Q. Working of Electrically erasable programmable rom
EEPROM is used for remote-area applications. The device is provided with special pins which, when activated electrically, alter the rewriting of selected memory locations most common disks have 11-in diameters and 200 tracks (concentric rings of data) per surface, numbered from 0 to 199, starting with the outside perimeter of the disk, with a typical track packing density of 4000 bits per inch. Disks are mounted on a common spindle, and all disks rotate at a typical speed of 3600 revolutions per minute (rpm). A typical disk has 17 sectors of fixed size per track and 512 bytes (1 byte = 8 bits) of information per sector. Any desired sector can be quickly accessed.
Floppy disks, also known as flexible disks, are the low-cost, medium-capacity, nonvolatile memory devicesmade of soft flexiblemylar plasticwithmagnetically sensitive iron-oxide coating.
The original 8-in standard floppy is no longer in popular use. The 5 1/4 -in minifloppy has a disk and a disk jacket covering the mylar media for protection, along with a write-protect notch and index hole. The present-day minifloppy disks are either double-sided/double-density (DS/DD) with 9 sectors per track and 40 tracks per side or double-sided/quad-density (DS/QD) with 9 or 15 sectors per track and 80 tracks per side. In a DS/DD minifloppy disk, about 720 kbytes of data can be stored; whereas in a DS/QD mini-floppy disk, about 1-2 Mbytes of data can be stored.
The 3 1/2-in microfloppy disk, also known as a microdiskette, is enclosed in a rigid protective case and is provided with a write-protect notch. Microdiskettes are recorded in quad-density format with a capacity of 2 Mbytes; 4- to 16-Mbyte 3 1/2-in diskettes are being developed. Also, 2-in diskettes are introduced in electronic cameras and portable personal computers.
Magnetic tapes are ideal devices for storing vast quantities of information inexpensively. However, the access time is slow because the entire tape must be read sequentially. The most commonly used tapes are ½-in wide, 2400 or 3600 ft long, and contained in a long 10 ½ -in reel. Tape densities of 200, 556, 800, 1600, 6250, and 12,500 bits per inch (BPI) are standard. In addition to these magnetic storage devices, two newer types of secondary storage have come into use:winchester disks and videodisks (also known as optical disks).The former are sealed modules that contain both the disk and the read/writemechanism, requiring littlemaintenance and allowing higher-density recording. The latter have been introduced recently, with high reliability and durability and a storage capacity of 1 Gbyte of data (equivalent to almost 400,000 typewritten pages of information). A typical 14-in optical disk has 40,000 tracks and 25 sectors per track, with each sector holding up to 1 kbyte of information. While a write-once optical-disk drive is currently available, a read-and-write drive is being developed.