Because the ancestors of the PC limited the memory used by DOS to a 640KB frame, everyone tried their best to break the 640KB tight band to get rid of the 640KB limit, and let some DOS programs get rid of the 640KB barrier . In the 1MB memory space that DOS can control, conventional memory occupies 640KB, and the remaining 384KB of upper memory (UMB) is reserved for BIOS ROM, graphics card and various other expansion cards, but there are some reserved spaces that are not used, so Versions above DOS 5.0 have the ability to break through 640KB, allowing the use of 384KB of upper memory UMB (address 0A0000H～OFFFFFlH) above conventional memory, but to surpass the traditional 640KB, there must be some conditions and operations, and its conditions and operations As follows:
◎Computers above 386 and extended memory above 384KB.
◎DOS 5.0 or above.
◎CONFIG.SYS set Devuce=C:\DOS\HIMEM.SYS (extended memory XMS driver).
◎CONFIG.SYS set Device=C:\DOS\EMM386. EXE, NOEMS (extended memory EMS simulation driver).
◎CONFIG.SYS set DOS=HIGH, UMB.