] At the nerve center of each MER rover is a 6-by 9-inch
] electronics board containing one computer responsible for
] every process that goes into a mission, whether it be
] monitoring spacecraft health in transit, deploying
] parachutes during landing or roving about the red planet.
] The computer, called a RAD6000, is a tried and true
] component for NASA space mission that has formed the
] brains of past Mars missions in the, as well as the
] recent Stardust comet encounter.

More interesting technical details about the rovers.

Thinking on Mars: The Brains of NASA's Red Planet Rovers