计算机引论Chapter8

Computing Essentials
Chapter 8 Secondary Storage
Zhixing Huang huangzx@
Chapter 8

To be a competent end user you need to be aware of the different types of secondary storage. You need to know their capabilities, limitations, and uses. There are four widely used storage media: floppy disk, hard disk, optical disk, and other types of secondary storage.
ASCII
Traditional Floppy Disk


The traditional floppy disk is the 1.44 MB 31/2 inch disk. 2DH indicates two-sided high density disk. Disks have shutters that open to provide access to the recording surface, labels to record disk content, and write-protection notches to allow or prohibit writing to the disk. Files are organized according to tracks (concentric circles) and sectors (wedge-shapes)
Secondary Storage

Secondary storage devices hold data and programs even after electrical power to the computer system has been turned off.

Floppy Disks Hard Disks Optical Disks
How a Floppy Disk Work

Step 2: Receive Instruction. Via cables connected to the system board, the drive receives a request to save a copy of a file located in RAM.
How a Floppy Disk Work

Step 5: Position Heads. The drive rotates the disk and positions the read/write heads to an open or free area on the disk's surface.

Characteristics



Media or medium is the actual physical material that holds the data and programs. Capacity measures how much a particular storage medium can hold. Storage devices are hardware that reads data and programs from storage media. Most also write to storage media. Access speed or access time measures the amount of time required by the storage device to retrieve data and programs.
Chapter 8
After reading this chapter, you should be able to Describe the following kinds of hard disks: internal hard disks, hard-disk cartridges, and hard-disk packs. Describe ways to improve hard-disk operations: disk caching, redundant !arrays of inexpensive disks, data compression and decompression.
How a Floppy Disk Work

Step 4: Check Filename. Working with the operating system, the new file name is compared to those on the disk. If a duplication occurs and data could be lost, the user is given the option to rename the new file or overwrite (lose) the existing file.
Chapter 8


Describe the different types of optical disks. Describe other kinds of secondary storage including solid state storage, Internet drives, and magnetic tape.
How a Floppy Disk Work

The size of the cluster is determined by the operating system. For example,a cluster with two sectors has a capacity of 1024 bytes (512 bytes per sector multiplied by 2 sectors). When a file is stored, it will use an entire cluster even for small files. If the file is larger that the capacity of one cluster, it will use additional clusters. As discussed in Chapter 5, the operating system will attempt to store a large file in adjacent sectors and clusters. If it cannot, the file is broken up into parts or fragments and stored wherever possible.
Storage

Primary storage

most RAM provides only temporary or volatile storage Secondary storage provides permanent or nonvolatile storage.

Secondary storage
How a Floppy Disk Work

Step 3: Check Write Protection. The ve directs a light at the disk's write protection notch. If the light passes through (notch open) the drive refuses the request. If the light does not pass through (notch closed), the drive accepts the request.
Floppy Disks


They use flat circular pieces of. Mylar plastic that have been coated with a magnetic material. Floppy disk. drives (FDD) store data and programs by altering the electromagnetic charges on the disk's surface to represent ones and zeros.
Floppy Disks


Floppy disks are widely used to store and transport data from one computer to another. They are called floppy because date is stored on a very thin flexible, or floppy, plastic disk.
Codes

ASCII

American Standard Code for Information Interchange Extended Binary-Coded Decimal Interchange Code

EBCDIC


Unicode binary codes
How a Floppy Disk Work

For most microcomputer systems, the floppy disk drive is mounted in the system unit and designated as the A drive. When a floppy disk is inserted into the drive, the disk's shutter opens and the drive's read/write head moves across the exposed disk's surface to either retrieve (read) information or save (write) information. The smallest amount of disk space that can be read from or written to a floppy disk is called a cluster or allocation unit.