A digital computer system has the following main parts as 1>  Input Devices 2>  Central Processing Unit ( CPU ) 3>  Memory Unit 4>  Output Devices   1>>   INPUT DEVICES :   We enter programs and data into the computer system through input devices . There are two types of input devices . ~~>> Direct Human - Computer interaction :   A keyboard is directly connected to the computer system and allows the data to entered directly into the computer . ~~>>  Indirect Interactions :   These input devices required the data / programs to be recorded on an input medium before it enters into the computer , such as floppies , magnetic tapes etc. ~~>> Other Input devices :   Punched cards , floppy disks , visual display unit ( VDU ) , Magnetic lnk Characters Reader ( MICR ) , Optical Character Recognition (OCR) , Keyboard , mouse , Light Pen , Magnetic Tape etc .   2>>   Contral Processing Unit ( CPU )  It is the heart of computer system . It has two main sections :  ! >>  Arithmetic And Logic Unit ( ALU )  !! >>  Control Unit ( CU ) !>>   Airthmetic And Logic Unit ( ALU ) :   All the calculations and decision are performed in the ALU . Initially The Data / programs are fed into the main memory through the input devices . Whenever the ALU needs these , the data is transferred to it . No calculation areperform in the main memory .The intermediate results generated in the ALU are sent to the working storage area of the main memory , where they are kept temporarily . The data can move many times between the main memory and the ALU before the final results are obtained .   Once the calculations are over , the final result are sent to the output storage section of the main memory . From here the results are sent to the printers or the other output devices .  !!>>  Control Unit or Control Sections ( CU ) :   This unit work like the Centrol Nervous System for all parts of a computer system . The entire operations of a computer system is controlled by the CU  . This unit given diraection and orders to all parts of a computer system so that they can work properly in a systemtic way . This unit also look into the execution of instructions . The CU selects , interpret program instructions and look into the execution of these instructions .   3>> Memory Unit :   The function of stroage unit is to store information i.e. it provides spce for storing data and instructions , space for intermediate results and space for the final results .   To sum up , the storage unit perform following functions : ~~>>  It stores the data and the program ( set of instruction ) ~~>>  It holds the intermediate results of processing. ~~>>  It stores the final result of processing before they are passed onto the output unit .  Basically we can divide the memory unit into two parts . ~~>>  primary memory ~~>>  secondary memory     4>> Output devices : These devices take the output results from the CPU in the binary coded from and convert them into easily readable from . Generally , printers and the monitors are used to show output results . Other output devices : ~~>> Printers : Line printer , Dot matrix printer , Letter Quality Printer , ( or Daisy Wheel Printer ) , Drum Printer , Chain Printer , Laser Printers etc. ~~>>  Plotters : Drum Plotter , Flat Bed plotter . Floppy , Magnetic tape , Punched Cards , Visual Display Unit etc .