DOS extender

A DOS extender is a computer software program running under DOS that enables software to run in a protected mode environment even though the host operating system is only capable of operating in real mode.

In addition to setting up the environment and loading the actual program to be executed, the DOS extender also provides (amongst other things) a translation layer that maintains buffers allocated below the 1 MB real mode memory barrier.

Included with Watcom's C, C++, and Fortran compilers for 386 class processors, it soon became a ubiquitous mainstay of PC applications and games such as id Software's successful Doom.

While initially it was the memory-hungry business applications that drove the development of DOS extenders, it would be PC games that truly brought them into the spotlight.

[1] They provided an API through which an extended program could interface with real mode software, allocate memory, and handle interrupt services.