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Training target: In this part ， our target is to train your reading speed. You should pay more attention to “word group”. Treating a group of words as a whole unit is a short cut to you’re your time. We have underlined the word groups in some sentences ， and the rest can be practiced by yourself. If there are some new words ， you may cover the note area with a piece of paper and try to guess their meanings without looking them up in the dictionary.

 Text  The basic building block of a modern computer is a chip ， a complex ， integrated electronic circuit etched on a tiny square of silicon no bigger than a fingernail. Since individual chips are fragile and difficult to handle ， they are normally packed and mounted on boards. A computer is assembled by sliding the right boards into a machine box. One board might hold the processor ， which in fact is the heart of a computer; another might hold memory store which provides the computer with data and instructions; while the third might hold the electronics to link a particular input device or output device to the system.

..  Let us use the system shown in Pic2.3 to show you how a typical computer works.A computer is controlled by a stored program ， so if we want to use a computer ， the first step is copying the program from diskette into memory. Now the processor can begin executing instructions; the data input from the keyboard are stored in memory. The processor processes the data and then stores the results back into memory. At last ， we can get the result.

Pic 2.3

.  Now we can see that a computer system consists of four basic components. An input device provides data. The data are stored in memory ， which also holds a program. Under the controls of the program ， the computer’s processor processes the data. The results flow from the computer to an output device. Let us come closer to the system components one by one ， beginning with the processor.

.  The processor ， usually called the central processing unit （ CPU ） or main processor ， is the heart of a computer. It is the CPU that in fact processes or manipulates data and controls all the rest parts of the computer. How can it manage its job? The secret is software. Without a program to provide control ， a CPU can do nothing. How can a program guide the CPU through the processes? Let us consider from the basic element of a program— instruction. An instruction is composed of two parts: an operation code and one or more operands.The operation code tells the CPU what to do and the operands tell the CPU where to find the data to be manipulated.

.  The processor contains four major components:a clock ， an instruction control unit ， an arithmetic and logic unit （ usually shorted for ALU ）， and several registers. The clock generates precisely timed pulses of current that synchronize the processor’s other components. Then the instruction control unit determines the location of the next instruction to be executed and fetches it from the main memory.

.  The arithmetic and logic unit performs arithmetic operations （ such as addition and subtraction ） and logic operations （ such as testing a value to see if it is true or false ）， while the registers are temporary storage devices that hold control information ， key data and some intermediate results. Since the registers are located in the CPU ， the processing speed is faster than the main memory.

.  Then which is the key component to a computer’s speed? It is the clock! In more detail ， it is the clock’s frequency that decides a computer’s processing speed. When we buy a computer ， we usually consider the main frequency first ， and that means a clock’s frequency.

.  Now we will talk more in detail about the Microprocessors and Central Processing Units （ Pic2.6 ）.Microprocessors are central processing units etched on a tiny chip of silicon and ， thus ， are called microchips. Microprocessors contain many electronic switches ， called transistors ， which determine whether electric current is allowed to pass through or not. Transistors are the basic building blocks of microprocessors. A single microchip may contain millions of transistors. When electric current is allowed to pass through ， the switch is on. This represents a1bit.If the current does not pass through ， the switch is off. This represents a0bit.Different combinations of transistors represent different combinations of bits ， which are used to represent special characters ， letters ， and digits.

.  When we talk about the computer ， we usually meet the topic: motherboard （ Pic2.7 ）.The main circuit board in a computer is called the motherboard. It is a flat board that holds all of the key elements that make up the“brain”of the system ， including the microprocessor or CPU ， RAM （ electronic or primary memory ）， and expansion slots which are sockets where other circuit boards called expansion boards may be plugged in.

Pic 2.6 CPU Pic 2.7 motherboard

 The End