Computer BasicsLesson 6 - Storage

Main Memory

Storage refers to the media and methods used to keep information available for later use.

Main Memory = Primary Storage Computers use RAM, or Random

Access Memory. Main memory keeps track of what is

currently being processed.  It's volatile, meaning that turning the power off erases all of the data.

Auxiliary Storage

Auxiliary Storage = Secondary Storage

Auxiliary storage holds what is not currently being processed. This is the stuff that is "filed away", but is ready to be pulled out when needed.

It is nonvolatile, meaning that turning the power off does not erase it.

Used for: Input - data and programs Output - saving the results of processing

Magnetic Disks

Advantages:   high storage capacity reliable gives direct access to data

Types Hard Disk Floppy Disk Zip Disk

Disk Format

All magnetic disks are similarly formatted, or divided into areas, called tracks, sectors and cylinders.

A track is a circular ring on one side of the disk. Each track has a number.

The diagram shows 3 tracks.

Disk Format

A disk sector is a wedge-shape piece of the disk, shown in yellow.

A track sector is the area of intersection of a track and a sector, shown in yellow.

Disk Format

A cluster is a set of track sectors, ranging from 2 to 32 or more, depending on the formatting scheme in use.

A cylinder is a set of matched tracks.What happens when a disk is

formatted? 1.  All data is erased. 2.  Surfaces are checked for physical and

magnetic defects. 3.  A root directory is created to list where

things are on the disk.

Disk Capacity

Capacity of a Disk depends on: 1. # of sides used: single-sided or double-sided

2. Recording density - how close together the bits can be on a track sector of the innermost track

3. # of tracks on the disk

Disk Capacity

5¼" floppy - 360 KB or 1.2 MB

3½" floppy - 720 KB or 1.44 MB

Hard disk       early ones = 20 MB       Currently (July 2010) up to 3 TB Internal hard drive where 1 TB  = 1 terabyte = 1000 GB

Accessing Data

The process of accessing data has 4 steps.

1.  Seek - move the head to proper track2.  Rotate - rotate disk under the head to the correct sector3.  Settle - head lowers to disk; wait for vibrations from moving to stop (actually touches only on floppies)4.  Data transfer - copy data to main memory

Accessing Data

ms stands for millisecond = .001 secondkbs is kilobytes per second Total time to transfer a kilobyte:

for floppies, 175 - 300 ms for hard drive, 15 - 80 ms     new hard drives (Jun. 2009) .0032 ms   (300

MB per sec).

Caring for Disks

Most problems occur when the read/write head (looks like a pointer in the photo) damages the metal disk by hitting or even just touching it. This is called a head crash.

Don‘t ever: Jar the computer while the disk is spinning.

  Turn the computer off and quickly back on before

spinning has stopped.  

Drop it - ever.

Computer BasicsLesson 6 – Cont.

Protect Data

How to safeguard your data: Write protect - This keeps your files

from being overwritten with new ones. Backup - Make multiple copies of

important data often. Anti-malware - Use a set of

programs that continuously look for an attack by a virus, trojan, or worm .

Magnetic Tape

Magnetic tape uses a method similar to that of VCR tape for storing data.

Types of Tape:

Tape Reel

Tape Cassette

Tape Formats

Density - Higher density means more data on shorter tape. Measured as bpi = bits per inch

Blocks - The tape is divided into logical blocks, as a floppy is divided into tracks and sectors.

Gap - Two kinds of blank spots, called gaps, are set on the tape. Interblock gap which separates logical

blocks. Interrecord gap which is wider and

separates records.

Optical Disks

An entirely different method of recording data is used for optical disks. These include the various kinds of CD and DVD discs.

How optical disks are similar Formed of layers Data in a spiral groove on starting from the

center of the disk Digital data (1's and 0's) 1's and 0's are formed by how the disk absorbs

or reflects light from a tiny laser.

Optical Disks

The materials used for the data (recording) and metal (reflecting) layers are different for different kinds of optical disks.

The most common type of optical disk is the CD-ROM, which stands for Compact Disc - Read Only Memory.

DVD used to stand for Digital Video Device or Digital Versatile Device, but now it doesn't really stand for anything at all! DVDs are used for recording movies and large amounts of data.

Optical Disks

The optical disks that you can record on your own computer are CD-R, DVD-R, and DVD+R discs, called writable or recordable disks.

The data layer is an organic dye that the writing laser changes. Once the laser modifies the dye, it cannot be changed again.

Manufacturers say that these disks have a shelf-life of 5 - 10 years before they are used for recording.

Optical Disks

An option for backup storage of changing data is rewritable disks, CD-RW, DVD-RW, DVD+RW, DVD+RAM.

The data layer for these disks uses a phase-changing metal alloy film. This film can be melted by the laser's heat to level out the marks made by the laser and then lasered again to record new data.

In theory you can erase and write on these disks as many as 1000 times, for CD-RW, and even 100,000 times for the DVD-RW types.

Advantages

Advantages of Optical Disks1. Physical: An optical disk is much sturdier

than tape or a floppy disk. It is physically harder to break or melt or warp. It's somewhat harder to lose than a USB flash drive.

2. Delicacy: It is not sensitive to being touched, though it can get too dirty or scratched to be read. It can be cleaned!

3. Magnetic: It is entirely unaffected by magnetic fields.

4. Capacity: Optical disks hold a lot of data, especial the double-sided DVDs.

Disadvantages

Disadvantages of Optical Disks1. Cost: The main disadvantage has

been cost – both for the drive and the disks.

2. Duplication: It is not quite as easy or as fast to copy an optical disk as it is to copy files to a USB flash drive. You need the software and hardware for writing disks!

Data Loss

Data loss comes from:Physical damage - breaking, melting, scratching...

Blocking of laser light by dirt, paint, ink, glue...

Corrosion of the reflecting layer

Cleaning

Cleaning: Keep it clean! Handle by the edges or center hole. Put it back in its case as soon as you are finished

with it. No laying around on the desktop!! Remove dirt and smudges with a clean cotton cloth

by wiping from the center to the outer edge, NOT by wiping around the disk. Wiping in a circle can create a curved scratch, which can confuse the laser.

For stubborn dirt, use isopropyl alcohol or methanol or CD/DVD cleaning detergent.

Labeling

Labeling: Don't use an adhesive label. The adhesive can

corrupt your data in just a few months! Don't write on or scratch the data side of the

disk - ever! Don't scratch the label side. Don't write on the label side with a pencil or pen

(scratches!) Don't write on the label side with a fine-point

marker or with any solvent-based marker. Use markers for CDs. (Solvent may dissolve the protective layer.)

Storage

Storage: Store optical disks upright on edge, like a book, in a plastic case designed specifically for them. Not flat for long periods!

Store in a cool, dark environment where the air is clean and dry. NO SMOKE! Low humidity.

How to treat it

How you treat it: Keep away from high heat and high

humidity which accelerate corrosion. Keep out of sunlight or other sources of

ultraviolet light. Keep away from smoke or other air

pollution. Don't bend it! Don't use a disk as a coaster or a frisbee

or a bookmarker!

What to do when recording

Recording Check disk for flaws and dirt BEFORE recording on it.

Only open a recordable disk just before you plan to record on it.

After recording, make sure the disk works as you expect: Read data; run programs.