ALTDEV WITH RESPECT TO AIRSPD DEV ON STAR ARR. A-320 FMC DESIGN.

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DSNDING INTO BOSTON ON GARDNER ARR. GIVEN CLRNC BY ZBW TO CROSS LOBBY INTXN AT 8000 FT 230 KTS. SWITCHED TO BOS APCH; PASSING ROUGHLY 13000 FT MSL; WITH ALL XING SPD RESTRS SET IN FMGC. CONSTRAINTS SHOWED ON NAV DISPLAY SCREEN. PUT IN RWY 15 ILS APCH IN MCDU; WHICH PROMPTLY DROPPED THE GARDNER 2 ARR AND ALL CONSTRAINTS. QUICKLY SELECTED ANOTHER APCH AND REENTERED GARDNER 2 ARR AND ORIGINAL CONSTRAINTS. ASKED FO TO CHK AND SEE WHAT WAS WRONG. WHEN HE RESELECTED THE ILS RWY 15 R APCH; THE VERT SPD WHICH I HAD SET AT MINUS 500 FPM TO SLOW SINK RATE AND SPD TO 250 KTS FOR BELOW 10000 FT MSL FAULTED TO AN OPEN DSCNT MODE DROPPING OUT THE VERT SPD (DSCNT RATE) MODE AND CONSEQUENTLY ACFT DSNDED BELOW 10000 FT MSL AT 330 KTS. DISCONNECTED AUTOPLT AND MANUALLY SLOWED DSCNT RATE SO ACFT WAS BELOW 250 KTS BY 9300 FT MSL. COMPUTER CTLED (ELECTRONIC) ACFT ARE REAL NICE BUT CAN GET ONE IN TROUBLE IN A HURRY WHEN THE DATABASE IS SET UP DIFFERENTLY THAN CTR OR APCH CTL CLRNCS. IN THIS CASE THE DATABASE IS SET UP TO DROP THE GARDNER ARR AND USE THE TRANSITION TO RWY 15R ILS WHICH IS DIFFERENT. DO NOT SEE ANY SOLUTION TO THIS UNLESS THE COMPANY MAKES KNOWN TO ATC WHAT THE CONSEQUENCES ARE IF THE ACFT DATABASE AND ATC CLRNCS DON'T MATCH UP. THIS COULD HAVE BEEN A REAL PROB AT A LOWER ALT OR IN A MOUNTAINOUS AREA.