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DOS extenderSoftware that enabled a DOS application to run in extended memory, which is beyond 1MB (that's right... "one megabyte"). Windows 3.0 included a DOS extender. It may seem ridiculous today, but by breaking the 1MB limit, Windows 3.0 solved a very thorny issue and became very popular.
To gain access to extended memory, the extender ran the application in Protected Mode. When the application requested DOS services, the DOS extender handled them. For functions such as a disk access, it reset the machine to Real Mode so DOS could handle them and then switched back to Protected Mode. See VCPI, DPMI and Protected Mode.