What was found
AT CRUISE ALTITUDE, AFTER 30 MINUTES OF FLIGHT, SIMULTANEOUS LOSS OF CAPTAIN`S FMC, FIRST OFFICER FMC, CAPTAIN`S NAVIGATIONAL DISPLAY WENT BLANK WITH MAP FLAG, VERTICAL TRACK FLAG, CAPTAIN`S FLIGHT DISPLAY AND FIRST OFFICER`S FLIGHT DISPLAY INVALID ENTRY MESSAGE DISPLAYED ON LOWER LT OF SCREEN. FMC/CENTER DRIVE UNIT WERE INOPERATIVE EXCEPT LIMITED FUNCTION. ALSO UNABLE TO COUPLE THE WORKING AUTOPILOT TO ANY NAVIGATIONAL TYPE MODE. EMERGENCY DECLARED AND FLIGHT DIVERTED AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE FMC IAW AMM 34-61-02 AND PERFORMED SATISFACTORY OPS CHECK.