A programmable electronic calculator operates in a programming mode, wherein a desired optional program is introduced into the programmable electronic calculator, or in a run mode wherein the stored program is executed. The introduced optional program is stored in a read/write memory (RAM). A mode selection switch is provided on a keyboard for selectively placing the programmable electronic calculator in the run mode or the programming mode. When a lock command is introduced through the keyboard, the mode selection switch is disabled so that the programmable electronic calculator will not be placed in the programming mode, thereby protecting the optional program stored in the read/write memory (RAM) from being unintentionally erased.