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14  CHAPTER 1 / INTRODUCTION


                           COMPUTER


                        I/O         Main
                                   memory

                            System
                            bus


                              CPU

                                                                    CPU


                                                            Registers
                                                                           ALU

                                                                  Internal
                                                                  bus


                                                                   Control
                                                                   unit

                              CONTROL
                              UNIT
                      Sequencing
                      logic
                            Control unit
                            registers and
                            decoders


                            Control
                            memory


                 Figure 1.4  The Computer:Top-Level Structure



                       But of greater concern in this book is the internal structure of the computer
                  itself, which is shown in Figure 1.4.There are four main structural components:

                     • Central processing unit (CPU): Controls the operation of the computer and
                       performs its data processing functions; often simply referred to as processor.
                     • Main memory: Stores data.
                     • I/O: Moves data between the computer and its external environment.
                     • System interconnection: Some mechanism that provides for communica-
                       tion among CPU, main memory, and I/O. A common example of system
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