the input unit 'digests' the program as it is received and passes it on to the store in the form of magnetic recording. the store can supply information, or data, either to the arithmetic unit, the control unit or directly to the output unit, as it is needed. all the calculation are done in the arithmetic unit. the control unit, which is the 'nerve center' of the computer, carries out the instruction which are received in a program. the arithmetic unit and the control unit each have small stores of information called registers. the output unit present the results of each computer operation, usually as a printout.