【正文】 re and Function The CPU reads and interprets the instructions, reads the data required by each instruction, executes the action required by the instruction, and stores the results back in memory. One of the action that is required of the CPU is to read data from or write data to an external device. This is carried out using the input/output system. The memory of puter consists of a set of sequentially numbered locations. Each location is a register in which binary information can be stored. The ?number? of location is called its address. The lowest address is 0. The manufacturer defines a word length for the processor that is an integral number of locations long. In each word the bits can represent either data or instructions. For the Intel 8086/87 and Motorala MC68000 microprocessors, a word is 16 bits long, but each memory location has only 8 bits and thus two 8bit location must be accessed to obtain each data word. In order to use the contents of memory, the processor must fetch the contents of the right location. To carry out a fetch, the processor place (enables) the binarycoded address of the desired location onto the address line of the external processor bus. The memory then allows the contents of the addressed memory location to be read by the processor. The process of fetching the contents of a memory location does not alter the contents of that location. In a real instruction set there are many more instructions. There is also a much larger number of memory locations in which to store instructions and data. In order to increase the number of memory locations, the address fields and hence the instructions must be longer than 16 bits if we use the same approach. There are a number of way to increase the addressing range of the microprocessor without increasing the instruction length: variable instruction field, multiword instructions, multiple addressing mode, variable instruction length. We will not discuss them in detail. The bus is the most important munication system in a puter system. Under control of the CPU, a data source device and data destination device are “enabled” onto (equivalent to being connected to ) the bus wires for a short transmission. The internal processor bus described in is connected to the external processor bus by a set of bus buffers located on the microprocessor integrated circuit. C: Fundamentals of Computer and Networks The operating system must ensure correct operation of the puter system. To prevent user programs from 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 instruction) 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. Although the physical separation of the municating puters may vary considerably from one type of application to ano