory space. This is the opposite of virtual memory since logical memory is smaller than physical memory! The 386 processor includes virtual memory support(proper page tables) that MS-Windows is capable of taking advantage of, but ther
e is still the 64Kb limit without changing segment registers. Windows
95 now supports a 32-bit address space.