The original Super Mario Bros is a platform game developed by Nintendo in late 1985 and published for the Nintendo Entertainment System, a sequel to the 1983 game, Mario Bros.
In Super Mario Bros, the titular character Mario must save Princess Toadstool of the Mushroom Kingdom from the evil King Koopa, king of the Koopas. In two-player mode, Mario is aided in his quest by his younger brother, Luigi.
In order to save Princess Toadstool, the Mario Bros must conquer the eight worlds that comprise the Mushroom Kingdom. Mario or Luigi must make his way to the castle in each world and defeat one of King Koopa's evil minions.
In order to reach each castle Mario or Luigi must battle through three "sub-worlds" by either destroying or avoiding King Koopa's henchmen. If Mario or Luigi successfully fights his way through the castle and defeats the evil minion, a Mushroom Retainer, is freed. Inside the eighth castle, the Mario Bros. will find Princess Toadstool.
There is no DOS port for Super Mario Bros, but due to tremendous number of fans around, there are some fan-made Super Mario Bros around. John Carmack and team have had produced a DOS port as well, but never accepted by Nintendo.
You can download one of the fan-made Super Mario Bros here.