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3/6/2010

TYPES OF RAM TYPES OF DRAM

• DRAM (Dynamic Random Access Memory) • SDRAM (Synchronous DRAM) • A 3.3 volt, 168-pin memory medule • Used in most • SDRAM started out running at 66 MHz and is able to scale to 133 MHz (PC133) • Least expensive type of RAM • DDR (Double Rate SDRAM) • • DDR basically doubles the rate of data transfer of standard SDRAM, this is achieved by transferring data on both the DRAM is recharged again and again to maintain its data rising edge and the falling edge of a clock cycle. • The cannot access the data when it is refreshed, that makes it slow. • No of pins: 184, 2.5 Volts • Operates at ; 133 – 400 MHz • SRAM (Static Random Access Memory) • DDR2 SDRAM • Memory cells are made from digital gates • DDR2 memory operates at 400 to 1066 MHz. • Each cell can store data without any need to refresh data as long as power is • DDR2 is a 1.8 volt that uses 240 pins in its DIMMs. available. • DDR3 SDRAM • DDR3 SDRAM is an improvement over its predecessor, DDR2 SDRAM, and the two are not compatible. The primary • CPU doesn’t need to wait to access it making it quickly accessible, usually used benefit of DDR3 is the ability to transfer at twice the data rate of DDR2 as Memory. • Uses 1.5 V operates from 800-1600 MHz with 240 Pins

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ROM (READ ONLY MEMORY) TYPE OF ROM

• Read-only memory is a class of storage media used in computers and other • Classic mask-programmed ROM or simply ROM : electronic devices. • Integrated circuits that physically encode the data to be stored, and thus it is impossible to change their contents after fabrication. • Because data stored in ROM cannot be modified (at least not very quickly or • easily), it is mainly used to distribute firmware (software that is very closely Programmable read-only memory (PROM), or one-time programmable ROM (OTP): tied to specific hardware, and unlikely to require frequent updates) • ROM which can be written to or programmed via a special device called a PROM programmer. • Every stored-program requires some form of non-volatile, or • Typically, this device uses high voltages to permanently destroy or create internal links erasable, storage to store the initial program that runs when the computer is (fuses or antifuses) within the chip. Consequently, a PROM can only be programmed once powered on or otherwise begins execution (a process known as • Erasable programmable read-only memory EPROM: bootstrapping, often abbreviated to "booting" or "booting up"). • EPROM can be erased by exposure to strong ultraviolet light (typically for 10 minutes or longer). • Repeated exposure to UV light will eventually wear out an EPROM, but the endurance of most EPROM chips exceeds 1000 cycles of erasing and reprogramming.

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TYPE OF ROM CPU BUS INTERCONNECTION

• Electrically erasable programmable read-only memory (EEPROM) : • The electrical lines or path that connects CPU, main memory and I/O units. • EEPROM is based on a similar semiconductor structure to EPROM, but allows • Buses can be classified as : its entire contents (or selected banks ) to be electrically erased, then rewritten • System Bus: Connects Main memory and CPU. electrically, so that they need not be removed from the computer (or camera, • Types of system buses are: MP3 player, etc.). • Address Bus : A collection of wires between the processor and main memory used to • Writing or flashing an EEPROM is much slower (milliseconds per ) than communicate the physical location (address) of data in main memory. from a ROM or writing to a RAM (nanoseconds in both cases). • Data Bus : A collection of wires between the processor and all internal devices, and is • used to send and receive both data and instructions. Usually Bidirectional. • Flash memory (or simply flash) is a modern type of EEPROM invented in 1984. • Control Bus : A collection of wires between the processor and devices used to send commands and get status signals from the devices. For example, it could show • Flash memory can be erased and rewritten faster than ordinary EEPROM, and whether the computer was reading or writing to memory. newer designs feature very high endurance (exceeding 1,000,000 cycles) • Expansion buses: • Connects CPU and Peripheral Devices.

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SECONDARY STORAGE DEVICES HARD DISK

• Secondary storage (or external memory) differs from primary storage in that it is not • A (often shortened as hard disk directly accessible by the CPU. hard drive, or HDD) is a non-volatile storage device that stores digitally encoded data on • The computer usually uses its input/output channels to access secondary storage rapidly rotating rigid (i.e. hard) platters with magnetic surfaces. and transfers the desired data using intermediate area in primary storage. • HDDs record data by magnetizing • Secondary storage does not lose the data when the device is powered down—it is ferromagnetic material directionally, to non-volatile. represent either a 0 or a 1 binary digit. • • A typical HDD design consists of a spindle that Per unit, it is typically also an order of magnitude less expensive than primary holds one or more flat circular disks called storage. platters, onto which the data are recorded. • Types of Secondary Storage : • The platters are made from a non-magnetic material, usually aluminum or , and • Magnetic are coated with a thin layer of magnetic material, typically 10–20 nm in thickness. • Optical • Semi Conductor

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HARD DISK

• HDD can contain one or more disks • Optical storage is a term from engineering referring to the storage of data on • For addressing, Each disk is divided into 2 sides an optically readable medium. • Each side is divided into tracks . A disk drive • Data is recorded by making marks in a pattern that can be read back with the is a circular path on the surface of a disk or diskette on which information is magnetically aid of light. recorded and from which recorded information • is read. A common modern technique used by computers involves a tiny beam of light precisely focused on a spinning disc • Tracks are sub divided into Sectors . Each sector stores a fixed amount of data. The • typical formatting of these media provides An (ODD) is a disk drive that uses laser light or space for 512 (for magnetic disks) electromagnetic waves near the light spectrum as part of the process of • A cluster is the unit of disk space allocation for reading or writing data to or from optical discs. files and directories. To reduce the overhead of managing on-disk data structures, the • Compact discs, and Blu-ray discs are common types of optical media filesystem does not allocate individual disk which can be read and recorded by such drives. sectors, but contiguous groups of sectors, called clusters.

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DVD COMPACT DISK (DIGITAL VERSATILE DISC OR DIGITAL DISC )

• Capacity : Typically up to 700 MB (up to 80 minutes audio) • Capacity: • Read: 780 nm wavelength • 4.7 GB (single-sided, single-layer) • A CD is made from 1.2 mm thick, almost-pure polycarbonate and weighs 15–20 grams. 8.5 GB (single-sided, double-layer) • CD ROM is written with the help of a laser of higher intensity 9.4 GB (double-sided, single-layer) which makes small "pits" in the reflecting surface 17.08 GB (double-sided, double-layer) • For reading the information of the disk a laser of lesser intensity is used when the laser encounters a pit the laser is refracted and • Read when it doesn't encounters a pit the intensity remains the same, the reflected laser coming out of the disk surface is received by • 650 nm laser, 10.5 Mbit/s (1×) a lens and converted to binary digits or which the computer can understand.

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BLU RAY FLASH MEMORY

• Media Type • Flash memory is a non-volatile computer storage that can be electrically erased and reprogrammed. • High-density optical disc • It is a technology that is primarily used in memory cards and • Capacity USB flash drives for general storage and transfer of data • 25 to 50 GB (single-layer) between computers and other digital products. 50 to 100 GB (dual-layer) • Since flash memory is non-volatile, no power is needed to • Read maintain the information stored in the chip. • 400 nm laser • Flash memory offers fast read access times • It is more kinetic shock resistant than hard disks. • A USB consists of flash memory device integrated with a USB (Universal Serial Bus) 1.1 or 2.0 interface. • Most flash drives ship preformatted with the FAT or FAT 32 . • Like all flash memory devices, flash drives can sustain only a limited number of write and erase cycles before failure.

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