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Memory and Storage Devices

Memory and Storage Devices. Connecting to the Processor. a bus is a connection between components classifying buses data width speed early designs featured a single system bus. Internal Memory.

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Memory and Storage Devices

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  1. Memory and Storage Devices

  2. Connecting to the Processor • a bus is a connection between components • classifying buses • data width • speed • early designs featured a single system bus

  3. Internal Memory • Random Access Memory (RAM)- this is where programs and data are stored while they are being worked on by the computer, while the CPU needs extremely fast access to them. • Cache memory or sometimes called RAM Cache, is special high speed memory that gives the CPU even faster access to data. • CMOS memory (Complementary Metal Oxide Semiconductor) • Read only memory (ROM) - a set of chips that contain permanent instructions to help the computer prepare for processing.

  4. RAM or Main Memory • The most common computer memory • Can be used by programs to perform necessary tasks while the computer is turned on • An integrated circuit memory chip allows, information to be stored or accessed in any order • All storage locations are equally accessible • Access is extremely fast • Memory is divided into units -called words • Each word is fixed size -32 bits or 64 bits. • Each word has a unique address in RAM

  5. RAM Types • DRAM (Dynamic RAM) - must be refreshed many times / sec. Small, slow and cheap. Used for standard main memory. Access time 50-70 ns • SRAM (Static RAM) - holds its data without external refresh, is larger, faster and more expensive than DRAM. Used for caching. Access time 5-15 ns • SDRAM - is synchronized to the system clock therefore it’s time is dependant with best access times of 7-12 ns . Used in newer machines for main memory. • DRDRAM – direct rambus DRAM or simply Rambus DRAM works more like an internal bus than conventional memory in which a 16 bit bus runs at 400MHz giving a theoretical limit of 1.6 GHz / sec. The narrower bus is capable of running at a much faster speed than the 64 bit bus.

  6. Role of Cache in a PC • In early PC’s all components were quite slow. • Memory and memory subsystems have increased by a factor of ten or more. • Processors however have increased performance by a factor of over 1000. • It would be too expensive to build an entire memory system out of SRAM or to make hard drives that would access data 100’s of times faster than now. • It is much better to have a small piece of very fast memory and a smart algorithm to allow you to get nearly as much benefit from this as from a large amount of slow memory.

  7. Direct Access Storage Devices • Long-term storage that will be retained when the computer is shut off. • Magnetic Storage • Floppy disk Storage • Zip Disk Storage • Hard Disk Storage • Optical Storage • CD Storage • DVD Storage

  8. Storage Terminology • Medium: the item that contains the data i.e. disk, tape, CD-ROM, paper • Device: The mechanical apparatus that records or reads back the data from the medium i.e. disk drive, CD drive, tape drive. • Capacity: Maximum amount of data the medium may store, measured in bytes ( 1 byte = 1 character ) and expressed as KB, MB, GB • Access Time: The average time it takes the computer to locate and read the data from the medium. Measured in ms (1 / 1000 sec) • Transfer Rate: The average number of bytes / sec that can be transferred. Expressed in KB / sec or MB / sec.

  9. Disk vs. File Organization • data is stored in clusters • clusters occupy sectors • sectors on tracks • files have names • files are indefinite in size • files may be updated (in part or whole) • directory entries record file data • file allocation table (FAT) keeps track of file pieces

  10. Optical Storage • Compact Disc–Read Only Memory (CD-ROM) • archived and published information • high capacity 650 MB, basic transfer rate 150 KBps • Transfer rate expressed as multiples of basic (ie 40X is 40 * 150KBps = 6000 KBps or 6.0 MBps • Compact Disc – Record able (CD-R) – user may burn their own CD-Rom • Compact Disc–Record able/Write able (CD-R/RW) • Record able and reWrite able • readable using CD-ROM technology

  11. CD-ROM • based on CDDA (compact disc – digital audio) technology • CLV geometry • density: 16,000 tpi • up to 650 MBytes • nonerasable, nonwriteable storage • discs are mastered, pressed (mass production) • multispeeds drives common

  12. CD–R • discs are “burnt” one at a time • high intensity laser beam used for recording pregrooved tracks • low intensity beam for reading • attributes similar to CD-ROM

  13. CD-RW • CD-ReWritable--writable, erasable disc • optical phase-change recording • Erased, written up to 1,000 times • UDF (Universal Disk Format) • variable-length packets • fixed-length packets

  14. Optical Storage – cont. • Digital Versatile Discs (DVD) • Five physical formats of DVD: • DVD-ROM is a high-capacity data storage medium • DVD-Video is a digital storage medium for feature-length motion pictures • DVD-Audio is an audio-only storage format similar to CD-Audio • DVD-R offers a write-once, read-many storage format akin to CD-R – used for home video DVD burning • DVD-RAM was the first rewritable (erasable) flavour of DVD to come to market and has subsequently found competition in the rival DVD-RW and DVD+RW format.

  15. Optical Storage – cont. • Four versions of DVD storage formats: • DVD-5 is a single-sided single-layered disc with a capacity of 4.7GB • DVD-9 is a single-sided double-layered disc offering 8.5GB • DVD-10 is a 9.4GB dual-sided single-layered disc • DVD-18 will increase capacity to 17GB on a dual-sided dual-layered disc.

  16. Optical Storage – cont. • DVD's seven-fold increase in data capacity over the CD has been largely achieved by tightening up the tolerances throughout the predecessor system • Tracks placed closer together and smaller pits give a 4 fold increase. • Shorter wavelength of laser light. • Double-layering the data by using a translucent layer above an opaque layer • Using thinner disks meant they could bond two discs together and still only be the same thickness as a CD, giving double sided. • The data structure was made more efficient by using a better, more efficient error correction code system.

  17. Access Speed – RAM vs. Hard Drive • 1 nano sec = 1/1,000,000,000 sec. • 1 milli sec = 1/1000 sec. • Standard RAM access time = 50 - 100 nsec • Hard drive = 8 - 20 msec • Assume RAM = 100 nsec and drive = 20 msec. • 20 msec = 20,000,000 nsec • Therefore the RAM access is 200,000 X as fast

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