DOS/4G is a 32-bit DOS extender developed by Rational Systems (now Tenberry Software). It 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.
Functioning as a highly flexible and reusable memory extension library, DOS/4G allowed programmers to access extended memory without programming specialized code. It embeds itself in the executable file at linking time and executes before main application code, so usually DOS/4G initialization messages show up at launch. It can in principle 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. However, in practice few DOS/4G games or other applications will run on non DOS based versions of Windows, including Windows NT, Windows 2000, Windows XP since none of these allow direct access to the hardware as was used for rendering video in those days.
Initial versions of DOS/4G had trouble with secondary DMA channels on the ISA bus, which prevented 16-bit devices like Gravis Ultrasound series from normally functioning; Gravis even had to develop PREPGAME, a patch utility which updated the game executable with a new version 1.97 to fix the incompatibility.
In case of problems, DOS/4G or DOS/4GW can be replaced with the newer and free DOS/32; a patch utility can even replace DOS/4G code embedded inside a compiled executable file.