Data processing experts in Mission Control have deemed space shuttle Atlantis’ General Purpose Computer (GPC) 4 to be healthy after reviewing data from its failure on Thursday evening and observing its stable operation ever since. They believe an unknown hardware glitch caused the computer to shut down. Commander Chris Ferguson has been asked to reassign GPC 4 to the shuttle systems management role it had served prior to the failure. GPC 1 will serve as the guidance, navigation and control computer. GPCs 2 and 3 will be in the standby mode.
Here is a NASA web page that explains GPCs:
http://www.nasa.gov/mission_pages/shuttle/flyout/flyfeature_shuttlecomputers.html