Both A321 FMGC's transitioned to the basic startup mode with only FMGC; ATSU; AIDS and CIDS selections available when clean up selections were made for the final approach after an RNAV arrival. The failure appears to be the result of a software update error.

Narrative

While turning and descending during approach at 5;000 FT; Pilot Not Flying asks 'Clean it up?' Pilot Flying responds; 'Yes'. Both FMGCs go to basic display of FMGC; ATSU; AIDS; CIDS. At first I (as Pilot Flying) thought we had dual FMGC failure. Pushed FLT PLN button on right FMGC (2L) and only had P POS. ATHR went off but ATHR light was still illuminated. I was a bit confused about what I had and didn't have. Conditions were VMC; runway in sight. Turned off autopilot and landed uneventfully. ATHR light remained illuminated; even after pushing Instinctive Disconnect button on THR levers. Event occurred during 'clean up' of flight plan during approach phase into a major airport. [I am] unsure why event occurred. Perhaps improper sequence used during 'clean up' of flight plan; or deleted too much (including runway). Pilot not flying has already stated he knows what he did. On return flight; we were going to review the event sequence; but work load prevented it during approach and landing. If it is possible to get yourself into this situation; this would be a good event for practice in annual simulator CQT.