An adaptable programmable calculator is provided by employing a modular read-write and read-only memory unit capable of being expanded to provide the calculator with additional program and data storage functions oriented towards the environment of the user, a central processing unit capable of performing both serial binary and parallel binary-coded-decimal arithmetic, and an input-output control unit capable of bidirectionally transferring information between the memory or central processing units and a number of input and output units. The memory, central processor, and input-output control units are controlled by a microprocessor included in the central processing unit.The input and output units include a keyboard input unit with a plurality of alphanumeric keys, a magnetic tape cassette reading and recording unit capable of bidirectionally transferring programs and data between a magnetic tape and the calculator, and a solid state output display unit capable of displaying every alphabetic and numeric character and many other symbols individually or in combination. All of these input and output units are included within the calculator itself. An output printer, and X-Y plotter, a typewriter, a teletypewriter, a magnetic or paper tape reading and recording unit, an extended read-write memory unit, a magnetic disc reading and recording unit, a modem for connecting the calculator via telephone lines to a remotely located computer, and many other peripheral input and output units may also be employed with the calculator.The calculator may be operated manually by the user from the keyboard input unit or automatically by a program stored within the memory unit to perform calculations and provide an output indication of the results thereof. It may also be employed to load programs into the memory unit from the keyboard input unit, to separately or collectively transfer data and programs bidirectionally between the memory unit and an external magnetic tape and to code programs or sections thereof stored in the memory unit as being secure when they are transferred to an external magnetic tape, thereby preventing users of the calculator from again transferring them to an external magnetic tape or obtaining any indication of the individual program steps once they are reloaded into the calculator. In addition, the calculator may be employed to edit programs stored in the memory unit and to print out program lists, labels, and messages.The calculator employs an extended version of BASIC computer language and allows the user to enter a line comprising an alphanumeric statement into the calculator from the keyboard input unit while visually observing an alphanumeric display of that line to check for errors therein, permitting the user to cause the entered lines to be immediately executed by the calculator or stored as part of a program within the memory unit, and permitting the user to subsequently recall the executed or stored line so that it may be reinspected, reevaluated, and, if necessary, edited and executed or re-executed, or restored in edited form. Any entered or recalled information may be edited by employing the keyboard input unit to selectively delete or replace incorrect or undesired portions of the information or to selectively insert corrected or previously omitted portions thereof on a line-by-line or character-by-character basis. Syntax errors are automatically detected by the calculator when the entered statement is terminated, and execution errors are automatically detected upon attempted execution of the statement or statements. Both types of errors are indicated to the user via error messages displayed by the output display unit. In the event the calculator is being used in combination with an external printer unit indications of syntax or execution errors may, if desired, be printed.The calculator employs a compiler for converting each statement entered into the calculator in BASIC language into an internal stored format. It also employs an uncompiler for regenerating in the BASIC language statement any entered line converted to the internal stored format. The compiler and uncompiler operate on a line-by-line basis.The magnetic tape cassette reading and recording unit employed in the calculator allows the user to chain together several program segments and allows program manipulation of several blocks of data on an individual basis, thereby providing more efficient utilization of the available calculator memory. An interrupt feature of the cassette unit facilitates searching for a particular file located on a magnetic tape at the same time the calculator is performing other functions.