Data Storage Lecture 3 CSCI 1405, CSCI 1301 Introduction to Computer Science Fall 2009

Main memory cells Cell: A unit of main memory (typically 8 bits which is one byte)

Main memory & address

Memory techniques Random Access Memory (RAM): Memory in which individual cells can be easily accessed in any order Capacity: –Kilobyte: 2 10 bytes = 1024 bytes ~ 10 3 –Megabyte: 2 20 bytes = 1,048,576 bytes ~ 10 6 –Gigabyte: 2 30 bytes = 1,073,741,824 bytes ~10 9

Mass Storage (Secondary Storage) 1.Magnetic Systems (Disk, Tape) 2.Optical Systems (CD, DVD) 3.Flash Drives Larger capacity Less volatility Slower: Requires mechanical motion- more time On-line: connected and readily available to the machine Off-line: requires human intervention before being accessed

Magnetic systems Two kinds: 1.sequential access secondary storage: searching for a certain data means starting from the beginning of the file sequentially till the data is reached ( like magnetic reel tape and cassette ) 2.Direct access secondary storage: Data can be reached directly without going through the stored data (like magnetic disk )

Magnetic Disk The most widely used storage medium in computers today Data is stored by magnetizing particles on the storage medium. Capacity of disk storage system depends on the number of disk used and the density in which the tracks and sectors are placed –Lower-capacity system consists of a single plastic disk known as diskette or floppy disk (off-line storage). 3 1/2 inch have a capacity of only 1.44 MB –High capacity system like hard-disk (consist of 5-10 rigid disks mounted on a common spindle )

Magnetic Disk Head, track, sector, cylinder Access time = Seek time + Rotation delay/latency time Transfer rate: the rate at which data can be transferred to or from the disk

Floppy Disks and Drives –Single plastic flexible disk –Typically 3½ inches in diameter. –Typically hold 1.44 megabytes. –Inserted into floppy disk drive to be read from or written to.

Floppy Disks and Drives, Cont’d. Disk is divided into tracks, sectors, and clusters.

Hard Disk System: Usually (5-10) rigid disks mounted on a common spindle and combined into a disk pack Disk drive - a device that allows data to be read from or written to a disk Disk drive for personal computers contained within computer housing Can be internal or external. Large computer systems may have several external disk drives Capacity in gigabytes Accessing files much faster than accessing files on diskettes

Disk Packs Each disk has its own access arm with read/write head Most disk packs combine disks, access arms, and read/write head

Reading/Writing Data Access arm moves read/write head over particular location Read/write head hovers a few millionths of an inch above disk and doesn’t touch the surface of the disk. –If head touches platter, a head crash occurs and data is destroyed –Data can be destroyed if head touches miniscule foreign matter on surface of disk

How Data Is Organized Organized into tracks, sectors, clusters, and cylinders Track Sector Cluster Cylinder

Track The circular portion of the disk surface that passes under the read/write head –Floppy diskette has 80 tracks on each surface –Hard disk may have 1,000 or more tracks on each surface of each disk Return

Sector Each track is divided into sectors that hold a fixed number of bytes –Typically 512 bytes per sector Zone recording assigns more sectors to tracks in outer zones than those in inner zones –Uses storage space more fully Return

Cluster A fixed number of adjacent sectors that are treated as a unit of storage –Typically two to eight sectors, depending on the operating system Return

Cylinder The track on each surface that is beneath the read/write head at a given position of the read/write heads –When file is larger than the capacity of a single track, operating system will store it in tracks within the same cylinder Return

Disk Access Speed Access time - the time needed to access data on disk Three factors –Seek timeSeek time –Head switchingHead switching –Rotational delayRotational delay Once data found, next step is data transferdata transfer

Seek Time The time it takes the access arm to get into position over a particular track –All access arms move as a unit –All simultaneously in position over a set of tracks that make up a cylinder Return

Head Switching The activation of a particular read/write head over a particular track –All access arms move together, but only one read/write head can operate at any one time Return

Rotational Delay The time it takes for the desired data on the track to rotate underneath the read/write head –On average, half the time for a complete revolution of the disk Return

Data Transfer The process of transferring data between its location on the disk track and memory Measures of performance –Average access time About 10 milliseconds (in electric circuits in nanoseconds!) –Data transfer rate - how fast data can be transferred once it has been found Stated in terms of megabytes per second Return

Optical Systems: Like Compact Disk (CD): These disks are 12 cm (approximately 5 inches) in diameter and consist of reflective material covered with a clear protective coating The capacity of the CD about 600 to 700 MB. Information is recorded on them by crating variations in their reflective surfaces. This information can be retrieved by means of a laser beam. That monitors irregularities on the reflective surface of the CD. (Laser scans disk and picks up light reflections from disk surface)

CD storage

Optical Systems: Information on these CDs is stored on a single track that spiral around the CD, this track divided into units called sectors. Each with its own identifying marking s and a capacity of 2KB of data The distance around the spiraled track is greater from the outer edge of the disk than the inner portion To maximize the capacity of a CD: information is stored at a uniform linear density over the entire spiraled track which means that more information is stored in a loop around the outer portion of spiral than in a loop around the inner portion

Optical Systems: Compact Disks Categorized by read/write capability: CD-ROM - drive can only read data from CDs –stores up to 700 MB per disk –Primary medium for software distribution CD-R - drive can write to disk once –Disk can be read by CD-ROM or CD-R drive CD-RW - drive can erase and record over data multiple times –Some compatibility problems trying to read CD-RW disks on CD-ROM drives

Optical Systems: Digital Versatile Disk (DVD) Constructed from multiple, semi-transparent layers –DVD drive can read CD-ROMs –Capacity up to 17GB –Allows for full-length movies –Sound is better than on audio CDs Several versions of writable and rewritable DVDs exist

Flash memory media Bits are stored by sending electronic signals directly to the storage medium where they causes electrons to be trapped in tiny chambers of silicon dioxide.

Flash memory media Flash memory media, cont’d. –Common forms of flash memory: Flash memory sticks and cards. Commonly used with digital cameras, digital music players, handheld PCs, notebook computers, smart phones, etc. Flash memory drives, such as USB flash drives. –with capacities of up to a few GB, are available for general mass storage applications –packaged in small plastic cases approximately three inches long with the removable cap on one end to protected the unit's electrical connector when the drive is off-line

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