Expanded or extende

In the case of the 8088/86 processor, supporting expanded memory involves adding extra hardware in the form of an expanded memory card. You could add RAM in the usual way to bring the machine up to the 640KByte limit. After this any memory that you add has to be expanded memory because the entire address space has already been allocated. In later 

PC/XT designs the expanded memory hardware was often built into the mainboard and so the user wasn’t quite so a ware of the distinction between the two types of memory and the lengths to which the system hand to go in supporting expanded memory.

In the case of 286  and 386 machines, there is a choice of how to add memory once you move beyond the first 640KBytes. As the address space in protected mode extends to at least 16MBytes you could add memory beyond 640KBytes in the form of extended memory. However, as already discussed, this extended memory would be inaccessible to all real mode programs. Alternatively, you could use the same sort of hardware found on PC/XT machines to add expanded memory. This would work in exactly the same way as expanded memory on a PC/XT-that is real mode programs that   had been modified to make use of it could break the 640KByte barrier-but it wouldn’t be used as extended memory in protected mode.

So which type of memory should you are to add memory beyond 640KBytes-expanded or extended? This is a very real problem because if you opt for expanded memory then protected mode applications will ignore it but if you opt for extended memory then real mode applications will ignore it!