When purchasing a computer， the purchaser should “look under the hood” to ensure that the computer contains high-quality components. Low-quality components have a greater tendency to fail after a few years of operation. If a critical component fails， the computer may not be usable for several weeks while the component is replaced. If the failed components are obsolete and no longer available， the computer must usually be rebuilt using new components. If the hard drive fails， the contents of the hard drive may be lost. Although name-brand computers tend to contain high-quality parts， this is not always the case. The buyer must ultimately ensure that her computer contains high-quality components.
Computer users generally desire computers that perform well in typical usage situations. A slow computer may significantly decrease the user’s productivity. In stand-alone usage situations， computer performance is largely determined by three computer components：
The CPU
The random access memory （RAM）， and
The computer monitor
The CPU
The CPU processes computer instructions to perform work， just as an automobile engine processes fuel to produce power. CPU performance is measured in terms of the maximum number of instructions the processor may carry out per second， expressed in megahertz. One MHz equals one million instruction cycles per second. Current-generation CPUs are incredibly fast. If a CPU operates at or above 350 MHz， most PC users do not appreciate much of a difference if additional speed is added. Sure， the difference is there， but it is subtle， not obvious. CPU speed will be very important in the context of voice recognition， but the maximum CPU speed is not yet high enough to enough to enough to enable most voice recognition applications to work well.
RAM
Operating systems， applications and files must be moved from disk to RAM before they may be used. The computer’s RAM size largely determines how many programs may run simultaneously without performance degradation. RAM size is measured in megabytes. One MB equals one million bytes. A byte stores one character （e. g. a letter or a number）. Many programs are memory hogs—they take up a lot of RAM， and do not let it go when other programs request it. RAM speed plays a role in determining a computer’s overall speed， but RAM speed is generally less important here than is CPU speed. RAM speed is measured in nanosecond. Currently， three types of DRAM （dynamic RAM） are commonly used in computers：
EDO （Extended Data Output） DRAM
SDRAM （Synchronous DRAM）， and
RDRAM （Ram bus DRAM）
EDO DRAM is the cheapest and slowest of the three， and is mostly used in older computers. RDRAM is the fastest and most expensive of the three. Overall computer performance falls off precipitously as the computer’s RAM utilization approaches 100%. In such situations， the user has three choices： the user may add more RAM， the user may close certain programs， or the user may continue to compute slowly. Today， RAM is so inexpensive that it makes little sense to skimp on RAM. Today， the computer purchaser should only consider machines that offer at least 128 MB of RAM.
Computer Monitors
Three monitor characteristics play a large role in determining the quality of the user’s computing experience， and may significantly influence the user’s productivity. These 3 characteristics are：
The screen size
The dot， and
The refresh rate
Screen size is measured diagonally， in inches. For any given screen resolution， a larger screen allows the user to see more of a file without scrolling. Lower resolutions may be used effectively with larger screens， which helps users with poor near-vision， and allows users to read screens from a farther distance.
Monitors produce colored dots， and these dots are given meanings （e.g. curves， lines， letters， and numbers） by our brains. Dot is measured in millimeters （mm）， and indicates the diagonal distance between same-colored phosphor dots on the screen. Generally speaking， the lower the pitch is， the better （e.g. 0.22 mm is better than 0.28 mm）.
The refresh rate indicates how many times per second the monitor cycles through the screen image. The refresh rate is measured in hertz （Hz）. One hertz equals one cycle per second. A monitor’s refresh rate usually depends upon the resolution at which the monitor is being used. Higher refresh rates are better than lower refresh rates. If the refresh rate is too low， the screen may flicker noticeably， which usually annoys and fatigues the user. Flickering tends to be more noticeable under fluorescent lighting.
【New Words】
assemblage 与会者，集会
tendency 取向，倾向
obsolete
荒废的，陈旧的
diagonal 斜的，对角线
2.2 Computer Processor
A processor is a function unit that interprets and carries out instructions. This is the central nervous system of the computer， and it is often contrasted to the devices that surround the processor and memory， such as keyboards， display screens， and disk， which are known as peripherals. Every processor comes with a unique set of operations such as ADD， STORE， or LOAD that represent the processor’s instruction set. Computer designers are fond of calling their computers machines， so the instruction set is sometimes referred to as machine instructions and the binary language in which they are written is called machine language. You shouldn’t confuse the processor’s instruction set with the instructions found in high-level programming languages， such as BASIC or PASCAL.