Auxiliary Memory

Presented by:Nazir Ahmad 110215 Shaakir Mozzam 110222Junaid Javeed 110229

Presented to:Mr. Aafaq Alam Khan

Contents

• Memory hierarchy• Magnetic disks• RAID• Optical disks• References

Memory hierarchy

Magnetic disk

• Based on the concept of magnetism.

• A disk is a circular platter constructed of nonmagnetic material,

called the substrate, coated with a magnetizable material like iron

oxide. Particles are magnetized to store data.

• Substrate used to be aluminum.

• Nowadays, glass is used because:

improved magnetic film surface uniformity, thus reliability.

reduction in surface defects, helps to reduce read/write errors.

Support lower fly heights.

Magnetic disk- write mechanism

• Data recorded and retrieve via a conducting coil, called a head.

• Head, consists of a core, with an air gap and a set of coils.

• May be separate read and write head.

• During read/write, the head is stationary, the platter rotates

beneath it .

• For writing data onto the medium, the data is converted into

current and is passed through the read/write head coils. This

generates a magnetic flux. This magnetic flux, is used to

magnetize the particles on the surface.

Magnetic disk- read mechanism

• During the read operation, when the flux transitions pass under a

head gap, a voltage is introduced in the read/write coils. This

voltage is converted into the data pulses.

• New method:

separate read head, positioned close to write head for

convenience.

Read head consists of a partially magnetoresistive (MR) sensor.

MR material has an electrical resistance, which depends on the

direction of magnetic field moving under it.

Read mechanism contd…

• By passing current through MR sensor, resistance

changes are detected as voltage levels.

• Allows greater storage density , high frequency

operation and high speeds.

Disk organization and formatting

• Concentric Rings or tracks

Each track has the width same as of disk head.

Tracks separated by gaps, minimizes errors in case of interference or

head misalignment.

• tracks divided into sectors- pie shaped segments, data transferred to

and from the disk sectors.

• Minimum block size is one sector; 512 bytes in most systems.

• May have more than one sector per block.

Disk data layout

Disk Velocity

• Bit near centre of rotating disk passes fixed point slower than bit on outside of disk

• Increase spacing between bits in different tracks • Rotate disk at constant angular velocity (CAV)

Gives pie shaped sectors and concentric tracks Individual tracks and sectors addressable Move head to given track and wait for given sector Waste of space on outer tracks

Lower data density

• Can use zones to increase capacity Each zone has fixed bits per track More complex circuitry

Disk layout diagrams

Multiple Platter

• One head per side

• Heads are joined and aligned

• Aligned tracks on each platter form cylinders

• Data is striped by cylinder

reduces head movement

Increases speed (transfer rate)

• Cylinder- set of all tracks in same relative position on the

platter.

Magnetic disk types

• Floppy disks

• Hard disks

o Zip/bernoulli disk- on platter, 100 MB capacity

o Winchester disk- two or more platters, capacity in GBs.

Speed/ Access time

• Seek time: time taken by head to move to a specific track

• Latency/Rotational delay: time required to spin the desired sector

under the head.

• Transfer time: time taken to transfer data.

• Access time=seek time + latency+ transfer time

RAID

RAID

•Redundant array of independent disks.

•Coined at University of Berkeley in 1987 as redundant array of inexpensive disks.

•Simultaneous data access by using multiple heads.

•Separate physical drives viewed by os a single drive.

Stripping:Data is segmented into equal-size partitions (the striping unit), which are transparently distributed across multiple disks.Mirroring:Data is replicated across multiple redundant disks

RAID levels

•RAID Level 0•RAID Level 1•RAID Level 2•RAID Level 3•RAID Level 4•RAID Level 5..........

RAID controller

Software Controller: Stored program concept

Hardware Controller: Hardware based

RAID 0

•Striping•No error correction •No redundancy•High speed transfer

RAID 1

•Mirroring•Mirroring makes exact copy of data•Each write to both the disks•No error correcting mechanism•Provides fault tolerance

RAID 2

•Parity used to introduce redundancy•Error correcting•Fault tolerance•Parallel access•Small data stripes for high speed transfer

RAID 5

•Independent access technique.•Block parity in distributed manner.•Fault tolerance

OPTICAL MEMORY

Compact Disk

• First made in 1983.• Compact Disk types

• CR-Read only• CD-Recordable• CD-R/W

• Data is encoded and read optically with a laser• Can store around 600MB to 700MB data

Data representation in CD-ROM• Digital data is represented as a series of Pits and

Lands:– Pit = a little depression, forming a lower level in the

track– Land = the flat part between pits, or the upper levels

in the track

Organization of data

• Reading a CD is done by shining a laser at the disc and detecting changing reflections patterns.– 1 = change in height (land to pit or pit to land)– 0 = a “fixed” amount of time between 1’s

LAND PIT LAND PIT LAND ...------+ +-------------+ +---...

|_____| |_______|

..0 0 0 0 1 0 0 1 0 0 0 0 0 0 1 0 0 0 1 0 0 ..

• Note : we cannot have two 1’s in a row!

CD-ROM

• Because of the heritage from CD audio, the data is stored as a single spiral track on the CD-ROM, contrary to magnetic hard disk’s discrete track concept.

• Thus, the rotation speed is controlled by CLV-Constant Linear velocity. The rotational speed at the center is highest, slowing down towards the outer edge. Because, the recording density is the same every where.

• Note that with CLV, the linear speed of the spiral passing under the R/W head remains constant.

• CLV is the result for the poor seek time in CD-ROMs• The advantage of CLV is that the disk is utilized at its best

capacity, as the recording density is the same every where.

CD-ROM

• Note that: Since 0's are represented by the length of time between transitions, we must travel at constant linear velocity (CLV)on the tracks.

• Sectors are organized along a spiral• Sectors have same linear length• Advantage: takes advantage of all storage space

available.• Disadvantage: has to change rotational speed when

seeking (slower towards the outside)

Digital Versatile Disc

The DVD (Digital Video Disc or Digital Versatile Disc) technology is based on CD technology with increased storage density.

• The DVD’s come with a storage capacity of up to 17GB

• Multi-layer• Very high capacity (4.7GB to 17GB)• Has same three types as CD

Why DVD has higher data capacity?

References

• Stalling William, Computer Organization

and Architecture, Pearson Publications,

2010

• Govindarajalu B, Computer Architecture

and Organization, Tata McGraw Hill, 2009

• Wikipedia/RAID.html