Under software control, up to 4 of these 16K banks was visible at any one time. By switching in and out these 16K blocks of RAM, it could appear as though there was a large amount of memory available, all accessed through this expanded memory window. In effect, they designed two processors inside a single chip. The first processor looked and behaved like a standard 8086, whilst the other acted as a super improved chip with extra functions and larger memory capability. The default mode of operation is a standard 8086, but a special software instruction can switch the chip into the new advanced mode.
Extended memory is ONLY accessible in protected mode .
This special mode of operation occurs when the chip is switched to act as a 286-486. Several operating systems have been written to take advantage of this mode, and thus access to the larger amounts of memory available. In a PC based computer, the remaining 640K below 1MB is reserved for system use, examples being the ROM BIOS and peripherals like networking and hard disk cards. When the 286-486 computers were designed, intelligent support chips were added to provide sophisticated memory manipulation and management. These are called the AT Chip Set , now common on virtually all PC based computers.