Passage Three Storage Devices

Training target: In this part ， you should try your best to form good reading habits. In order to avoid your ill habits such as regression ， you should read with the card showed to you in the preface of this book. Keep reading the paragraphs from the beginning to the end without stopping.

Text We have known that the CPU controlled by program can process data. Then where are the data and the program from? The answer is storage devices. We usually divide the storage devices into two types: the main memory and the secondary storage. A CPU can only execute the instructions of a program which has already been in the main memory.

The main memory of most computers is composed of random access memory ， or RAM.A programmer can read and write RAM. We can store data and programs into RAM. When we have finished using them ， we can let new ones occupy the position of the main memory ， destroying the old ones. In a word ， the content of RAM is easy to change. Sometimes we don’t want the content of memory to be changed ， for example ， the automatic teller terminals used in many banks.

They are controlled by a small computer ， which is controlled by a program. If someone can modify the data to give free access to certain accounts ， the bank would not allow such things to happen. In fact ， these programs are stored in read only memory ， or ROM ， which we can only read but cannot modify. In a word ， ROM is permanent memory that can be read ， but not be written. How can a program or data enter the computer system? We often use diskette drive to copy them into the main memory. Then we come to the concept of secondary storage.

Floppy Disk The most common secondary storage is a floppy disk. A floppy drive stores in formation on a floppy disk or diskette. We usually call the floppy disk drive “drive A”.A floppy disk is a removable device that magnetically stores data. We usually use floppy disks to transfer data from one computer to another. Floppy drives use3.5-inch floppy disks. It is a thin circular piece of flexible polyester coated with a magnetic material. Data are recorded on one or both flat surfaces.

In order to avoid damage ， it usually has a protective jacket ， just as Pic2.8 shows. （ a ） floppy disk （ b ） floppy drive Pic2.8

Hard Disk The hard drive （ Pic2.9 ） is the primary device that a computer uses to store information. Most computers have one hard drive located inside the computer case. If a computer has one hard drive ， it is called“drive C”.If a computer has additional hard drives ， they are called“drives D ， E ， F” ， and so on. Pic 2.9 hard drive

And the hard drive light is on when the computer is using the hard drive. Do not move the computer when this light is on. The hard drive is also called the hard disk ， hard disk drive or fixed disk drive.

The hard drive magnetically stores data on the stack of rotating disks ， called platters. And a hard drive has several read/write heads that read and record data on the disks. A hard drive can store your programs and data files.

How shall we choose a hard drive? The first factor is the capacity. The amount of information a hard drive can store is measured in bytes. A hard drive with a capacity of 2GB to 20GB will suit most home and business users. Purchase the largest hard drive you can afford. New programs and data will quickly fill a hard drive. For example ， Microsoft Word is a word processing program that requires about16 MB of hard drive space. The second factor is average access time.

The average access time is the speed at which a hard drive finds data. The average access time is measured in milliseconds （ ms ）.One millisecond equals 1/1000 of a second. Most hard drives have an average access time of 9 to 14 ms.The lower the average access time is ， the faster the hard drive will be. There are several connection types such as IDE ， EIDE SCSI and so on.

Removable Hard Disk Trays An interesting compromise between internal and external hard disks is the removable hard disk drive tray. A tray is installed into a standard PC case drive bay that allows regular internal hard disks to be placed into it. You then can swap the internal hard disk with another one without opening up the case, allowing you to use hard disks as a removable storage medium. In a way, the concept is

similar to the way a removable car stereo is designed. These trays are also commonly called mobile racks, drive caddied and several other names. For certain applications, this is the ideal removable storage device: it uses regular hard disks, which are very fast, highly reliable, very high capacity and very inexpensive. They can be used for backup purposes ( secondary to a

regularly-installed standard internal hard disk). Removable trays can also be used to allow the use of different operating systems on the same PC (though there are simpler ways to do this) and to allow different people to use the same PC while keeping their data and programs totally separate.

If you decide to use a mobile rack system, be sure to check out the specifications of the unit you are considering carefully. Different models come with support for different speed drives, some are made primarily of metal and others of plastic, and so on. Metal units will provide better cooling than plastic ones. Some also come with an integrated cooling fan, which is a good idea especially for higher-RPM drives.

Other Secondary Storage Media Magnetic tape is one of the most common backup media. Tape is fast with data transfer rates comparable to disk. Its storage capacity is quite high ， and a reel of tape is relatively inexpensive. Unfortunately ， data can be read or written only in a fixed sequence ， which limits tape to a few application areas.

Another secondary storage medium is videodisk. These disks are read and written by a laser beam; there is no physical contact between the recording surface and the read/write mechanism. Fast ， accurate ， compact ， and easy to use ， videodisk has become more and more popular.

Accessing Secondary Storage Media On a small computer ， secondary storage media are linked to the system through interface boards. The interface board physically controls the disk drive ， accepting seek ， read and write commands from the processor ， positioning the access mechanism ， and controlling the flow of data between the disk surface and the main memory.

On a large computer ， channels and I/O control units are used. The channel communicates with the computer. The control unit communicates with the external device.

The End