In 386 and above microcomputers, there are two memory working modes, one is called real address mode or real mode, and the other is called protection mode. In real mode, the physical address still uses 20 bits, so the maximum addressing space is 1MB to be compatible with 8086. The protection mode adopts 32-bit physical address, and the addressing range can reach 4GB. The DOS system works in real mode, and the memory space it manages is still 1MB, so it cannot directly use the extended memory. To this end, Lotus, Intel, AST, and Microsoft have established a standard for the use of extended memory under MS-DOS, that is, the extended memory specification XMS. Himem.sys, which we often see in the Config.sys file, is the driver that manages the extended memory.
The appearance of the extended memory management specification is later than the extended memory management specification.
