A popular DOS extender from the early 90's. Back then, most advanced PC OSes, such as Microsoft Windows, OS/2 or various flavors of UNIX, didn't allow the programmer full direct access to the hardware, something a lot of game developers relied on for performance-enhancing tricks. Their own APIs were generally suited to the needs of more serious applications, not video games. That left a PC programmer with MS-DOS as a system of last resort, as it did allow direct access to hardware, but then another problem reared its ugly head: the infamous 640K barrier.