Two Storage Devices 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. Hard Disk The hard drive is also called the hard disk, hard disk drive or fixed disk drive. The hard drive (Pic2-10) 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, it is called “drive D, E, F ”, and so on. 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 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 about 16 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 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 caddies 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 sued for backup purposes 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 . Videodisk Videodisk is 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. --