【文章內(nèi)容簡介】 . After pleting the service subroutine, the processor will continue the task from which it was originally interrupted. The third type of input is the bus request, or direct memory access(DMA) request. It is possible to have a terminal interface that stores up all the characters in a line of text until it receives a carriage return. Then the interface requests the use of the system bus, at which time the plete line of data is transferred to memory as fast as possible. In this way the processor simply bees inactive until the transfer is pleted. 13 Buses The bus is the most important munication system in a puter system. Under control of the CPU, a data source device and a data destination device are enabled onto(equivalent to being connected to) the bus wires for a short transmission. External processor bus The internal processor bus described in Sec. is connected to the external processor bus by a set of bus buffers located on the microprocessor integrated circuit. System bus The microputer board can municate with other boards by connecting its bus to an external system bus through a connector. 14 Computer Input and Output A set of registers external to the CPU is associated with what is known as the input/output (I/O) system. The I/O system is connected to the external processor bus using control, address, and data buses through an I/O registers in an interface. There are basically two ways that are used to address I/O register. In the first method, called I/Omapped input/output, the operation code itself has 無錫科技職業(yè)學(xué)院 中英文翻譯 8 special I/O instructions that address a numbered register in the interface called an I/O port. The second method of addressing I/O registers gives the I/O ports addresses that lie within the memory address range of the CPU. This is called memorymapped I/O. Of course there must not be any memory locations at the same address as I/O locations. One of the benefits of the memorymapped approach is that the full range of memory addressing modes is available to the addressing of I/O registers. 2 Fundamentals of Computer and Networks 21 Organization of Computer System A puter is a fast and accurate symbol manipulating system that is anized to accept, store, and process data and produce output results under the direction of a stored program of instructions. This section explains why a puter is a system and how a puter system is anized. Key elements in a puter system include input, processing, and output devices. Let39。s examine each ponent of the system in more detail. Input Devices Computer system use many devices for input purpose. Some INPUT DEVICES allow direct human/machine munication, while some first require data to be recorded on an input medium such as a magizable material. Devices that read data magically recorded on specially coated plastic tapes or flexible or floppy plastic disks are popular. The keyboard of a workstation connected directly to (or ONLINE to) a puter is an example of a direct input device. Additional direct input devices include the mouse, input pen, touch screen, and microphone. Regardless of the type of device used, all are ponents for interpretation and munication between people and puter systems. Central Processing Unit The heart of any puter system is the central processing unit (CPU). There are three main sections found in the CPU of a typical personal puter system: the primary storage section, the arithmeticlogic section, 無錫科技職業(yè)學(xué)院 中英文翻譯 9 and the control section. But these three sections aren39。t unique to personal puters. They are found in CPUs of all sizes. Output Device Like input units, output device are instruments of interpretation and munication between humans and puter system of all size. These device take output results from the CPU in machinecoded form and convert them into a form that can be used (a) by people (. a printed and /or displayed report) or (b) as machine input in another processing cycle. In personal puter systems, display screen and desktop printers are popular output devices. Larger and faster printers, many online workstations, and magic tape drives monly found in large systems. The input/output and secondary storage units are sometimes called peripheral devices (or just peripherals). This terminology refers to the fact that although these devices are not a part of the CPU, they are often located near it. Besides, a puter system also includes buses, ROM(read only memory), RAM(random access memory), parallel port and serial port, hard disk, floppies and CD(pact disk)drive, and so on. 22 Operating System Operating systems have developed over the past thirty years for two main purposes. First, they provide a convenient environment for the development and execution of programs. Second, operating systems attempt to schedule putational activities to ensure good performance of the puting system. The operating system must ensure correct operation of the puter system. To prevent user programs form interfering with the proper operation of the system, the hardware was modified to create two modes: user mode and monitor mode. Various instructions (such as I/O instructions and halt instructions) are privileged and can only be executed in monitor mode. The memory in which the monitor resides must also be protected from modification by the user. A timer prevents infinite loops. Once these changes (dual mode, privileged instructions, memory protection, timer interrupt) have been made to the basic puter architecture, it is possible to write a correct operating system. 無錫科技職業(yè)學(xué)院 中英文翻譯 10