DOSBox

DOSBox is an open-source emulator for accurately running MS-DOS games under modern operating systems. It supports many games, and its results are often more accurate than the legacy support provided by more recent versions of Windows. It also has the added bonus of being able to run on various non-Windows platforms.

Emulators use quite a lot of processing power and memory, since they have to translate all requests, event notifications, etc. from the sending software's "language" into the receiving software's "language". But this seldom causes difficulties, since emulators are mostly used with old games and modern platforms are vastly more powerful than those for which the games were designed.

Commercial uses
DOSBox also used by commercial companies to re-release game under DOS like GOG.COM and some author release their older game in DOSBox.