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    While performing a processing operation, the CPU needs a place to temporarily hold instruction to be executed and data to be used with those instructions. Memory, which is composed of one or more chips on the motherboard, holds data and instruction while the CPU is processing them.

The two basic type of memory are volatile and nonvolatile. The contents of volatile memory, such as RAM are lost (erased) when the power to the computer is turned of. The contents of nonvolatile memory, however, are not lost when power is removed from the computer. For example, once instruction has been record onto a nonvolatile ROM chip, they usually cannot be erased or change, and the contents of the chips are not erased when power is turned off. 



Storage also called secondary storage, auxiliary storage, or mass storage. Holds items such as data, instruction, and information for future use.

Think of storage as a filling cabinet used to hold file folder, and memory as the top of your desk. When you need to work with a file, you remove it from the filling cabinet (storage) and place it on your desk (memory). When you are finished with the file, you return it to the filling cabinet (storage).

Storage is nonvolatile, which means that items in storage are retained even when power is removed from the computer (fig 6-1). A storage medium (media is the plural) is the physical material on which item are kept. One commonly used storage medium is a disk, which is a round, flat piece of plastic or metal with a magnetic coating on which items can be written. A storage device is the mechanism used to record and retrieve items to and from a storage medium.

Storage devices can function is sources of input and output. For example, each time a storage devices transfer data, instruction, and information from a storage medium into memory-a process called reading-it function as an input source. When a storage device transfer these item from memory to a storage medium-a process called writing-it function as an output source. The speed of the storage device is defined by its access time, which is the minimum time it takes the devices to locate a single item on a disk.

Compared to memory, storage devices are slow. The access time of memory devices is measured in non second (billionths of a second), while the access time of storage devices is measured in million seconds (thousands of a second).

The size, or capacity, of storage devices, is measured by the number of bytes (characters) it can hold, fig 6-2 lists the terms used to define the capacity of storage devices. For example, a typical floppy disk can store 1.44 MB of data (approximately 1,440,000 bytes) and a typical hard disk can store 8GB of data (approximately 8,000,000,000 bytes).

Storage requirements among user vary greatly. Users of smaller computer, such as small business users, might need moth store a relatively small of amount data, for example, a field sales representative might have a list of names, addresses, and telephone number of 50 customers, which he or she uses on a daily basis. Such a list might require no more than several thousand bytes of storage. Users of larger computer, such as banks, libraries or insurance companies, process data for millions of customers and thus might need to store trillions of bytes worth of historical or financial record in their achieves.

To meet the needs of a wide range of users, numerous types of storage media and storage devices exist, figure 6-3 show how different types of storage media and memory compare in terms of relative cost and speed. The storage media included in the pyramid are discussed in this chapter.



     A floppy disk, or diskette, is a portable, inexpensive storage medium that consists of a thin, circular, flexible plastic disk with a magnetic coating enclosed in a square-shaped plastic shell (figure 6-4). In the early 1970s, IBM introduced the floppy disk a new type or storage. Because these early 8-inch wide disk had flexible plastic cover, many users referred to them as floppies. The next generation of floppies looked much the same, but was only 5.25-inches wide. Today, the most widely used floppy disk is 3.5-inches wide. The flexible cover of the earlier floppy disk has been replaced with a rigid plastic outer cover. Thus, although today’s 3-5 inches disk are not at all floppy. The term floppy disk still used.

As noted, a floppy disk is a portable storage medium. When discussing a storage medium, the term portable means you can remove the medium from one computer and carry it to another computer. For example, you can insert a floppy disk into and remove it from the floppy disk drive on many types of computer (figure 6-5). A floppy disk drive is a device that can read from and write to a floppy disk.



To protect them from accidentally being erased, floppy disk have a write-protect notch. A write-protect notch is a small opening in the corner of the floppy disk with a tab that you slide to cover or expose the notch (figure 6-8). The write-protect notch works much like the reading tab on VHS tape: if the recording tab is removed, a VCR cannot record onto the VHS tape.

On a floppy disk, if the write protect notch is exposed, or open, the drive cannot write on the floppy disk. If the write-protect north is covered, or closed, the drive can write on the floppy disk. The write protect notch only affect the flopper disk drive’s capability of writing on the disk: a floppy disk drive car read from the floppy disk whether the write-protect notch is open or closed. Some floppy disk has a second opening on the opposite side of disk that does not have the small tab: this opening identifies the disk as a high-density floppy disk



As noted, a floppy disk drive (FDD) is a device that can read from and write on a floppy disk. Desktop personnel computers usually have a floppy disk drive installed inside the system unit. Many lap top computer have removable floppy disk drives that can be replaced with other types of drives or devices, or they use an external floppy disk drive that plugs into the laptop (see figure 6-5 on page 6.4).

If a computer has one floppy disk drive, the drive usually is designated drive A; if the computer has two floppy disk drive; the second one usually is designated drive B. To read from or write on a floppy disk, a floppy disk drive must support that   floppy disk’s density. That is, to use a high-density floppy disk, you must have a high-density floppy disk drive. Floppy disk drive is downward compatible, which means they recognize and can use earlier media. Floppy disk drive is not upward compatible, however, which means they cannot recognize newer media. For example, a lower-density floppy disk drive cannot read from or write on a high-density floppy disk.

On any 3.5-inch floppy disk, a piece of metal called the shutter covers an opening in the rigid plastic shell. When you insert a floppy disk into a floppy disk drive, the drives slides shutter to the side to expose a portion of both sides of the floppy disk’s recording surface.

The read/write head is the mechanism that actually read items from a write items on the floppy disk. Figure 6-0 illustrates the steps for reading from and writing on a floppy disk. The average access time for current floppy disk drives to locate an item on the disk is 84 ms, or approximately 1/12 of a second.

On the front of most floppy disk drives is a light emitting diode (LED) that lights up when the drive is accessing the floppy disk. You should not remove a floppy disk when the floppy disk drive is accessing the disk.Sometimes, a floppy disk drive will malfunction when it is attempting to access a floppy disk and will display an error message on the computer’s monitor screen, if the same error accurse with multiple floppy disks, the read/write heads in the floppy disk drive may have a buildup of dust or dirt. In this case, you can try cleaning the read/write heads using a floppy disk cleaning kit.  



When personnel computer (PC) were introduced, software programs and their related files required small amount of storage and fit easily on floppy disk. As software became more complex and included graphical user interfaces and multimedia, file size and storage requirement increased. Today, hard disk-which provide far larger storage capacities and much faster access time than floppy disk-are one of the primary media for storing software programs and file. Current personnel computer hard disk can store 4 to 50 gb of data, instruction and information.

A hard disk usually consist of several inflexible, circular disk, called platter, on which item are stored electronically. A platter in a hard disk is made of aluminum, glass, or ceramic and is coated with a material that allow item to be magnetically recorded on it surface. On hard disk, the platter, the read/write heads, and the mechanism for moving the heads across the surface of the disk enclosed in a airtight, sealed case that protect the platters from contamination

The hard disk in most desktop personnel computer is housed inside the system unit. Such hard disk, which are not portable, are considered fixed disk (figure 6-13). Hard disks also can be removable. Removable hard disk is discussed later in the chapter.