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COMPUTER MEMORIES
Posted Date: 22 Feb 2008 Resource Type: Articles/Knowledge Sharing Category: Computer & Technology
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Posted By: syam s kurup Member Level: Silver
Rating:  Points: 5
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COMPUTER MEMORIES
Computer Memory is a mechanism that stores data for use by a computer. In a computer all data consist of numbers. A computer stores a number into a specific location in memory and later fetches the value. Most memories represent data with the binary number system. In the binary number system, numbers are represented by sequences of the two binary digits 0 and 1, which are called bits. In a computer, the two possible values of a bit correspond to the on and off states of the computer's electronic circuitry.
Computer memory may be divided into two broad categories
* Internal memory
* External memory
Internal memory operates at the highest speed and can be accessed directly by the central processing unit (CPU)—the main electronic circuitry within a computer that processes information. Internal memory is contained on computer chips and uses electronic circuits to store information. There are two types
• Internal RAM
Random access memory is also called main memory because it is the primary memory that the CPU uses when processing information. The electronic circuits used to construct this main internal RAM can be classified as dynamic RAM (DRAM), synchronized dynamic RAM (SDRAM), or static RAM (SRAM). DRAM, SDRAM, and SRAM all involve different ways of using transistors and capacitors to store data.
• Internal ROM
Read-only memory is the other type of internal memory. ROM memory is used to store items that the computer needs to execute when it is first turned on. For example, the ROM memory on a PC contains a basic set of instructions, called the basic input-output system (BIOS). The PC uses BIOS to start up the operating system. BIOS is stored on computer chips in a way that causes the information to remain even when power is turned off.
Information in ROM is usually permanent and cannot be erased or written over easily. A ROM is permanent if the information cannot be changed—once the ROM has been created, information can be retrieved but not changed. Newer technologies allow ROMs to be semi-permanent—that is, the information can be changed, but it takes several seconds to make the change. For example, a FLASH memory acts like a ROM because values remain stored in memory, but the values can be changed.
External memory consists of storage on peripheral devices that are slower than internal memories but offer lower cost and the ability to hold data after the computer’s power has been turned off. External memory uses inexpensive mass-storage devices such as magnetic hard drives. This can be classified as
• Magnetic media
A magnetic storage device, such as a computer's hard drive, uses a surface coated with material that can be magnetized in two possible ways. The surface rotates under a small electromagnet that magnetizes each spot on the surface to record a 0 or 1. To retrieve data, the surface passes under a sensor that determines whether the magnetism was set for a 0 or 1. Magnetic disks are classified as either hard or floppy, depending on the flexibility of the material from which they are made. A floppy disk is made of flexible plastic with small pieces of a magnetic material imbedded in its surface. The read-write head touches the surface of the disk as it scans the floppy. A hard disk is made of a rigid metal, with the read-write head flying just above its surface on a cushion of air to prevent wear. Inside a Computer Hard Drive The inside of a computer hard disk drive consists of four main components. The round disk platter is usually made of aluminum, glass, or ceramic and is coated with a magnetic media that contains all the data stored on the hard drive. The yellow arm like device that extends over the disk platter is known as the head arm and is the device that reads the information off of the disk platter. The head arm is attached to the head actuator, which controls the head arm.
• Optical media
Optical external memory uses a laser to scan a spinning reflective disk in which the presence or absence of non reflective pits in the disk indicates 1s or 0s. This is the same technology employed in the audio CD. Because its contents are permanently stored on it when it is manufactured, it is known as compact disc-read only memory (CD-ROM). A variation on the CD, called compact disc-recordable (CD-R), uses a dye that turns dark when a stronger laser beam strikes it, and can thus have information written permanently on it by a computer.
• Magneto-optical media
Magneto-optical (MO) devices write data to a disk with the help of a laser beam and a magnetic write-head. To write data to the disk, the laser focuses on a spot on the surface of the disk heating it up slightly. This allows the magnetic write-head to change the physical orientation of small grains of magnetic material (actually tiny crystals) on the surface of the disk. These tiny crystals reflect light differently depending on their orientation. By aligning the crystals in one direction a 0 can be stored, while aligning the crystals in the opposite direction stores a 1. Another, separate, low-power laser is used to read data from the disk in a way similar to a standard CD-ROM. The advantage of MO disks over CD-ROMs is that they can be read and written to. They are, however, more expensive than CD-ROMs and are used mostly in industrial applications. MO devices are not popular consumer products.
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Responses
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| Author: Moncy 24 Feb 2008 | Member Level: Silver Points : 2 | Thanks for that piece of information..Try to include main differences between SD,DDR1,DDR2,DDR3 memories...
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