DOS/4GW is a 32-bit DOS extender that allows DOS programs to eliminate the 640 KB conventional memory limit by addressing up to 64 MB of extended memory on Intel 80386 and above machines. It can operate within MS-DOS, PC-DOS, DR-DOS, and other DOS clones; the DOS boxes of OS/2, Microsoft Windows, Windows NT, and Windows 95; and DOS emulators such as DOSBox. DOS/4GW was developed by Rational Systems (now Tenberry Software) as a subset of DOS/4G and was distributed with the Watcom C compiler.
Functioning as a highly flexible and reusable memory extension library, it allowed programmers to access extended memory without programming specialized code. Several games used it either internally (like the Doom engine did) or externally (being started with a batch file that contained a command along the line of @DOS4GW GAME.EXE rather than directly with the exe; Shadowcaster is such an example). DOS4/GW implements the DOS Protected Mode Interface (DPMI), an API for DOS extension which originated with the development of Windows 3.0.
An alternative DOS extender, DOS/32A, is available. It can be used as a replacement of DOS/4GW in some cases.