B737-300 COURSE DEV ON APCH TO DFW DUE TO FMC ENTRY OF A LAST MIN RWY CHANGE.

Narrative

WE WERE ON THE BONHAM 5 ARR INTO DFW. THE ACFT WAS ON AUTOPLT AND THE FMC WAS NAVING. WHEN WE CONTACTED APCH CTL WE WERE TOLD THAT WE WOULD BE ASSIGNED A RWY INTXN. I LOADED THE ILS RWY 36L INTO THE FMC SINCE THIS IS THE ONE WE REQUESTED BECAUSE WE PARK ON THAT SIDE OF THE FIELD. ADDITIONALLY; THIS IS NECESSARY SO THAT THE FMC WILL NAV TO DIRKK INTXN AFTER STONZ. I CHKED THAT ALL OF THE FIXES WERE LOADED PROPERLY AND WAITED FOR A RWY ASSIGNMENT. FINALLY AS WE WERE APCHING STONZ; APCH CTL ASSIGNED US THE ILS RWY 35C. I QUICKLY LOADED THIS INTO THE FMC AND STARTED LOOKING FOR THE APCH CHART FOR RWY 35C. THIS TOOK A FEW SECONDS SINCE DFW HAS ABOUT 20 DIFFERENT APCHS. AS I WAS SETTING UP THE RADIOS FOR THE APCH; WE GOT AN 'ENTERING OPEN SEGMENT' MESSAGE FROM THE FMC. I LOOKED AT THE LEGS PAGE OF THE FMC AND OUR ENTIRE RTE HAD DISAPPEARED. BEFORE I COULD RE-ENTER THE RTE; ATC ISSUED US A HDG TO FLY WHICH I ASSUME MEANT THAT WE HAD OVERFLOWN STONZ. THIS INDICATED THAT INSTEAD OF TURNING AT STONZ TO PROCEED TO DIRKK; WE WOULD HAVE CONTINUED SW. FROM NOW ON I'LL HAVE ALL OF THE APCH PLATES OUT (IN THIS CASE; THERE WERE 4 RWYS IN USE -- RWY 36L; 35C; 35R; AND 31). ALSO; IT WOULD BE HELPFUL IF ATC WOULD GIVE US A RWY ASSIGNMENT EARLIER. ALSO; I DON'T KNOW WHY THE FMC DROPPED OUR RTE OUT WHEN I ENTERED THE NEW APCH. WHEN I GET HOME I'LL CALL OUR TRAINING DEPT.