Abstract: A portable electronic device is disclosed. The device has a first program stored in memory contained within the device and a second program stored on transposable memory which is releasably connectable to the device. The transposable memory comprises a plug-in module having a EEPROM. The second program stored on the transposable memory comprises a series of groups of instructions which are downloaded, as required, by the device and executed in order. The specific groups to be downloaded are determined by a jump table which accesses different groups of instructions in response to input signals. The instructions to be downloaded are identified by variable addresses in the jump table, and, the variable addresses are updated by means of the first program and the second program.