The HP 39g and the HP40g were released during the year 2000. They are made on the same hardware platform with 1MB of ROM, 256KB of RAM and has 51 keys. The physical difference between the HP 39g and the HP40g is the IR port - 40g does not have IR port. The software knows if it’s running on an HP 39g or an HP40g and will support aplets and programs accordingly. On the software side, the HP 39G has CAS (Computer Algebra System) disabled.