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The essential components of a PC are the mainboard, processor and memory. The memory stores the computer program the processor is running and tile data the program needs. The mainboard is the printed circuit board that the processor and memory is mounted on. It connects the processor and memory together. Let's look at these in more detail. Remember, a computer can only work with numbers. The processor does all the calculations with the numbers, obeying the instructions in the computer program. The memory consists of tiny circuits called a memory cell. Each circuit can store a zero or a one. To make a byte of memory, you need eight of these circuits. So when you buy a 128 Meg memory strip (128 million bytes of memory), it contains 128,000, 000×8 memory cells that is, 1, 000, 000, 000 memory cells. Yet a 128 Meg memory strip is not much bigger than your finger！
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A memory strip actually contains other circuitry as well. This circuitry allows the processor to select which memory cell it wants to get data from, or to store data into. The processor can jump directly from the first memory cell to the last, then to a cell in the middle, or to anywhere else to get or store data. That's why this kind of computer memory is called Random Access Memory, or RAM for short. RAM needs electricity to be able to remember data. When you switch off your computer, all the data stored in RAM is lost. For this reason, computers need some kind of permanent storage. That's why computers have disk drives. Computer people call disk drives, magnetic tape, CDs, DVDs and so on secondary storage.
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The mainboard connects all the components of the computer together. It has electrically conducting tracks which allows electricity to flow from one component to another. For example,from the processor to the RAM.
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A computer takes some kind of input, processes it in some way, then outputs the result. For example, you scan a picture into your PC. This is the input. You then modify the graphic — you adjust the color balance, the contrast and the brightness. This is the processing. Finally, you print the graphic. This is the output.
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一台PC的主要部件有主板、处理器和存储器。存储器存储处理器正在运行的计算机程序和程序所需的数据。主板是一块印制电路板，处理器和存储器都安装在上面。它将处理器和存储器连接起来。现在就让我们来更详细地了解一下。记住，一台计算机只能处理数据。处理器遵循计算机程序中的指令对数据进行各种计算。存储器是由被称作存储单元的许多微电路组成的。每一个微电路能存储一个0或1。要构成一个字节容量的存储器，你需要8个这样的微电路。因此当你购买一块128 MB的内存条时，它包含了128 000 000×8个存储单元——也就是1 000 000 000个存储单元，然而一块128 MB的内存条还不如你的手指大。
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一块内存条实际上还包含其他电路。这些电路允许处理器选择从哪个存储单元取数据或向哪个存储单元存储数据。处理器可以从第一个存储单元直接跳转到最后一个存储单元，再到中间的存储单元，或到任意一个存储单元取数据或存储数据。这就是这类计算机存储器称作随机存取存储器，或简称RAM的原因。RAM需要有电流才能记住数据。当你关掉计算机时，在RAM中存储的所有数据就丢失了。正因为如此，计算机需要某种永久存储器。这就是为什么计算机要有磁盘驱动器。使用计算机的人将磁盘驱动器、磁带、CD、DVD等称作辅助存储器。
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主板将所有计算机部件连接在一起。它有导电电路，可使电流从一个部件流到另一个部件，例如，从处理器到RAM。
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一台计算机接收某种输入，以某种方式处理它，然后输出结果。例如：你将一幅图片扫描到你的PC中，这就是输入。接着你修改图片——调整颜色平衡、对比度和亮度，这就是处理。最后，你打印图片，这就是输出。
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